T A J ,116:293 1998.TheAmericanAstronomicalSociety ...migall.fastmail.fm/astronomy/stars_and_nebulae/orion/objects/orion_s_m...THE ASTRONOMICAL JOURNAL,116:293È321,1998July (1998.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A

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EXTERNALLY ILLUMINATED YOUNG STELLAR ENVIRONMENTS IN THE ORION NEBULA:HUBBL E SPACE T EL ESCOPE PLANETARY CAMERA AND ULTRAVIOLET OBSERVATIONS

JOHN RALPH S. DAVID AND DOUGBALLY,1,2 SUTHERLAND,3 DEVINE,2,4 JOHNSTONE5Received 1997 September 29 ; revised 1998 April 1

ABSTRACTWe present new resolution (22 AU) narrowband Hubble Space Telescope Planetary Camera0A.05

images of externally illuminated young stellar objects embedded in the Orion Nebula. We also presentresolution (9 AU) UV images of seven externally illuminated protostellar environments and the Ðrst0A.02

UV spectra that cover the spectral range between 1400 and 3000 We discuss 43 objects for which theÓ.angular resolution has been improved over previous data by more than a factor of 2. These young stellarobjects are either embedded inside the Orion Nebula and externally illuminated by the Trapezium starsor located in front of the nebula and seen in silhouette. The visibility of young stars surrounded bydi†use matter is dominated by intense line emission from ionization fronts (IFs) propagating intomaterial photoablated from circumstellar disks by soft UV radiation. Near the Trapezium stars, the elec-tron density at these IFs is around 105 to 106 cm~3 and the radial intensity proÐles of emission linesdecrease roughly as r~3, consistent with an approximately constant-velocity diverging Ñow with an r~2density proÐle. However, some radial intensity proÐles are better Ðtted with an exponential function.Low ionization fraction near the IF and the heating and acceleration of the photoablation Ñow canexplain deviations from the r~3 intensity proÐles. Many young stars located within 30A of h1 Ori C aresurrounded by concentric arcs of [O III] and Ha emission located to 3A from the IF facing h1 Ori C.0A.5These arcs may trace bow shocks formed by the interaction of the expanding photoablation Ñow withthe fast stellar wind from h1 Ori C. The [O III] emission may be enhanced by UV radiation from h1 OriC, thermal conduction, and/or turbulent mixing of weakly shocked, photoablated gas with the therma-lized shocked stellar wind. About 30% of the bright externally illuminated young stellar objects containdark regions seen in silhouette against background nebular emission in Ha and the forbidden transitionsof common ions that may trace circumstellar protoplanetary disks. In some sources, the regions seen insilhouette in Ha and ionic transitions are bright in the 6300 [O I] line. Most externally illuminatedÓyoung stellar objects have dusty tails pointing radially away from the source of ionizing photons. Tailshave an average length of 500 AU, independent of the projected distance from h1 Ori C, are limb-brightened in emission lines, and are sometimes seen in silhouette against background nebular light, indi-cating that they contain large column densities of gas and dust. We discuss a variety of tail formationmechanisms and conclude that initial conditions probably play a key role in their formation. The largefraction of young stellar objects with extended circumstellar structure, the mass limits on this structure,and the estimated mass-loss rates are combined to produce an estimate for the photoionization age ofthe Orion Nebula. The derived photoionization age of the Orion Nebula is less than 105 yr and possiblyas short as 104 yr.Key words : ISM: individual (Orion Nebula) È stars : formation È stars : preÈmain-sequence

1. INTRODUCTION

The Orion Nebula (M42, NGC 1976) is the nearest H II

region and site of ongoing massive star formation. Locatedat a distance of about 430 pc & Hesser it is(Warren 1977),

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 Department of Astrophysical and Planetary Sciences and Center for

Astrophysics and Space Astronomy, University of Colorado, Boulder, CO80309-0389 ; bally=casa.colorado.edu.

2 Visiting Astronomer, Kitt Peak National Observatory, NationalOptical Astronomy Observatories, operated by the Association of Uni-versities for Research in Astronomy, Inc., under cooperative agreementwith the National Science Foundation.

3 Astrophysical Theory Centre and Mount Stromlo and Siding SpringObservatories, Australian National University, Canberra, ACT 0200,Australia ; ralph.sutherland=anu.edu.au.

4 NASA Goddard Space Flight Center, Code 681, Greenbelt, MD20771 ; devine=h2.gsfc.nasa.gov.

5 Canadian Institute for Theoretical Astrophysics, University ofToronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada ;johnstone=cita.utoronto.ca.

the ““ Rosetta stone ÏÏ for the study of young stellar objects(YSOs). The principal sources of ionizing radiation are theO6p star h1 Orionis C in the Trapezium cluster and the O9star h2 Orionis A located several arcminutes to the south.Their youth and proximity make the Orion Nebula thehighest apparent surface brightness H II region in the solarvicinity. Near-infrared images of the nebula and the molec-ular cloud core that lies less than 1 pc behind it reveal anexceptional concentration of young stars (McCaughrean

& Burton & Stau†er1988 ; Allen 1993 ; McCaughrean 1994 ;et al. The Trapezium cluster contains overProsser 1994).

700 stars that formed in a region less than 0.5 pc in diam-eter. Modeling of the stellar distribution has led to an esti-mated core density of about 50,000 stars per cubic parsec,which corresponds to an average nearest neighbor distanceof about 6000 AU & Stau†er The(McCaughrean 1994).ages of several hundred stars in the Trapezium cluster havebeen estimated by Ðtting theoretical isochrones to theobserved H-R diagram of the cluster & Terndrup(Herbig

& Walker The1986 ; Jones 1988 ; Hillenbrand 1997).

293

294 BALLY ET AL. Vol. 116

median cluster age is between 3] 105 and 106 yr, with fewstars older than 106 yr. Because the birth of the massivestars probably halted further star formation within theionized region, the photoionization age of the nebula mustbe less than 3] 105 yr, making it one of the youngest H II

regions known.Young stars that have formed from the Orion molecular

core (OMC-1) have been uncovered by the advancing H II

region and are now embedded in its interior. The remnantsof the circumstellar environments from which these youngstars were born are illuminated from the outside by theintense radiation Ðelds of the massive stars. Since theHubble Space Telescope (HST ) Faint Object Cameraimages can resolve structure as small as 9 AU in the OrionNebula, HST provides an opportunity to investigate theproperties of young stars and their environments, includingthe dense protoplanetary accretion disks, with more than 1order of magnitude improvement in angular resolution overother techniques, such as millimeter-wavelength interferom-etry or near-infrared imaging.

Externally illuminated YSOs were Ðrst detected in theOrion Nebula by & Vidal who found sevenLaques (1979),compact knots bright in Ha in the immediate vicinity of theTrapezium cluster (the LV sources). Free-free radio contin-uum emission from these objects and other subarcsecond-diameter condensations both inside the H II region andembedded in the molecular gas in the background weredetected by the Very Large Array (VLA) radio telescope inthe mid-1980s by et al. andChurchwell (1987) Garay,Moran, & Reid More detailed radio observations(1987).(Felli et al. resulted in the discovery of over 301993a, 1993b)compact free-free emitting sources. Some are resolved andexhibit cometary tails pointing away from Trapezium whileothers show arc-shaped emission facing these hot stars.

obtained high-quality near-infraredMcCaughrean (1988)images of the Orion Nebula and demonstrated that nearlyall compact VLA sources coincide with infrared sources.

Wen, & detected intense line emissionOÏDell, Hu (1993)that wraps around the residual dense gas that surroundsover 50 YSOs born from the Orion molecular cloud. TheHST observations support the identiÐcation of theseobjects as externally illuminated circumstellar environ-ments. et al. coined the phrase ““ proplyd ÏÏ (anOÏDell (1993)acronym for ““ protoplanetary disk ÏÏ) to describe them. Afterthe Ðrst HST service mission, in 1993 December, the OrionNebula was reobserved by & Wen The dra-OÏDell (1994).matic improvement in the performance of HST led to thedetection of previously unseen features and many newYSOs, including six objects that lie in the foreground andare seen only in silhouette. Several of the bright objects andsome of the silhouetted objects have been detected at 10 kmin emission Houck, & Miles &(Hayward, 1994 ; HaywardMcCaughrean providing further evidence that these1997),YSOs are surrounded by disks. A catalog of 153 externallyilluminated YSOs was presented by & WongOÏDell (1996),and a detailed analysis of the silhouetted objects was givenby & OÏDellMcCaughrean (1996).

In this paper, we present follow-up observations of theexternally illuminated YSOs in the Orion Nebula obtainedwith HST in 1994 and 1995 with the PC portion of theWide Field Planetary Camera 2 (WFPC2), with the FaintObject Camera (FOC), and with the Faint Object Spectro-graph (FOS). Our observations and those of & WenOÏDell

and & OÏDell were carefully(1994) McCaughrean (1996)

coordinated to maximize the coverage of the core of theOrion Nebula and to obtain the highest angular resolutionobservations possible of many YSOs. The Ðnal mosaic ofthe Orion Nebula, which has appeared in various pub-lications, used the combined observations obtained fromour and OÏDellÏs observing programs.

2. OBSERVATIONS

2.1. HSTWe observed four Ðelds near Trapezium with the 0A.05

angular resolution of the Planetary Camera (PC) portion ofWFPC2 on HST on 1995 March 17. The angularresolution of these images is more than a factor of 2 betterthan previous WF camera images and provide 22 AU linearresolution at a distance of 430 pc, which we adopt for theOrion Nebula. Observations were obtained through nar-rowband Ðlters centered on the [S II], [N II], Ha, [O I], and[O III] lines and in an emission-lineÈfree continuumchannel. Three identical exposures were obtained througheach Ðlter for each pointing. Since the set of three images ineach Ðlter was always obtained within the same orbit andusing one set of guide stars, the registration of star imageson individual exposures was found to be better than 0.1pixels. Data received from STScI had been Ñat-ÐeldÈcorrected and calibrated for a variety of e†ects by the stan-dard WFPC2 processing pipeline.

The three identical exposures were median-combined toyield a single image for each Ðlter and each target. Thisprocedure removed most cosmic rays from the data.Remaining cosmic rays, hot pixels, and other artifacts wereeliminated by hand. We used stars in the images to registerframes taken through di†erent Ðlters. Three-color 24 bitred-green-blue (RGB) images were made with AdobePhotoshop by combining data taken through three di†erentÐlters in various combinations. Coordinates for objectslisted in Tables and below were obtained with the2 3STSDAS routine METRIC, which converts raw pixel coor-dinates into distortion-corrected J2000.0 coordinates refer-enced to the Guide Star Catalog.

The data numbers (DNs) were converted into spectralline Ñux densities in units of ergs cm~2 s~1 pixel~1 by(Fj)

Fj \ Ne

AT

hcj

1QTmax

, (1)

where is the number of electrons detected per second byNeeach pixel in the focal plane of the telescope and is given by

where G is the gain (in electrons per DN) andDNG/texp, texpis the exposure time in seconds. is the e†ective aperture,ATQ is the throughput efficiency of telescope, corrective optics,

and camera, and is the Ðlter transmission at the wave-Tmaxlength of a spectral line.This method is only approximate, since it assumes that

contamination by additional lines and continuum withinthe passband can be ignored. Therefore, this method worksonly when the emission in the Ðlter is dominated by theobserved emission line. For example, the F656N Ðlter usedto observe Ha is contaminated by partial transmission ofthe two [N II] lines near the Ha line. For equal Ñuxes in the[N II] and Ha lines, the resulting error in the Ha Ñux wouldbe about 15%. However, typically the [N II] lines are morethan 3È10 times fainter than Ha, and the resulting error inthe Ha Ñux is less than about 5%. Since the [N II] Ðlter onlytransmits about 2% of the incident Ha Ñux, the contami-

No. 1, 1998 EXTERNALLY ILLUMINATED YSOs IN ORION 295

TABLE 1

OBSERVATIONS OF THE ORION NEBULA

ExposureTarget a (B1950.0) d (B1950.0) Filter/Grating (s) Comments

WFPC2 PC1a

LV 3 . . . . . . . . . . . . . . . . . . . . . . 5 32 48.83 [5 25 10.07 F673N 3] 100 Both [S II] linesF658N 3] 100 [N II]F656N 3] 60 HaF631N 3] 100 [O I]F502N 3] 100 [O III]F547M 3] 30 Continuum

HST 1 . . . . . . . . . . . . . . . . . . . . 5 32 50.23 [5 25 33.7 F673N 3] 180 Both [S II] linesF658N 3] 180 [N II]F656N 3] 60 HaF631N 3] 100 [O I]F502N 3] 180 [O III]F547M 3] 30 Continuum

HST 3 . . . . . . . . . . . . . . . . . . . . 5 32 48.49 [5 25 43.5 F673N 3] 100 Both [S II] linesF658N 3] 100 [N II]F656N 3] 60 HaF631N 3] 100 [O I]F502N 3] 100 [O III]F547M 3] 30 Continuum

HST 10 . . . . . . . . . . . . . . . . . . . 5 32 50.75 [5 26 06.1 F673N 3] 180 Both [S II] linesF658N 3] 180 [N II]F656N 3] 60 HaF631N 3] 180 [O I]F502N 3] 180 [O III]F547M 3] 30 Continuum

FOC/96b

LV 6ÈVLA 10 NW . . . . . . 5 32 48.32 [5 25 18.0 F253M 2] 60 C II]F231M 2] 60 O II]F190M 2] 60 C III

HST 16 . . . . . . . . . . . . . . . . . . . 5 32 50.89 [5 25 57.5 F253M 2] 60 C II]F231M 2] 60 O II]F190M 2] 60 C III

HST 10 . . . . . . . . . . . . . . . . . . . 5 32 50.75 [5 26 06.1 F253M 2] 60 C II]F231M 2] 60 O II]F190M 2] 60 C III

FOSc

HST 4 (LV 6) . . . . . . . . . . . . 5 32 48.32 [5 25 20.15 G130H 300 On-sourceG190H 300G270H 300

HST 4 (SKY 1) . . . . . . . . . . 5 32 48.39 [5 25 20.15 G130H 300 1A eastG190H 300G270H 300

HST 3 . . . . . . . . . . . . . . . . . . . . 5 32 48.49 [5 25 43.5 G130H 300 On-sourceG190H 300G270H 300

HST 3 (SKY 2) . . . . . . . . . . 5 32 48.52 [5 25 42.56 G130H 300 1A.0 o†-sourceG190H 300G270H 300

HST 3 (SKY 3) . . . . . . . . . . 5 32 48.54 [5 25 41.62 G130H 300 2A.0 o†-sourceG190H 300G270H 300

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcmin-utes, and arcseconds.

a All WFPC2 observations were obtained on 1995 March 21.b All FOC observations were obtained on 1994 October 21.c All FOS observations were obtained on 1994 October 8. The FOS aperture used is circular and 1A in diameter.

nation by Ha may raise the estimated [N II] Ñux by about20% at most. None of the other narrowband Ðlters su†erserious line contamination ; however, serious contaminationcan arise in the presence of bright continuum sources suchas stars. We have attempted to use the F547M data toremove the continuum sources from the narrowbandimages. Unfortunately, variations in the intrinsic colors ofstars in the Ðeld are large, and the results of global image

subtractions are less than satisfactory. Therefore, in thispaper we restrict our analysis to emission-line surfacebrightness measurements of regions that are sufficientlydistant from each continuum source so that continuum con-tamination is small. We leave more sophisticated analysesin which we attempt to compute precise continuum-corrected Ñuxes to future work. The Ñuxes reported in Table

below are restricted to regions of the images well away3


from stars, and we estimate that the quoted surface bright-ness per pixel is accurate to about 5%È25%.

UV images of two Ðelds were obtained in 1994 Novemberusing FOC. The FOC pixel scale of corresponds to a0A.014linear scale of about 6 AU in the Orion Nebula. Weobtained images through a medium passband Ðlter(F253M) that transmits the semiforbidden lines O II] (2326

and C II] (2470 We also obtained images through aÓ) Ó).medium-band Ðlter (F190M) centered on the 1900 C IIIÓline. A preliminary report of these observations was pre-sented by Devine, & SutherlandBally, (1995).

Moderate-resolution (R\ 1300) UV spectra wereobtained of the externally illuminated YSOs 158[327 (LV6, HST 4) and 159[350 (HST 3) and three positions in thenebula near these objects in 1994 December with FOS usinga 1A circular aperture. Three di†erent FOS gratings(G130H, G190H, and G270H) were used to cover the spec-trum from 1150 to 3300 provides a summary ofÓ. Table 1these observations including exposure dates, times, Ðlters,number of pointings, and pointing coordinates.

3. OBSERVATIONAL RESULTS

3.1. HST FOS SpectraTwo externally illuminated YSOs, 158[327 (HST 4, LV

6) and 159[350 (HST 3) and three adjacent SKY positionslocated several arcseconds away were targeted for UV spec-troscopic observations with FOS. Three settings of FOScovered the spectral range from 1150 to 3300 Details ofÓ.the observations are given in shows theTable 1. Figure 1FOS spectra of the Ðelds listed in Table 1. The results of theobservations, including the peak surface brightness mea-sured in our aperture in the brighter emission lines and thecontinuum level at three wavelengths, are given in Table 2for both externally illuminated YSOs and the three adjacentSKY positions. The spectral line Ñuxes have beencontinuum-subtracted. The exact pointing of these obser-vations is uncertain by about since HST was able to0A.2track on only one guide star. However, both targets aresufficiently small that in the on-source pointings the slitscontained the bright emission from the externally illumi-nated YSOs, while the o†-source pointings (1A away) werenot contaminated by this emission and, therefore, samplethe nebular background. Emission from the externally illu-minated YSOs was detected in C III, C II], O II], and Mg II.

Continuum emission is observed in all spectra ; however,the continuum toward the externally illuminated YSOs ismarginally brighter than toward the adjacent sky referencepositions. The UV continuum is much brighter than thetotal emission from spectral lines over the observed portionof the spectrum. The continuum is very blue and reachesmaximum intensity [(2È7)] 10~14 ergs cm~2 s~1 atÓ~1

FIG. 1.ÈFOS spectra showing three separate settings of FOS and Ðvepointings listed in Tables and Top to bottom : FOS spectrum of1 2.158[327 (LV 6), FOS spectrum of the nebula at position listed as LV 6SKY 1 in Table 2 near 158[327 (LV 6), FOS spectrum of 159[350 (HST3), FOS spectrum of sky nebula at position listed as HST 3 SKY 1 in Table2 near 159[350 (HST 3), and FOS spectrum of sky nebula at positionlisted as HST 3 SKY 2 in Table 2 near 159[350 (HST 3).

1300 at the shortest observed wavelengths, where it isÓ]about 10 times stronger than at 3000 Ó.

The Lya proÐle exhibits a broad absorption dip towardall observed positions. A strong emission spike, centered inthe absorption feature, is observed toward all SKY posi-tions but is absent or much weaker toward the externallyilluminated YSOs. A P Cygni proÐle is seen in the 1549

C IV line in the spectrum of 158[327 and faintly in theÓadjacent SKY position.

The C III line is visible in all of our spectra and is 5 timesstronger toward the externally illuminated YSO 158[327

TABLE 2

SUMMARY OF FOS OBSERVATIONS

Lya Continnum C III Continuum C II] [O II] Mg II Continuum He I He I

Source 1216 Ó 1300 Ó j1909 2000 Ó j2329 j2471 j2796/j2803 3000 Ó j2946 j3189

158[327 (LV 6) . . . . . . . 15 70 50 12 35 42 4 6.5 2 5LV 6 SKY 1 . . . . . . . . . . . . 150 65 7 12 6 6.5 0 4 1.2 2159[350 (HST 3) . . . . . . 25 25 27 5 16 18 26 5 0 2.5HST 3 SKY 1 . . . . . . . . . . 145 20 30 5 8.5 12 3 2.5 3 4HST 3 SKY 2 . . . . . . . . . . 14 20 11 4 2.8 2.6 0 2 0.5 1

NOTES.ÈFlux units are in 10~15 ergs cm~2 s~1 in a 1A diameter circular aperture. Lya has a broad and saturated absorption dip. The quoted ÑuxesÓ~1refer to the central emission spike that is seen toward the sky reference positions. Line Ñuxes are continuum-subtracted.


(HST 4, LV 6) than the nearby SKY reference position. Thisline was also detected in the spectrum of 159[350 (HST 3)and the two adjacent reference positions. The C III line isfainter toward the Ðrst reference position but brightertoward the second. The source 158[327 lies at a projecteddistance of from h1 Ori C, while 159[350 lies about10A.6

from h1 Ori C, so the strong C III in the Ðrst object may38A.5be due to its proximity to h1 Ori C.

The C II] and O II] lines have large, similar Ñuxes in bothexternally illuminated YSOs. The two lines are signiÐcantlybrighter toward the externally illuminated YSO than thesurrounding nebula (about a factor of 10 in 158[327 andabout a factor of 2È5 in 159[350). The Mg II resonance lineis faint in the externally illuminated YSOs and not visible inthe sky reference positions. Two other faint emission lines,which are produced by neutral helium, appear near 2950and 3190 Ó.

3.2. HST Planetary Camera and FOC Images

Figures through show the PC images of the exter-2 5nally illuminated YSOs embedded in the Orion Nebulanear the Trapezium stars. The color images are RGB com-posites in which the Ðgure captions describe the colorassignments.

is centered on the Trapezium and contains theFigure 2original objects discovered by & VidalLaques (1979).About a dozen externally illuminated YSOs are visible inthis image. All have very bright emission-line fronts on theside facing h1 Ori C (overexposed in this reproduction) andhave tails ranging in length from to over 2A that point0A.4radially away from h1 Ori C. The tails are limb-brightenedin low-excitation lines, and parts of the tails are visible insilhouette against the nebular background in [O III] and[N II]. Some tails (cf. 158[327 \ LV 6\ HST 4) show

FIG. 2.ÈTrapezium region as a color mosaic of the PC images from the Cycle 4 WFPC2 HST observations of the Orion Nebula in narrowband Ðlters.Red is [N II], green is Ha, and blue is [O I].


slight wiggles and other distortions near their ends, andsome point a few degrees away from h1 Ori C. Arcs consist-ing of bright [O III] and Ha emission are seen on the sides ofthe externally illuminated YSOs facing h1 Ori C and, inevery case, are concave toward the externally illuminatedYSOs and convex toward h1 Ori C. The [O III] arcs aredisplaced from the externally illuminated YSO ionizationfronts (IFs) by 1AÈ3A toward h1 Ori C.

is centered about 30A southeast of the Tra-Figure 3pezium, where there is a prominent ring of externally illumi-nated YSOs (which we dub ““ Corona Ori ÏÏ). Of the ninebright objects in the ring, all but two exhibit prominent tailswith lengths ranging from under 1A to 4A. On average, theseobjects are larger than those closer to h1 Ori C. The objectin the center of the Ðeld, 177[341 (HST 1), sports thelargest emission-line crescent (which we will show to be anIF) and the longest tail. The parts of the IFs that face h1 OriC are nearly hemispherical. The tails are strongly limb-

brightened in most emission lines, especially in [N II], andtend to be wider in the higher excitation species than in thelower excitation lines. The tails connect smoothly to thehemispherical IFs and become progressively narrower““ downwind.ÏÏ Most tails end in very sharp points that haveradii of curvature less than The second brightest object0A.1.in 170[337 (HST 2), has a Ðlament of [O III]Figure 3,emission that extends directly from the star to about 0A.5beyond the projected edge of the IF facing h1 Ori C. A chainof [O III]-bright knots that are probably Herbig-Haro (HH)objects lies several arcseconds farther northeast, roughlyalong the axis of the Ðlament. As discussed by et al.OÏDell

these [O III] knots are redshifted by about 100 km(1997),s~1 with respect to the nebular emission. The source171[340 (HST 11) is a stubby, nearly round object severalarcseconds south of 170[337. Unlike most YSOs close toTrapezium, it exhibits a very stubby tail and a prominentsilhouetted disk in Ha, [N II], and [O III].

FIG. 3.ÈSame as but for the ““ Corona Ori ÏÏ region, about 30A south of Trapezium. Red is [N II], green is Ha, and blue is [O III].Fig. 2,


FIG. 4.ÈSame as but for the region around HST 3Fig. 2,

shows the Ðeld centered on the bright objectFigure 4159[350 (HST 3), one of the most luminous low-massmembers of the Trapezium cluster, located about 30A southof h1 Ori C. An unusual externally illuminated YSO,160[353 (HST 6), which has a cloverleaf geometry, liesabout 4A to the south of 159[350. The object 159[350(HST 3) lies near the axes of two chains of [O III] brightarcs, one of which extends to the northeast toward ““ CoronaOri ÏÏ at intervals of about 10A while the other extends alonga northwest-southeast axis. Unlike the [O III] arcs that arecentered on the externally illuminated YSOs near Tra-

pezium, these arcs are not centered on any visible objectand do not face h1 Ori C.

shows the Ðeld centered on 182[413 (HST 10),Figure 5about 60A to the south of Trapezium, which resemblesHerbigÏs ““ interstellar teardrops ÏÏ seen in the RosetteNebula This Ðeld also contains 183[405(Herbig 1974).(HST 16), the Ðrst of the dark silhouettes to be recognized

& Wen that do not show any evidence of(OÏDell 1994)external ionization. None of the externally illuminatedYSOs in this Ðeld, or for that matter any other Ðeld at acomparable projected distance from Trapezium, have


FIG. 5.ÈSame as but for the Ðeld containing HST 10 and HST 16. Red is [N II], green is Ha, and blue is [O I].Fig. 2,

associated [O III]] Ha arcs. The radii of the IFs in thisÐeld are on average larger than the IFs associated with theexternally illuminated YSOs closer to h1 Ori C, and the tailstend to be stubby.

The three largest bright, externally illuminated YSOs inthis Ðeld each contains opaque regions interior to the IFs.These regions are seen in silhouette against backgroundlight in Ha and the forbidden lines of ionized species.However, in the 6300 [O I] line the silhouetted regionsÓare bright and seen in emission.

shows a color composite mosaic of the OrionFigure 6Nebula core using all of the WFPC2 PC and WF imagesfrom our program. This Ðgure illustrates the generalappearance of the nebulaÏs core and contains several addi-tional objects that we discuss but which were not includedin our PC Ðelds of view. One of the largest externally illumi-nated YSOs discovered to date, 142[301, lies about 40Anorthwest of h1 Ori C (see We Ðrst detected thisFig. 7).

object from the ground on images taken in 1994 Januarywith the 3.5 m New Technology Telescope at the EuropeanSouthern Obervatory, La Silla, Chile, in seeing. This0A.6object has a tail nearly 4A long that comes to a very sharppoint downstream from h1 Ori C. It is one of the few bright,externally illuminated YSOs that does not have a hemi-spherical IF facing the bright star. The IF looks very Ñatand sharp on the side facing the Trapezium cluster. Figure 6also shows the largest pure silhouette, 114[426, located100A southwest of h1 Ori C.

shows a selection of externally illuminated YSOsFigure 7in each of the six Ðlters. The objects are arranged by wave-length (increasing to the right) and projected distance fromh1 Ori C (increasing toward the bottom). From top tobottom, the di†erent rows show the following objects :

1. The 168[326AB binary (LV 1), located at a projecteddistance of from h1 Ori C. Both stars are surrounded by6A.3


FIG. 6.ÈSame as but for the mosaic obtained by combining all of our Cycle 4 WFPC2 observations. The orientation of this image is as in Figs. 3È5.Fig. 2,Red is [N II], green is Ha, and blue is [O III].

prominent [O III]] Ha arcs, with the brighter arc lyingbetween the two sources. The tails point directly away fromh1 Ori C.

2. The source 163[317 (LV 3), located at a projecteddistance of from h1 Ori C. A long, skinny tail points6A.9directly away from h1 Ori C.

3. The object 167[317 (LV 2), located at a projecteddistance of from h1 Ori C (the brighter object), and7A.8166[316, located from h1 Ori C (fainter object). Both7A.2objects have prominent [O III]] Ha arcs facing h1 Ori Cand skinny tails pointing radially away.

4. The 158[327 (left) and 158[325 (right ; this object isalso known as LV 6 \ HST 4) group, at a projected dis-tance of and from h1 Ori C. Both objects show9A.6 10A.3central silhouettes in [O III]. Note the apparent bend in thetail of 158[325 on the right.

5. The object 170[337 (HST 2), which powers a stellarmicrojet et el. is located at the top left at a(OÏDell 1997),projected distance of from h1 Ori C. The small faint16A.1object to the right is 169[338 (HST 13) ; 171[340 (HST11), a much fainter object, contains an embedded silhouettein [O III].

FIG

.7.

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6. The source, 180[331 located at a projected distanceof from h1 Ori C. The small pure silhouette 182[33216A.3can be seen in the [O III] and Ha frames ; 180[331 has alarge [O III]] Ha arc that is slightly displaced to the rightof the line pointing toward h1 Ori C.

7. The large object 177[341 (HST 1), located at a pro-jected distance of from h1 Ori C. Note the faint com-25A.6panion to the lower left of the IF. This object has a largenearly 5A radius [O III]] Ha arc that lies outside the Ðeldshown here but is visible in A faint silhouette canFigure 3.be discerned on the [O III] image in the center of the object.

8. (WF image). The object 142[301, at a projected dis-tance of from h1 Ori C, has a Ñat head facing h1 Ori C.40A.7Note that the tail is seen in silhouette against backgroundnebular light and is very opaque.

9. The teardrop-shaped object 182[413 (HST 10) islocated at a projected distance of 57A from h1 Ori C andcontains an edge-on silhouette that is bright in [O I]. Alsonote the distinct [O I] emission component that coincideswith the IF. Note that the axis of symmetry is nearly alignedwith the minor axis of the silhouette and points about 20¡due east of the line pointing to h1 Ori C.

10. (WF image). The giant silhouette 114[426 islocated at a projected distance of 99A from h1 Ori C. Notethe large crescent of nebulosity to the upper right that isvisible in [O I], Ha, [N II], and [S II] but not in [O III]. Thecrescent faces away from Trapezium, and its emission-lineÑux ratios are consistent with a low-excitation HH object (ashock-excited nebula powered by an outÑow from a youngstar). The [N II] image shows a faint cone-shaped emission

structure that connects the polar reÑection nebula in themiddle of the silhouette to the brightest portion of the cres-cent. It is possible that the [N II] structure traces a jet thatpowers the brightest portion of the crescent.

shows the FOC UV images of the Ðeld contain-Figure 8ing the sources LV 4, 5, and 6 (161[322, 158[323, and158[327), as well as the sources 158[326 and 157[323.Unfortunately, this image was severely underexposed (evenwith a total 10 minute integration time), and we onlypresent the image formed by summing all of the individualexposures. The individual externally illuminated YSOs aredominated by bright crescents on the side facing h1 Ori C,with fainter tails extending away from this star. The mor-phology is similar to that in the PC images. The centralstars of several externally illuminated YSOs are easily seenin the UV images. In several objects, such as 158[326 and158[327, the central stars are surrounded by symmetricsilhouettes that appear darker than the general nebularbackground.

The pure silhouette 183[405 (HST 16) was also observedwith FOC, though our image is severely underexposed andtherefore not shown here. As in the visual-wavelengthimages, this YSO is surrounded by a symmetric and ellip-tical silhouette centered on a central star that is consider-ably fainter relative to the nebular background in the UVimages. The shape and elliptically averaged surface bright-ness proÐles of the silhouette are similar to those derivedfrom the visual images. The main new result is that thesilhouette appears to remain completely opaque as close to

FIG. 8.ÈUV image of the Ðeld containing LV 4, 5, and 6 obtained with FOC. These images have a pixel scale of 9 AU, the highest linear resolutionobservations of a candidate protoplanetary disk obtained so far by any method. The orientation of these images is the same as in Figs. 3È6.

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the central star as we can probe with FOC. Since the FOCis fully sampled and its point-spread function is about afactor of 2 smaller than that of the PC, and the central staris considerably fainter at UV wavelengths, we can trace theinner part of the silhouette to within about (10 AU) of0A.02the star.

3.3. Observed Features of the Externally Illuminated Y SOsIn this section, we review the main observed features of

externally illuminated YSOs within the Orion Nebula. Infollowing sections, we discuss the interpretations of thesefeatures.

Figures and show two ““ cartoons ÏÏ that illustrate the9 10discussion. shows a schematic drawing of theFigure 9Orion Nebula and its IFs, its stellar wind bubble and cavity,and several externally illuminated YSOs embedded withinthe various regions within the nebula. The features observedin an externally illuminated YSO depend on its location inthe nebula. All externally illuminated YSOs within thenebula should be surrounded by IFs. Externally illuminatedYSOs located in front of the nebula outside its Stro� mgrensphere (the ““ neutral lid ÏÏ ; & Wen will lack anOÏDell 1995)IF and can only be seen in silhouette if circumstellar dust ispresent. However, externally illuminated YSOs locatedinside the H II region sufficiently far from Trapezium mayalso be seen in silhouette if they have IFs that are fainterthan the nebular background. The Ha ] [O III] crescentsdiscussed below are only seen close to Trapezium, and it isargued below that they trace interactions with the h1 Ori Cstellar wind. is a schematic drawing of an exter-Figure 10

nally illuminated YSO in the inner portion of the nebulashowing the features discussed below.

3.3.1. Emission-L ine Crescents

& Wong catalog 153 YSOs in the OrionOÏDell (1996)Nebula that are associated with extended structure. Sixobjects are seen in silhouette against the nebula

& OÏDell and are either completely(McCaughrean 1996)dark or very faint in the observed emission lines. The restare bright cometary objects whose extended structure isdominated by strong emission in Ha, [N II], [S II], [O I],and, to some extent, [O III]. On the side facing the principalionization source, the projected shape of the brightest emis-sion is well described by a half-circle or a crescent, suggest-ing that the emission region is hemispherical. The radii ofthe crescents range from (22 AU) for sources near h10A.05Ori C, such as 163[323 and 161[322, to over 2A (860 AU)for the largest object, 252[457.

3.3.2. Emission-L ine Radial Intensity Gradients in the Crescents

shows the azimuthally averaged andFigure 11background-subtracted radial intensity proÐles for anumber of externally illuminated YSOs in the Ha line.

shows a similar plot for the continuum.Figure 12 Table 3lists the coordinates of all externally illuminated YSOs inthe PC Ðelds, as well as the larger and well-resolved objectsthat were imaged by the WF cameras of WFPC2. Table 4lists the observed properties of these externally illuminatedYSOs, including the dimensions of various emission andopaque components and the peak surface brightness of theobjects in each emission line. For sources near h1 Ori C, the

FIG. 9.ÈSchematic drawing of the Orion Nebula showing the h1 Ori C stellar wind bubble, the interface between the H II region and the backgroundmolecular cloud, the foreground ““ neutral lid, ÏÏ and externally illuminated YSOs embedded in various portions of the nebula. The line of sight to the Sun istoward the left.

306 BALLY ET AL.

FIG. 10.ÈSchematic drawing showing an externally illuminated YSO. The young star has a circumstellar disk and a bipolar outÑow or jet. FUV radiation(2000[ j [ 912 shines on the circumstellar disk, initiating a low-velocity (B3 km s~1) disk wind (cf. et al. At several disk radii, theÓ) Johnstone 1998).quasi-spherical disk wind passes through a weak D-type IF and is accelerated to about 10 km s~1 greater than the sound speed in the ionized gas. The weakshock associated with this front lies just inside the IF and may produce and slight density and column density enhancement that is seen in silhouette in someobjects. The electron density just outside the IF is about 105 cm~3 and is expected to decrease as r~2, where r is the distance from the low-mass star, resultingin an r~3 decrease in surface brightness of emission lines. For externally illuminated YSOs embedded in the ram pressureÈdominated portion of the h1 Ori Cstellar wind cavity, a stationary bow shock forms where the ram pressures of the photoablation Ñow and the stellar wind are approximately equal, producinga region that is visible in Ha and [O III]. For externally illuminated YSOs embedded in the hot stellar wind bubble, the photoablation Ñow is subsonic and noshock can form. In this region, heat conduction gradually raises the temperature of the photoablation Ñow until it merges smoothly with the hot stellar windbubble, and no Ha ] [O III] arcs concentric with the IF are expected. The surface brightness of emission lines is expected to decrease exponentially withincreasing distance from the source in this region.

radial proÐles are well Ðtted over about 2 orders of magni-tude in intensity by a power law of the form r~a, wherea D 3. However, for some objects an exponential proÐle ofthe form may yield a better Ðt with a length scalee~r@r0

cm There is a loose correlationr0B 7 ] 1014 (OÏDell 1998).between the cusp radius and the distance to h1 Ori C, withthe cusp radius increasing as r(d) P da with a \ 0.5È0.8,where d is the projected distance from h1 Ori C. The peaksurface brightness in each emission line decreases withincreasing distance from h1 Ori C roughly as d~2.

3.3.3. Cometary Tails

Most externally illuminated YSOs contain tails pointingdirectly away from the primary illuminator (either h1 Ori C,h1 Ori B, or h2 Ori A). Deviations between the tail axis andthe line connecting the externally illuminated YSO to themain source of ionization are at most a few degrees. Sometails show irregularities, especially toward the tail tips. Partsof many tails are seen as silhouettes in [N II] and [O III],and to some extent in the other images, providing evidencefor substantial amounts of dust in the tails. Externally illu-minated YSOs at greater distances from h1 Ori C exhibitteardrop shapes with larger radius IFs facing the illumi-nating star. The tails are limb-brightened in the brighteremission lines and especially in [N II]. As shown in Table 4,the tails have lengths ranging from to nearly 4A. There is0A.3no correlation between tail length and the projected dis-tance from the ionizing source. However, sources near h1Ori C tend to have tails with large aspect ratios (ratio oflength divided by the width), while objects at large projecteddistances tend to have stubby tails with relatively lowaspect ratios. This follows from the observation that the

tails always connect smoothly to the forward-facingemission-line crescents, which tend to have larger radii atlarger projected distances from h1 Ori C.

3.3.4. Silhouetted Disks Embedded in the Bright ExternallyIlluminated Y SOs

About 20% of the externally illuminated YSOs shown incontain dark and opaque cores seen in silhouetteFigure 7

against the di†use background light. The dark structurestend to be symmetric and centered on the embedded star(when the star is visible) and appear similar to the six sil-houettes discussed by & OÏDell TheMcCaughrean (1996).pure silhouettes, however, are not surrounded by crescentsor tails that emit in emission lines. The silhouettes found atthe centers of the bright externally illuminated YSOs tendto be much smaller (by up to a factor of 20) than the puresilhouettes such as 114[426 or 183[405 and are mostclearly seen in the larger objects that lie at relatively largeprojected distances from Trapezium. When both a silhou-ette and an emission-line crescent are seen, the ratio of theemission-line crescent radius divided by the semimajor axisof the silhouette ranges from about 1.5 to 4.

Most objects within 20A of h1 Ori C are so compact thatno central opaque regions are seen in the PC image. Darkcores may be hidden by the combined light of the nebularlines and the embedded star. However, in the shorter wave-length UV images obtained with FOC, evidence for acompact dark zone surrounding the central stars is clearlyseen. shows a UV image of Ðve objects locatedFigure 8roughly 10A west of h1 Ori C at a wavelength of 2530 Ótaken with FOC. Cometary tails point directly away fromh1 Ori C, which is located to the left. In the FOC image, all

FIG. 11.ÈAzimuthally averaged background-subtracted radial intensity proÐles for 14 externally illuminated YSOs in the Ha line. This diagram showsthe data on a log-log plot along with the best-Ðt power law that describes the correlation. The power-law index and formal uncertainty are shown.

FIG. 12.ÈAzimuthally averaged background-subtracted radial intensity proÐles for 14 externally illuminated YSOs in the continuum

EXTERNALLY ILLUMINATED YSOs IN ORION 309

TABLE 3

POSITIONS OF TRAPEZIUM STAR-DISK SYSTEMS

a (J2000.0) d (J2000.0) X YNumbera Star (05h35m) ([05¡) Frameb (pixels) (pixels) OWc Comments

1 . . . . . . . . . h1 C 16.286 23 23.14 LV 3 (1) 327 521 163[323 In glare of h1C2 . . . . . . . . . h1 C 16.026 23 24.80 LV 3 (1) 240 487 161[322 LV 43 . . . . . . . . . h1 C 16.788 23 26.45 LV 3 (1) 394 690 168[326 LV 1, fainter of two sources4 . . . . . . . . . h1 C 16.801 23 26.77 LV 3 (1) 392 698 . . . LV 1, brighter of two sources5 . . . . . . . . . h1 C 16.718 23 28.54 LV 3 (1) 345 707 168[3286 . . . . . . . . . h1 C 16.245 23 17.08 LV 3 (1) 411 416 163[317 LV 37 . . . . . . . . . h1 C 16.573 23 16.65 LV 3 (1) 495 485 166[3168 . . . . . . . . . h1 B 16.062 23 14.75 LV 3 (1) 404 337 161[314 Faces h1 B9 . . . . . . . . . h1 C 16.037 23 28.31 LV 3 (1) 188 545 161[328 Disk in [O I]10 . . . . . . . h1 C 16.703 23 16.95 LV 3 (1) 521 520 167[317 LV 2, tail kinks, multiple arcs11 . . . . . . . h1 C 15.796 23 22.98 LV 3 (1) 214 405 158[323 LV 512 . . . . . . . h1 C 15.805 23 26.01 LV 3 (1) 169 455 158[32613 . . . . . . . h1 C 15.753 23 27.08 LV 3 (1) 140 460 158[327 LV 6, disk in [O I]14 . . . . . . . h1 C 15.691 23 23.00 LV 3 (1) 189 381 157[32315 . . . . . . . h1 C 16.934 23 37.71 HST 1 (1) 294 204 170[337 Jet, [O I] disk16 . . . . . . . h1 C 17.507 23 25.55 HST 1 (1) 618 145 176[32517 . . . . . . . h1 C 16.844 23 38.66 HST 1 (1) 258 198 169[33818 . . . . . . . h1 C 16.020 23 28.38 HST 3 (1) 776 94 160[328 Disk in [O I]19 . . . . . . . h1 C 15.321 23 24.55 LV 3 (1) 77 320 154[32420 . . . . . . . h1 C 15.822 23 38.73 HST 3 (1) 576.2 213.7 159[33821 . . . . . . . h1 C 17.012 23 40.37 HST 1 (1) 271 264 171[340 End-on?22 . . . . . . . h1 C 15.175 23 19.30 LV 3 (1) 124 203 152[319 Silhouette in [O III]23 . . . . . . . h1 C 15.442 23 38.20 HST 3 (1) 495.0 117.2 155[33824 . . . . . . . h1 C 17.279 23 41.97 HST 1 (1) 309 351 173[34125 . . . . . . . h1 C 17.992 23 31.45 HST 1 (1) 640 351 180[331 [O III]] Ha arc one-sided26 . . . . . . . h1 C 17.628 23 41.59 HST 1 (1) 397 426 177[341 HST 127 . . . . . . . h1 C 18.141 23 32.17 HST 1 (1) 664 397 182[332 Silhouette28 . . . . . . . h1 C 15.889 23 50.32 HST 3 (1) 412.5 412.3 159[350 Faint companion to HST 329 . . . . . . . h1 C 15.867 23 50.73 HST 3 (1) 410.9 413.7 159[350 HST 330 . . . . . . . h1 C 15.916 23 53.68 HST 3 (1) 366.9 471.3 160[353 Cloverleaf-shaped IF31 . . . . . . . h1 C 16.923 22 49.40 HST 1 (4) 327 502 170[24932 . . . . . . . h1 C 17.493 23 55.61 HST 1 (1) 147 616 175[35533 . . . . . . . h1 C 17.913 23 54.06 HST 1 (1) 270 689 . . .34 . . . . . . . h1 C 14.140 23 01.27 HST 1 (4) 534 117 142[301 [O I] disk35 . . . . . . . h1 C 15.345 22 40.82 HST 1 (4) 555 392 154[24036 . . . . . . . h1 C 16.432 22 36.49 HST 1 (4) 472 540 165[235 Bipolar jet source?37 . . . . . . . h1 C 18.273 24 05.04 HST 10 (1) 561 323 183[405 HST 16 silhouette38 . . . . . . . h1 C 17.444 22 34.62 HST 1 (4) 379 633 173[23639 . . . . . . . h1 C 17.341 24 14.50 HST 10 (1) 202 249 174[41440 . . . . . . . h1 C 18.161 24 14.18 HST 10 (1) 400 434 182[413 HST 10, central star not visible41 . . . . . . . h2 A 18.250 24 19.48 HST 10 (1) 339 538 183[419 Faces h2 A, central star not visible42 . . . . . . . h1 C 18.297 24 27.38 HST 10 (1) 227 674 184[427 [O I] disk43 . . . . . . . h1 C 11.288 24 26.86 HST 10 (3) 698 511 114[426 Giant silhouette, central star not visible

NOTE.ÈUnits of right ascension are seconds, and units of declination are arcminutes and arcseconds.a Labels used to identify objects in Figs. and11 12.b Numbers in parentheses indicate which WFPC2 CCD was used : (1) is the PC and (2), (3), and (4) refer to one of the three available WFs.c Designation given in OÏDell & Wong 1996.

Ðve objects show unresolved stellar images at the centers ofthe cometary condensations. Two objects show darkregions inside the crescent-shaped bright rims that are sym-metric about the central stars.3.3.5. [O I] Emission from Silhouettes Embedded in Bright Objects

Some dark silhouettes (dark in [O III], [N II], [S II], orHa) embedded inside the bright, externally illuminatedYSOs surrounded by emission-line crescents are bright inthe [O I] images. The spatial extent, orientation, and struc-ture of the [O I] emission matches those of the dark silhou-ettes. However, in the six pure silhouettes (YSO disks seenonly in silhouette that are not surrounded emission-lineregions), the disks are also dark in [O I]. The largeteardrop-shaped object 182[413 (HST 10) has severalextended [O I] emission components. The central region isseen as a prominent elongated silhouette in ionized species([S II], [N II], and [O III]) and in Ha. However, this region isbright in the [O I] image. The teardrop-shaped outerboundary that is bright in Ha and in the ions is also bright

in [O I]. Finally, there is di†use [O I] emission in the entireprojected interior of 182[413 that is several times fainterthan the emission produced by the silhouette or the outerboundary.

3.3.6. Central Stars

The F547M continuum images show the presence ofcentral stars in about 80% of the externally illuminatedYSOs. Infrared images show that all except a few notableobjects (cf. 114[426 and 182[413) contain central stars.These stars are centered with respect to hemispherical IFsfacing the dominant illuminating UV source. The fewobjects in which no central stars have yet been directly seencontain highly elongated silhouettes at their centers. Theabsence of stars in these objects sets a minimum limit on thecolumn density along the line of sight. As discussed by Chenet al. the failure to detect a central star in 182[413(1998),implies a minimum visual extinction of 50 mag, or a columndensity of order N(H)\ 1023 cm~2. Dividing the columndensity by the major semidiameter of the silhouette (0A.175,

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EXTERNALLY ILLUMINATED YSOs IN ORION 311

or 75 AU) implies a minimum mean density of about 108cm~3 for the silhouette in 182[413.

3.3.7. [O III]] Ha Arcs

Within 30A of h1 Ori C, the HST observations show arcsof [O III]] Ha emission centered on the proplyds and con-centric with the Ha-dominated emission-line crescents.Examples include LV 1 (168[326), LV 2 (167[317), andLV 4 (161[322) in and the objects 168[326,Figure 2167[317, and 180[331 in The source 177[341Figure 7.has such an arc & Meaburn and(Fig. 3). Massey (1995) Hu

obtained long-slit spectra of several of these objects,(1996)and et al. measured their radial velocities withOÏDell (1997)a Fabry-Perot interferometer. The Ha ] [O III] arcs nearTrapezium all appear to be redshifted, with velocitiesranging up to about 100 km s~1. The nearly edge-on diskseen in silhouette as 114[426 bottom) is surrounded(Fig. 7,by a low-excitation arc bright in [O I], [N II], and [S II].Unlike the Ha ] [O III] arcs surrounding the otherproplyds, this arc faces away from Trapezium and may beshock-excited by a wind blowing from the central starembedded within the silhouette. The [O III]] Ha arcs arelocated between the proplyds and h1 Ori C about to 4A0A.5from their IFs with radii of curvature about 3 to 10 timeslarger than the radii of these fronts. The ratio of the project-ed distance between the proplyd and the arc divided by theprojected distance between the arc and h1 Ori C rangesfrom 0.05 to 0.2. These arcs are only visible in the Ha and[O III] images and are high-excitation features. Theirsurface brightness is comparable to that of the nebula,which implies that the product of the square of their den-sities and the observed path through them is comparable tothe emission measure of M42, where this product is about106 cm~6 pc. Since the observed path length is probablycomparable to about half of their observed sizes of 4A (i.e.,their lengths are about 0.004 pc), their densities must beabout 1.5 ] 104 cm~3.

A second family of [O III]] Ha arcs appears not to beassociated with stellar objects. These objects appear to haverandom orientations with respect to h1 Ori C, and manyexhibit large radial velocities et al. Meaburn(OÏDell 1997 ;

Both types of arcs have shapes reminiscent of1986, 1988).bow shocks.

4. INTERPRETATION

The observed properties of the externally illuminatedYSOs reÑect the evolutionary status of the YSOs and inter-actions with radiation and massive star winds in the OrionNebula. We review the environment of the Orion Nebulaand discuss the role that various physical processes play indetermining the appearance of young stars embeddedwithin it.

The region surrounding the Trapezium stars contains astellar wind bubble created by the wind from h1 Ori C, aphotoionized zone dominated by Lyman continuum radi-ation, and a predominantly neutral layer that separates theH II region from the molecular cloud that lies in the back-ground. Observations show that there is also a layer ofneutral gas in front of the nebula, the so-called neutral lid

& Wen shows a schematic drawing(OÏDell 1995). Figure 9of the Orion Nebula, illustrating the various features dis-cussed below, and shows a schematic drawing ofFigure 10an externally illuminated YSO. The various features shown

in this drawing are discussed below, starting with the fea-tures most distant from the YSO.

4.1. T he h1 Ori C Stellar W ind and Its Bubbleh1 Ori C has a strong stellar wind, with M0

wB 4 ] 10~7

yr~1 at a velocity of km s~1 &M_

vw

B 103 (HowarthPrinja which drives a bubble into the surrounding1989),medium. A stellar wind bubble contains two zones (Weaveret al. McCray, & Weaver The inner1977 ; Castor, 1975).region is dominated by the ram pressure of the wind,

Pramk

Bow

vw2

k\ M0

wvw

4nkd2 B 4 ] 107M0 ~7 v3 d17~2 cm~3 K ,

(2)

where is the stellar wind mass-loss rate in units ofM0 ~710~7 yr~1, is the wind velocity in 103 km s~1,M_

v3and is the distance from h1 Ori C in units of 1017 cm.d17The wind passes through a reverse shock at Rs\

cm, where is the undis-3 ] 1017M0 ~73@10n3~3@10v31@10t52@5 n3turbed gas density outside the wind bubble in units of103 cm~3 and is the age of the bubble in units of 105t5yr. This region is surrounded by a hot ([106 K) bubbleof shocked wind material dominated by thermal pres-sure. The expansion of this bubble drives a forwardshock into the surrounding medium at a radius Rfs\cm and velocity3 ] 1018M0 ~71@5n3~1@5v32@5t53@5 vfs\km s~1.6.0M~71@5 n3~1@5v32@5t5~2@5

An extended, high-velocity, blueshifted [O III] feature hasbeen found south of Trapezium in Fabry-Perot data cubesof the Orion Nebula, which may trace the forward shock ofthe h1 Ori C stellar wind bubble et al. In(OÏDell 1997).addition, high-dispersion long-slit spectra show anextended blueshifted [O III] feature north of Trapezium

& Meaburn that may also trace parts of this(Massey 1993)shock. This component of the [O III] emission has beentraced to about 60A to 100A (0.1 to 0.2 pc) from Trapeziumand has velocities of up to about [90 km s~1 with respectto the nebular emission. As discussed by et al.OÏDell (1997),the dynamic age of this feature is very short, less than 5000yr.

A condensation such as an externally illuminated YSOlying within the wind bubble will present an obstacle to theexpanding wind. In the ram pressureÈdominated interiorportion of the wind bubble, the stellar wind will passthrough a stationary bow shock that wraps around theobstacle. The location of this shock is determined by thebalance between the wind ram pressure and the pressurepresented by the obstacle. As we argue below, the obstaclepressure is dominated by the photoablation Ñow from theYSO environment.

In addition to the extended high-velocity gas, localizedhigh-velocity features have been known to exist in the coreof the Orion Nebula for some time. Some are HH objectspowered by young stars still deeply embedded within thedense cloud behind the nebula & Mu� nch(Taylor 1978 ;

& Taylor Others are associatedAxon 1984 ; Meaburn 1986).with the brightest externally illuminated young stars withinthe nebula, the & Vidal LV sourcesLaques (1979)

et al. While most HH(Meaburn 1988 ; Meaburn 1993).objects are blueshifted, the high-velocity emission associ-ated with the LV sources are predominantly redshifted byover 100 km s~1 and most readily visible in [O III] spectraand images. The brightest externally illuminated YSOs in


the immediate vicinity of Trapezium (LV 1, 2, 3, 5, and 6)are associated with high-velocity redshifted [O III] emis-sion. Our images show a nearly one-to-one correspondencebetween the location of this high-velocity gas and the pres-ence of [O III]] Ha arcs, and it is therefore tempting toassociate these two phenomena. Below, we explore this pos-sible connection further.

et al. argue that the h1 Ori C stellar windHenney (1996)reaches the surfaces of disks surrounding YSOs embeddedin the nebula and is responsible for the emission-line struc-ture. However, Hollenbach, & Bally haveJohnstone, (1998)shown that, in the presence of Lyman continuum andfar-UV (FUV) radiation with wavelengths longer than 912

a layer of photoheated material insulates the disk fromÓ,the wind. The expansion of this neutral layer drives the IFaway from the disk surface. Divergence of this Ñow allowsLyman continuum radiation to penetrate the outer portionsof this Ñow to establish a roughly hemispherical IF at adistance of several disk radii. The relative line strengths,radial emissivity proÐles, and morphologies indicate thatthe bright emission-line crescents surrounding the YSOsembedded in the Orion Nebula are IFs propagating into thesoft-UVÈpowered wind expanding away from a dense cir-cumstellar reservoir such as a disk. Once photoionized, thewind accelerates to about the sound speed in ionized gasand continues to expand until its ram pressure drops to thatof the stellar wind blowing from h1 Ori C, the thermalpressure of the surrounding wind bubble, or the pressure ofthe H II region.

Some YSOs may be embedded in the hot 106 K windbubble beyond the ram pressureÈdominated part of thewind. In this postÈwind-shock region (outside the reverseshock), the local sound speed is higher than the bulk veloc-ity of the outward-moving plasma. This stellar wind plasmaÑows past stationary obstacles at roughly the velocity of theforward shock driven into the surrounding H II region bythe expansion of the wind bubble. Thus, the Ñow of thestellar wind bubble around a YSO and its circumstellarenvironment is subsonic, and a wind shock will not form.

4.2. Ionization Fronts Associated with the ExternallyIlluminated Y SOs

UV radiation interacts with the YSO environment bydissociating molecules, heating and ionizing the gas, andthrough radiation pressure acting on dust. The Trapeziumcluster has a total luminosity of about 105 and h1 Ori CL

_,

has a Lyman continuum luminosity of aboutQ\ 1.5] 1049 photons s~1 Consider(L LycB 8 ] 104 L

_).

a static, spherical condensation in the path of the advancingIF. The advancing front will be retarded as it enters theregion of higher density because of the increased recombi-nation rate in the higher density medium. For a D-type IFadvancing subsonically into the medium, the total recombi-nation rate between the star and the IF is balanced by thephotoionization rate produced by the incident Ñux, FLyc\Q/4nd2, where d is the distance from h1 Ori C.

The Ha-dominated emission-line crescents in the exter-nally illuminated YSOs have typical radii of order 25 to 500AU. The emission-line Ñux ratios derived from the peaksurface brightness of various emission lines given in Table 4are consistent with emission produced by a photoionizedplasma (cf. If we assume that the crescentsSutherland 1997).trace the IFs at the surface of a spherical cloud and alsoignore the absorption by dust in the nebula, the electron

density at the IF is expected to be approximately n Bcm~3, where is the(FLyc/aB

L )1@2 B 7 ] 105L 15~1@2d17~1 L 15characteristic radius of the IF in units of 1015 cm, is thed17distance to the illuminating source in units of 1017 cm, andcm3 s~1 is the case B recombination coef-a

BB 2.6 ] 10~13

Ðcient for hydrogen.The electron density of the plasma just outside the exter-

nally illuminated YSO IF is one of the few physical param-eters that can be determined with a fair amount of certainty.The electron density has been estimated by three methods :from the radio continuum (Felli et al. from1993a, 1993b),the brightness of the Ha emission (corrected for extinctionby the mean to the Orion Nebula), and by Ðtting theA

Vobserved intensities of trace ions and atoms in the IF usingMAPPINGS2 (cf. et al. Sutherland 1997).Sutherland 1993 ;All methods yield electron densities at the IF that rangefrom to 106 cm~3.n

e\ 105

Most externally illuminated YSO crescents have peaksurface brightnesses about 2È20 times brighter than thebackground nebula (see The central density in theTable 4).Orion Nebula is about 8000 cm~3 et al. and(Pogge 1992),the emitting line-of-sight path is about 0.13 pc &(OÏDellWen Thus, if a typical externally illuminated YSO IF1995).has a radius cm B 0.0003 pc (67 AU), theL 15\ 1015density required to produce a surface brightness 10 timeshigher than that of the central part of the nebula is Sn

eT \

cm~3,8000(FYSO/FM42)1@2(0.13pc/L pc)1@2\ 5 ] 105L 15~1@2which is in excellent agreement with the predictions for anIF.

The pressure of plasma at the IF is about PI/k B nT \

cm~3 K. The plasma thermal pressure is com-109n5 T4parable to the radiation pressure acting on grains(total luminosity) and neutrals (Lyman continuum lumi-nosity), which is of order Prad\ L /(4ncd2) \ 7.7

cm~3 K. For most externally illuminated] 108L 5 d17~2YSOs, the thermal pressure at the IF is greater than the rampressure of the h1 Ori C stellar wind. The pressure ratio isgiven by

PI

Pw

\ kmH cII2 dM0

wvw

A4nQaBLB1@2 \ 113d17 M0 ~7~1v3~1Q491@2L 15~1@2 .

Since this ratio is greater than unity, the stellar wind cannotreach the IF. Photoionized plasma will expand outwardfrom the IF, forming a diverging wind at about the soundspeed in ionized gas.

To Ðrst order, the Ñow from the IF is isothermal andspherical (at least in the hemisphere facing the illuminatingstar) and has a constant velocity at about the sound speedin ionized gas, km s~1. The mass-loss rate throughcIIB 10the IF is and the electron densityM0

eB 2nkmH cII ne L 2,

outside the IF decreases as ne(r [ L ) \

where r is the distance from the centralM0e/4nkmH r2cII,low-mass star (or point of symmetry for a spherically diver-

gent Ñow). The mass-loss rate from typical externally illumi-nated YSOs is about 10~7 yr~1. et al.M

_McCullough

show that the IF radii scale as L P d2@3, which is(1995)consistent with the results shown in and with aTable 4constant mass-loss rate approximately independent of theprojected distance from the illuminating source. The ionizedgas Ñowing away from the IF forms a low-velocity ““ IFwind ÏÏ with a slightly steeper than r~2 density proÐlebecause of the slight acceleration of the Ñow by the radialpressure gradient.


The surface brightness at some projected radius from theIF is proportional to the emission measure, EM D n

e2 l,

where l is the path length through the emitting region. For aspherical wind, the surface brightness is proportional to

where r is measured from the center ofEM\ 2 /0= ne2 (r)dy,

the spherical expansion and y is measured along the line ofsight. For a line of sight that passes at a projected distance bfrom the central source, the emission measure is pro-portional to b~3. An exact integration shows the emissionmeasure at projected radial distance b through a sphericalwind to be

EM\ 2C2P0

= dyb2] y2 \ nC2

2b3 , (3)

where and y is the distance along theC\M0E/(4nkmH cII)line of sight measured from the point of closest approach

between the line of sight and the source. (We use the sub-script E to denote parameters pertaining to an externallyilluminated YSO.) A constant expansion velocity r~2density proÐle produces an r~3 emission measure and pro-jected radial intensity proÐle, roughly consistent with theobserved radial proÐles shown in Figure 11.

If we ignore absorption by dust in the H II region, the IFis located at a distance from the center of the clump, r

I,

given by

F\ Q4nd2\

PrI

=ne2(r)a

Bdr ,

which leads to

rI\C M0kmH v(r)

D2@3A aB

4nQBd2@3 .

The surface brightness of the ionized gas is proportional tothe emission measure, EW \ / where L is the line-n

e2(L )dL ,

of-sight path length. The ratio of the peak emission measureof a proplyd divided by that of the H II region is given by

EWp/EWH II\ 13(rS/d)2 ,

where is the Stro� mgren radius of the H II region.rS OÏDellshows that the Ha surface brightness of the proplyds(1998)

is bounded by the inverse square of the projected distancefrom h1 Ori C, providing strong evidence that the proplydemission is dominated by photoionization.

If the externally illuminated YSOs are assumed to haveidentical power-law radial density proÐles, n(r) \ n0 r~a \

the radius of the IF is expected to scale asn0(r0)r0a r~a,

rI\C (1[ 2a)Q4na

Bn02 (r0)r02a

D1@(1~2a)d~2@(1~2a) ,

which for a \ 2 gives A scaling implies arIP d2@3. r

IP d1@2

radial power-law index a \ 5/3. If the ionized density dis-tribution is an exponential with the formaln(r) \ n0 e~r@r0,solution for the location of the IF is

rI\ r0 ln [(2na

Br0/Q)1@2n0 d] ,

where is the exponential scale height and is the densityr0 n0at location r \ 0 of the coordinate system in which theposition of is measured. For the parameters of the Orionr

INebula and an exponential scale height of cmr0\ 7]1014this reduces to where(OÏDell 1998), r

I\ r0 ln (0.87n6 d17),is the density in units of 106 cm~3 and is in units ofn6 d171017 cm.

For spherical clouds with sufficient density to trap an IF,if the electron density outside the IF decreases as r~2, thenthe electron density at the IF is given by n

e(I) \

where is the cloud IF radius and d(3Q/4naB)1@2r

I~1@2 d~1, r

Iis the true (not projected) distance from h1 Ori C. So long asa cloud has sufficient density to shield itself from Lymancontinuum radiation, the electron density at the IF is onlydetermined by the geometry once steady state conditionsare achieved. Since the peak surfacen

e(I) P r

I~1@2 d~1,

brightness of the IF, which is proportional to its emissionmeasure scales as d~2. This is roughly consistent with(n

e2 L ),

the results given in In principle, with precisionTable 4.calibration, the Ha images can be used to deduce the ratioof projected to true distance from h1 Ori C. For any givenobject, this yields two possible locations in three dimensionswith respect to the ionizing star. Careful inspection of theimages may be used to determine whether the object is infront of h1 Ori C or is in the background. In foregroundobjects, the circumstellar disks should be seen in silhouetteagainst the externally illuminated YSOÏs own IF, and weare probably looking at the side of the disk that is notdirectly illuminated by h1 Ori C. In this case the disk shouldshow up as a pure silhouette, even in the [O I] and in thenear-IR lines. On the other hand, for externally illumi-H2nated YSOs in the background, the circumstellar disk willbe located behind the YSO IF and not seen in silhouette. Inthis case we are probably seeing the face of the disk that isilluminated by soft UV radiation from h1 Ori C, whichshould result in bright near-IR lines and possibly [O I]H2emission from the irradiated disk face. Thus, it may be pos-sible to derive the distribution of externally illuminatedYSOs near Trapezium in three dimensions.

4.3. [O III]] Ha ArcsHST images in the [O III] and Ha Ðlters show arcs of

emission lying between h1 Ori C and many externally illu-minated YSOs at about 5% to 30% of the YSOÈh1 Ori Cdistance. We propose that the [O III]] Ha arcs trace theshock structure formed by the interaction of the photoa-blated material expanding from the YSO IF (the ““ IFwind ÏÏ) with the stellar wind from h1 Ori C.

To be visible against the bright background of the OrionNebula, the [O III]] Ha arcs must have an emissionmeasure comparable to that of the nebula, which in Ha hasan of about 105È106 cm~6 pc toward Trapezium. If wen

e2 L

assume that the [O III]] Ha arcs are seen predominantlywhere our line of sight is tangent to a sheet of emission, thepath length through the emission region, L , is likely to be atmost of order of the projected length of the emission region,or about 0.002È0.01 pc (corresponding to a projected lengthof 1AÈ3A). To produce an observed surface brightness com-parable to the nebula, the density in the emission regionmust be at least of order 104 cm~3. The density of the h1 OriC stellar wind declines with distance as M0

w/4nkmH d2v

wB

Upon passing through an adiabatic0.5M0 ~7 d17~2 v3~1 cm~3.shock, the density only increases by a factor of 4. Thus, theobserved [O III]] Ha arcs cannot correspond to the stellarwind shock itself, since the emission measure from thisregion is too low by many orders of magnitude.

We propose that the [O III]] Ha arcs are produced in adense layer where the IF wind is compressed by a mildreverse shock associated with the stationary bow shockstructure formed where the IF wind reaches ram pressurebalance with the h1 Ori C stellar wind. Two physical mecha-


nisms may contribute to the [O III] and Ha emission of thislayer : photoexcitation and turbulent mixing of this layerwith the hot, shock-heated stellar wind debris &(BegelmanFabian Shull, & Begelman1990 ; Slavin, 1993).

A stationary shock structure is formed around eachexternally illuminated YSO where the ram pressure of theIF wind equals the ram pressure of the h1 Ori C stellar wind(see As discussed above, the gas Ñowing away fromFig. 10).the IF acts to Ðrst order as a spherically symmetric windwith

nE\ M0

E4nd

E2 kmH v

E, (4)

where is the distance from the low-mass YSO,dE

M0ED

10~7 yr~1 as given by and is about 1È2M_

equation (3), vEtimes the sound speed in ionized gas (10È20 km s~1).

(Again, we use the subscript E to denote parameters per-taining to an externally illuminated YSO.) A similarformula holds for the massive stellar wind from h1 Ori Cwith yr~1 and km s~1M0

wD 4 ] 10~7 M

_vw

D 1 ] 103& Prinja On the axis connecting the(Howarth 1989).

massive star and the externally illuminated YSO, ram pres-sure balance is reached when

dE

dw

\ M0EvE

M0w

vw

B 0.1 . (5)

Raga, & Wilkin present analytic formulaeCanto� , (1996)that describe the shape of a bow formed at a wind-windcollision interface. Their formulae can be used to show thatfor conditions appropriate to the collision between theexternally illuminated YSO ionized photoablation Ñow andthe h1 Ori C wind, the separation between the externallyilluminated YSO and the stagnation point in the bowshock, is about 0.05È0.2 times the separation betweend

E,

this point and h1 Ori C.In general, we view the externally illuminated YSO and

its bow shock along lines of sight that are inclined withrespect to the line connecting the externally illuminatedYSO and h1 Ori C, and we see a bright arc in projectionfrom a portion of the bow shock structure where our line ofsight is nearly tangent to the shock structure. The observedprojected separation between the externally illuminatedYSO and the arc is always larger than the projected separa-tion between the YSO and the apex of the bow (which lieson the line connecting the YSO with h1 Ori C). For theestimated parameters of the IF winds and h1 Ori C, thepredicted location of the bow shock closely matches theobserved location of the [O III]] Ha arcs with a projectedd/L B 0.2.

While the shock into the h1 Ori C stellar wind is tootenuous to produce observable amounts of [O III]] Haemission, the density behind the reverse shock propagatinginto the externally illuminated YSO photoablation Ñowmay be high enough (n D 103È104 cm~3) to provide therequired emission measure. However, this shock is propa-gating at less than 30 km s~1, the expansion velocity of theexternally illuminated YSO wind far from the IF, and is tooweak to directly excite either Ha or [O III] emission. Thecompressed postshock layer may produce [O III] and Ha bytwo mechanisms : photoionization by the UV radiationfrom h1 Ori C, and excitation in a turbulent mixing layerformed by the mixing of the dense and cold (104 K) gas withshocked and hot (106 K) stellar wind. The externally illumi-

nated YSO wind shock is expected to be isothermal since itis a weak shock, and the temperature of the preshock andpostshock layers will be determined by photoionizationequilibrium. Thus, the compression ratio will be roughlygiven by the Mach number squared, or about 4È10. At adistance of 1A to 2A from an externally illuminated YSO(where most [O III] arcs are seen), the IF wind has a densityof about 102È103 cm~3, and the postshock density willrange from 103 to about 104 cm~3. When this layer is nearlytangent to our line of sight, enhanced Ha emission and, tosome extent, [O II] and [O III] emission will be observable,since the emission measure will be comparable to that of thenebular background.

There are several predictions that follow from this inter-pretation of the [O III] arcs. First, the arcs should be sta-tionary structures that show no proper motion. Second,these shocks represent standing surfaces through which gasÑows. Therefore, the radial velocity of Ha or [O III] in thearcs, which traces the motion of individual atoms or ions,should show Doppler shifts that are characteristic of theexternally illuminated YSO windÈabout 20 to 30 km s~1.The postshock stellar wind in the bow shock skirts willpreserve a large fraction of the 103 km s~1 velocity of themassive stellar wind. The postshock IF wind may be in partentrained by this hot plasma. Such entrainment, and theturbulent mixing that should result, will increase both theline width and the Doppler shift of the mixed gas. In thiscase, the [O III] arcs are expected to have a low Dopplershift with respect to stationary photoionized gas in theOrion Nebula if turbulent mixing does not play an impor-tant role. If turbulent mixing does play an important role,then the [O III] arcs are expected to have a large (up toseveral hundred km s~1) radial velocity and a large linewidth.

et al. proposed that the ionizing radiationHenney (1996)from h1 Ori C reaches the disk surface. The collisionbetween the resulting disk wind and the stellar wind fromh1 Ori C produces a standing shock where ram pressurebalance is reached. Henney et al. identify the bright hemi-spherical edges with the reverse shock (““Mach disk ÏÏ)formed where the IF wind passes through a roughly station-ary shock. This model appears to be incorrect for severalreasons : The azimuthally averaged radial proÐles of surfacebrightness are consistent with photoionization models (cf.

of these edges and not with shock models.Sutherland 1997)Furthermore, the observations provide no evidence ofphotoionization of the disk surface or the region betweenthe disk and the emission-line crescents. If the IFs are iden-tiÐed with the hemispherical arcs, then the implied pressureof this region is greater than the ram pressure of the stellarwind, and wind-wind interactions should be expected tooccur at a considerably greater distance from the externallyilluminated YSO.

4.4. Role of T hermal Conductionargues that an exponential radial intensityOÏDell (1998)

proÐle provides a slightly better Ðt to the Ha emission pro-Ðles of some proplyds than does a power law. However, thedi†erences are subtle, and both types of proÐle imply steepdensity and pressure gradients that will drive mass Ñowsfrom the proplyds unless there is an external force thatkeeps the Ñuid in place. Three distinct physical e†ects canproduce deviations from an r~3 intensity proÐle expected ina spherically expanding envelope : (1) Close to the IF, the


ionized fraction declines because of the decreasing Ñux ofLyman continuum photons. (2) Isothermal Ñows acceleratelogarithmically, producing density proÐles steeper than r~2and intensity proÐles steeper than r~3. (3) Thermal conduc-tion from the hot stellar wind bubble that may surroundsome proplyds will tend to accelerate the outer parts of thediverging photoablation Ñows. The Ðrst e†ect tends tolower the observed surface brightness just outside the IF.Both the second e†ect and the third e†ect lower the surfacebrightness at large radii when compared with an isothermal,fully ionized, and constant-velocity Ñow. These three e†ectsproduce radial intensity proÐles that deviate below the r~3radial dependence both near the IF and at large radii. SuchproÐles can approach an exponential form.

Externally illuminated YSOs located beyond the reverseshock of the h1 Ori C stellar wind will be embedded in thehot, postshock stellar wind bubble. The photoevaporatingYSO environment is very similar to the spherical cloudconsidered by & McKee and & CowieCowie (1977) McKee

in their treatment of thermal evaporation of clouds(1977)embedded inside 106È108 K bubbles produced by super-novae and stellar winds. Thermal conduction will, to Ðrstorder, result in strong deviations from the isothermalexpansion of the photoionized material that Ñows throughthe externally illuminated YSO IF. As shown by &McKeeCowie and & McKee the temperature(1977) Balbus (1982),of the photoionized plasma rises from its initial value of 104K at some inner boundary to the temperature of the windbubble as r2@5 in the limit that the Ñow remains subsonicrelative to the local sound speed. The thermal conductionÈpowered mass-loss rate is independent of the density ofthe wind bubble, but it does depend on its temperatureand on the radius of the cloud according to M0 B

yr~1, where " is the1.3] 10~7T 75@2 r~3(30/ln ") M_Coulomb logarithm, is the bubble temperature in unitsT7of 107 K, and is the radius of the evaporating cloud inr~3units of 10~3 pc & Cowie(McKee 1977).

Thermal evaporation becomes signiÐcant when theconduction-induced mass-loss rate at a cloud radius r

pbecomes comparable to the radiation-induced mass-lossrate of the externally illuminated YSO. Conduction-induced mass loss becomes comparable to the estimatedphotoablation rate at a cloud radius of about 200 AU, com-parable to the radii of the IFs at large distances from h1 OriC. Thus, at a distance of more than about 200 AU, thermalconduction may dominate the Ñow characteristics.

Thermal conduction results in a radial temperature gra-dient, which leads to a more rapidly declining radial densityproÐle than is characteristic of an isothermal Ñow. Asshown by & Cowie an exponential densityMcKee (1977),proÐle is expected, and this will result in an exponentialradial intensity proÐle. Therefore, large externally illumi-nated YSOs embedded in the hot stellar wind bubble areexpected to have exponential radial intensity proÐles. Asdiscussed by some externally illuminatedOÏDell (1998),YSOs exhibit signiÐcant deviations from the r~3 intensityproÐles expected for an isothermal Ñow, and some objectscan be Ðtted reasonably well with exponential radial inten-sity proÐles. Future studies of the radial intensity gradientsof embedded condensations might be used to map theextent of the thermal wind bubble.

4.5. FUV RadiationSoft UV radiation with wavelengths between 912 and

D2000 penetrates into the neutral gas between the YSOÓand the IF, establishing a photon-dominated (orphotodissociation) region (PDR; & HollenbachTielens

The intensity of soft UV radiation in the core of the1985).Orion Nebula is about in units of the mean energyG0\ 105density of soft UV radiation in the vicinity of the Sun (about1.6] 10~3 ergs cm~3, also known as the Habing Ðeld). SoftUV is primarily attenuated by dust, which, between 1000and 2000 produces about 3 to 5 mag of extinction forÓ,every 1 mag of visual extinction Thus, the soft-UV(A

V).

energy density drops to solar-vicinity values when is 2È3AVmag, which for a standard gas-to-dust ratio corresponds to

a hydrogen column density of (2È3) ] 1021 cm~3. The softUV radiation Ðeld heats the gas in the PDR to tem-peratures of order 102È103 K with corresponding soundspeeds of 1È3 km s~1 & Sto� rzer For any(Hollenbach 1998).conÐguration that is in approximate hydrostatic balanceprior to irradiation, the soft-UV heating will drive anoutward expansion of gas at approximately the local soundspeed. This expansion is fueled by the reservoir of materialin the YSO environment and can maintain a static (in theframe of the YSO) IF at for as long as the reservoir canr

Isustain the FUV-powered disk wind et al.(Johnstone 1998).et al. present a model for externallyJohnstone (1998)

illuminated YSOs in which the FUV radiation powers awind expanding from the disk surface. The resulting diskmass-loss rate is of order yr~1M0

E\ 1.3] 10~7vr

d,15 M_at a velocity km s~1 (the expected sound speed in thev

EB 3

PDR), where v is a parameter of order unity and is therd,15outer radius of the disk in units of 1015 cm. By the time this

Ñow reaches several disk radii, it is approximately spherical.The column density between the disk outer radius and ther

dIF at is given byrI

N(H)BM0

E4nkmH v

ErEB (1È3)] 1021v3~1r10~1 cm~2 , (6)

where is the Ñow velocity in units of 3 km s~1 and isv3 r10the point to which the column density is being estimated inunits of 10 AU as measured from the central star.

The disk wind is self-regulating. If the column densitybetween the disk and the IF is decreased, the Ñux of soft UVradiation incident on the disk will increase and result in ahigher mass-loss rate. This pushes the IF outward untilN(H) settles down at a value close to that given above. Ifthis column density is much larger (as must be the casewhen the YSO is Ðrst exposed to UV radiation), too littlesoft UV radiation reaches the disk and the mass-loss rate islow. The D-type IF advances into the externally illuminatedYSO envelope until a sufficient Ñux of soft UV radiation isavailable at the disk surface to stall the IF in the frame ofthe externally illuminated YSO. In steady state, the propa-gation velocity of the IF into the neutral photoablation Ñowexactly equals the outward velocity of the photoablateddisk wind.

4.6. Implications of O I EmissionOur WFPC2 images of the Orion Nebula externally illu-

minated YSOs show that many contain disklike featuresthat appear as dark silhouettes in [O III], [S II], [N II], Ha,and the continuum but are bright in [O I]. The best exam-ples are HST 10 and 17, shown in which have anFigure 5,[O I] surface brightness of about 8 ] 10~14 ergs s~1 cm~2arcsec~2 (cf. Tables and3 4).

In principle, it is possible for the 6300 [O I] line toÓ


resonantly scatter incident radiation. The maximum surfacebrightness caused by scattering is roughly S \ 1.8

ergs s~1 cm~2 arcsec~2 for a line] 10~14gsd17~2(*j/j)L 5width of 1 km s~1, where we have assumed that the 6300 Ó

Ñux emitted by the Trapezium stars is approximately givenby where is the Trapezium lumi-(*j/j)L 5, L 5\ 105 L

_nosity and is the scattering efficiency. For [O I] linegswidths of order 4È5 km s~1, the observed peak surface

brightness of the [O I] emission from the silhouettes liesclose to this value, so if it is produced by scattering, thescattering optical depth must be about unity. However, thescattering cross section for this forbidden transition isextremely low, requiring an O I column density of orderN(O cm~2 for an optical depth qB 1.I) B 2 ] 1021bkmsHere is the line-broadening parameter in units of kmbkmss~1. For normal metallicity this requires N(H)B 1024cm~2. Unless the dust is depleted by a factor of order 103,the observed [O I] emission cannot be produced by scat-tering.

We now consider the excitation of [O I] by thermal colli-sions. Collisions with a kinetic energy of 1.97 eV arerequired to pump the 6300 [O I] transition. For a Max-Ówellian distribution, this requires that the mean kineticenergy of the collision partners be at least 0.5 eV, corre-sponding to a temperature of about 3800 K. In the mid-plane of the disk, the temperature is likely to be nearly inthermodynamic equilibrium with the incident radiationÐeld. If the YSO embedded in the disk has luminosity L YSO,the disk temperature is dominated by the central starfor disk radii smaller than r

d\ d(L YSO/L 5)1@2B 1.3

cm, beyond which the minimum] 1015[L 5/(1 L_)]~1@2

disk temperature is set by the luminosity of Trapezium atabout K, where is the luminosity of theT

d\ 87L 51@4d17~1@2 L 5illuminating source in units of 105 and is the dis-L

_d17tance from this source in units of 1017 cm. The disk mid-

plane temperature at the radius where the escape speed isequal to the sound speed in ionized gas is about T (r

G) B

100 K for a Sun-like star. However, FUV photons withenergies in the range 6È13.6 eV can penetrate and heatregions where the column density is less than aboutN(H)B 3 ] 1021 cm~2. Models of photon-dominatedregions predict temperatures of about 1000 K for FUVradiation Ðelds in the range & Hol-104\ G0\ 106 (Tielenslenbach et al. & Sto� rzer1995 ; Johnstone 1998 ; Hollenbach

Hollenbach, & Tielens show that in1998). Burton, (1990)very dense PDRs, a thin surface layer immediately adjacentto the IF with N(H)\ 1020 cm~3 can be heated to about3000 K primarily by vibrational heating, in which pho-H2toexcited states of are collisionally de-excited and heatH2the medium.

HST 10 lies at a projected distance of 57A (4 ] 1017 cm)from h1 Ori C. The FUV Ñux from h1 Ori C at this distanceis calculated to be The et al.G0\ 5 ] 104. Burton (1990)model for and n \ 107 cm~3 predicts a total 6300G0\ 105

[O I] Ñux of 3 ] 10~16 ergs s~1 cm~2 in a PC pixel,Óabout twice the peak surface brightness near the substellarpoint on the IF of HST 10. Thus, the [O I] emission locatednear the IF probably traces the component of very hot gasmentioned above. Our observations also show a secondcomponent of [O I] emission associated with the silhouettein HST 10, which presumably lies at the base of the PDR.The electron density at the HST 10 IF is estimated to beabout 8 ] 104 cm~3. Assuming that the neutral hydrogendensity inside the IF remains at least this high, the column

density of H between the IF and the silhouette surface is atleast about nL \ 4 ] 1020 cm~2, where L is the separationbetween the silhouetteÏs surface and the IF (about 5] 1015cm, corresponding to 0A.7.)

We propose that the [O I] emission is excited by 2 eVcollisions with nonthermal particles that are not in ther-modynamic equilibrium at the PDR temperature. There aretwo physical mechanisms in a PDR that may produce sucha non-Gaussian high-energy tail on the Boltzmann distribu-tion : the ejection of electrons from grains by the photoelec-tric e†ect and the dissociation and reformation of onH2grain surfaces. The photoelectric e†ect of FUV radiationproduces electrons with energies ranging from 0 to about 6eV that can su†er many elastic collisions with neutrals inthe weakly ionized PDR prior to reaching an equilibriumdistribution. Photoelectrons have sufficient kinetic energyto excite the [O I] line. The formation of on grain sur-H2faces releases the 4.8 eV binding energy of molecular hydro-gen. About half of the time, a recently formed moleculeH2is ejected from the grain surface with an energy greater than2 eV, also sufficient to excite [O I]. Thus, in the IH2ÈHtransition zone, where the destruction of is in approx-H2imate equilibrium with reformation, the total reformationrate will be comparable to the Ñux of photons absorbed inthe Lyman and Werner bands of These two processesH2.might produce a sufficient population of suprathermal par-ticles to excite the observed [O I] emission. A detailedanalysis of these processes will be presented elsewhere. Aconsequence of this model is that any abundant neutralspecies that has a transition to the ground state in theoptical portion of the spectrum should produce detectableemission from the surface of the silhouettes embedded in thebright externally illuminated YSOs.

If the illuminating Ñux is about the observedG0\ 104,surface brightness of the [O I] emission implies that theefficiency of conversion of incident FUV Ñux in the 6È13.6eV range into [O I] line emission is about [v([O I]) sin

where v([O I]) is the [O I] emission fromi]/GFUV ¹ 10~4,the disk surface in ergs s~1 cm~2, sin i is a correction forforeshortening, and is the incident FUV radiationGFUVÐeld. Assuming that the FUV radiation responsible for gen-erating the [O I] emission is absorbed in a column densityof hydrogen of order N(H)B 1021 cm~2 and that the layeris no thicker than a PC resolution element the(0A.05),density in this layer is at least n(H)º 3 ] 106 cm~3, orabout 1 order of magnitude greater than the density at theIF.

The absence of forbidden line emission from other speciesin the [O I] emission zone associated with the silhouettesplaces another constraint on the physical characteristics ofthis region. The [O III], [N II], and [S II] lines originate instates that lie about 2 eV above the ground state and there-fore have similar excitation requirements as [O I]. Sincesulphur has an ionization potential of about 10.3 eV, theFUV photons will photoionize this species, and S II shouldbe present in the PDR. Since the red [S II] lines requirecollisions of about 1.8 eV to excite, collisions that excite[O I] will also excite [S II]. The absence of red [S II] emis-sion implies that sulphur is mostly in its neutral state,despite the presence of an intense FUV radiation Ðeld. This,in turn, implies that the recombination rate exceeds thephotoionization rate. Using the recombination rate coeffi-cients of & Van Steenberg which giveShull (1982), aS II\cm3 s~1, where is the temperature1.75] 10~12 T 3~0.63 T3


in units of 103 K, and the photoionization cross section of& Yakovlev which givesVerner (1994) pS I\ 1.63] 10~16

cm2, and assuming a fractional electron abundance, xe\

this leads to10~4,nS IInS I

\ FFUV pS IneaS II hl

\ 9 ] 1011nH

F12 cm~3 ,

where is the UV photon Ñux in units of 1012 photonsF12s~1 cm~2. At the high-density base of the PDR, the Lymanand Werner bands of block about half of the incidentH2photons above the ionization threshold for S I and dustattenuates the radiation Ðeld by about a factor of 10È100. Ifrecombinations deplete the S II, then the density in the [O I]emission zone must be around cm~3 or higher. InnH \ 1010the high-density and heated environment of the disksurface, it is possible that chemical reactions, such as thosethat lead to the formation of SO, also contribute to thedepletion of S II, or may produce O I in the metastable state.

4.7. TailsObservations of the 150 objects embedded within the

Orion Nebula show that most externally illuminated YSOshave tails with lengths ranging from to about 4A, with a0A.1median length of about T here is no systematic depen-0A.8.dence of tail length on projected separation from the primaryilluminator. The tails connect smoothly to the IFs that facethe primary source of radiation and, in most objects, pointdirectly away from the primary illuminator. The tailsassociated with sources lying within a projected distanceless than 1@ of h1 Ori C point to this star to better than a fewdegrees. However, there are several sources between h1 OriC and h2 Ori A that have very stubby tails with an axis ofsymmetry that tends to bisect the lines pointing to thesesources. In these objects, the tail orientations may reÑect theinÑuence of irradiation by both stars. Most tails have sharptips at the substellar point (the end of the tail most distantfrom the illuminator). The tail shapes are well representedby a conical shape linking the tips with the forward-oriented IFs. In the well-resolved tails, the IFs appear limb-brightened with some evidence for excess obscuration justinside the IF in the [N II] and [O III] images. The tails lyingclose to h1 Ori C tend to be long and skinny since theprojected radius of the forward-facing IF decreases withdecreasing projected distance from the illuminating source.

We have considered a variety of mechanisms for tail pro-duction ; however, we do not have a satisfactory explana-tion for their existence at this time. We list some possiblemodels and mechanisms for tail production and brieÑydiscuss their problems.

Radiation pressure.ÈThe Trapezium cluster has a totalluminosity of about 105 and h1 Ori C has a LymanL

_,

continuum luminosity of about Q\ 1.5] 1049 photonss~1. Soft UV and longer wavelength radiation couplesstrongly to dust, which exerts radiation pressure on themedium. This coupling is complex because of the role thatgrain charging plays in coupling the gas to the grains. Theradiation pressure is given by Prad\ L

*/4ncd2B 109d17~2

cm~3 K, where is the illuminator luminosity andL*,5 L

*is this luminosity in units of 105 and is thisL*,5 L

_, d17distance in units of 1017 cm. The drift velocity of maximally

charged grains through the gas is estimated to be of order0.1 km s~1 for the typical conditions in externally illumi-nated YSOs in the Orion Nebula Radiation(Spitzer 1978).pressure e†ects are likely to be important only in regionswhere the e†ect of other forces produces velocities less than

this drift velocity. Since the FUV radiation heats gas toabout 103 K, where the sound speed is around 3 km s~1,radiation pressure is expected to play only a minor role intail formation.

Impact of stellar winds.ÈThe cometary shapes of the tailssuggest that they may be formed by wind-wind interactions(Dyson However, the IF wind discussed pre-1968, 1973).viously will shield the IF from direct impact by the stellarwind. Within 30A of h1 Ori C, the Ha ] [O III] arcs provideevidence that the IF outÑow interacts with the h1 Ori Cwind in a zone well separated from the IF. Though thepostshock gas produced by these bow shocks may Ñow pastthe tails at subsonic velocities, the stellar winds are notlikely to directly impact the tail region.

Radiative ““ breaking ÏÏ of disks.ÈF. Bertoldi (1997, privatecommunication) has suggested that when the pressure of theIF ([109 cm~3 K) exceeds the midplane pressure in thecircumstellar disk, the disk material will be accelerated bythe rocket e†ect. This material will be pushed into theregion of the tail. Since the disk has angular momentumabout its central star, it is hard to see how the observedpointed tails can be produced by this mechanism alone.

IF shadow.ÈAssume that the YSO is only surrounded bya circumstellar disk during initial ionization. Once steadystate ionization is maintained, most of the photoablationoccurs in the forward direction, the direction from whichthe bulk of the UV radiation is arriving. Gas that is shad-owed from direct illumination can remain neutral.However, the di†use radiation Ðeld will ionize this regionquickly in the absence of rapid mass motions that can reple-nish the neutral medium.

PDR powered by the di†use L yman continuum.ÈRecombinations within an H II region e†ectively scatterLyman continuum photons. For case B recombination, thedi†use radiation Ðeld has a total Lyman continuum lumi-nosity about one-third that of the ionizing star. On average,the di†use Ðeld is constant within the Stro� mgren spheresince most of it is formed by recombinations near the IF.Thus, on average the back side of a disk is exposed to aroughly constant Ñux where is theFdiffB 16Q/4nrS2, rSStro� mgren radius of the H II region. This will heat the diskÏsback side to about 103 K, powering a PDR outÑow atabout 3 km s~1 from the portion of the disk where r

d[ r

G,

the radius at which the sound speed exceeds the local escapespeed. Since the backÑow is in the shadow of the disk andthe forward-facing IF, it is ionized only by the di†use radi-ation Ðeld at a rate that is roughly independent of locationwithin the nebula. Thus, the tail may be a result of a PDR-induced Ñow from the shadowed back side of the disk. Asthe di†use radiation Ðeld eats into the tail downstream fromthe disk, it becomes narrower, resulting in a conical shapeand a sharp tip. The forward-facing IF radius decreaseswith decreasing distance from the illuminator. Thus, thismodel predicts that tails closer to the illuminator areshorter because of the smaller radius of the shadow zone,contrary to observation et al.(Johnstone 1998).

L eftover wind wakes.ÈIt is possible that the tails are notformed by the present-day steady state processes, but ratherby processes that operated prior to ionization, or by tran-sient processes operating during initial ionization. High-mass stars produce powerful winds prior to photoionizingtheir environments. For example, the high-luminositysources embedded in the BN/KL core behind the OrionNebula produce one of the most spectacular protostellaroutÑows known. Dozens to hundreds of individual


““ Ðngers ÏÏ of shock-excited near-IR and [Fe II] emissionH2indicate a poorly collimated Ñow at several hundred km s~1from the high-luminosity region & Burton(Allen 1993 ;

& Mac Low The Near InfraredMcCaughrean 1997).Camera and Multiobject Spectrometer (NICMOS) EarlyRelease Observations of the BN region reveal several hemi-spherical fronts close to BN that might be recent examplesof fossil cloud cores surrounding embedded YSOs (Stolovyet al. When h1 Ori C was born, YSOs in its vicinity1998).were likely surrounded by similar envelopes. Tails couldhave been forged from these envelopes by powerful outÑowsemerging from the young h1 Ori C, resulting in a densewake downwind from h1 Ori C. Subsequent ionization ofsuch a wake could produce the observed tails. The mass inthe wake would be dependent on the formation history ofthe low-mass YSO, and probably independent of the pro-jected distance from the high-mass star, readily explainingthe observed independence of tail length on projected dis-tance from the illuminator.

Initial ionization instability of a spherical envelope.ÈForcircumstellar envelopes that have not been sculpted by aprotostellar wind from a high-mass star, the transient stageof initial ionization may also produce a dense tail that maysurvive subsequent photoionization for a long time. Initialionization of spherical clouds leads to cometary shapessince the downstream portion of the cloud is shielded by theupstream part of the cloud &(Bertoldi 1989 ; BertoldiMcKee For clouds with a power-law radial density1990).proÐle with a negative index, the IF will decelerate andwrap around a condensation, eventually forming an iso-lated ““ teardrop ÏÏ of neutral gas. To Ðrst order, the pressuregradient is always orthogonal to the IF. Therefore, thedensity contrast between a cloud and its surroundings isenhanced. The IF compresses gas downstream of the initialcloud centroid. Furthermore, the dense D-type IF is subjectto a Vishniac type of instability & Franco(Garc•� a-Segura

IFs that wrap around an isolated dense clump will1996).develop a tail whose density will be greatly enhanced by thecompression of the IF and the rocket e†ect. Such tails mayresist photoionization for a time comparable to that of theages of the externally illuminated YSOs.

If the Ðnal two scenarios are correct, they provide evi-dence that most of the sources now having tails were sur-rounded by massive protostellar envelopes at the time ofinitial photoionization by h1C Ori. If so, tails provide evi-dence for the extreme youth of the Trapezium members thatare now observed to be externally illuminated YSOs.

4.8. Interpretation of the FOS UV ResultsThe FOS spectra show a Lya spike in the middle of a

broad Lya absorption feature at most observed positions.There is a layer of neutral H I in front of the Orion Nebulawith a column density of about N(H)\ 1020È1021 cm~2that can be seen as an absorption feature against thenebular continuum in the 21 cm line and in a variety ofoptical absorption lines, such as sodium and magnesium (cf.OÏDell, Wen, & Hu 1993). The foreground H I is expected toproduce a broad damped Lya feature in the UV. Thenarrow Lya emission spike superposed on this feature maybe heliospheric emission that is backscattered by interstellargas in the solar vicinity or geocoronal Lya emission.However, it is also possible that a small fraction of theintrinsically very strong Lya emission produced by theOrion Nebula leaks through the damped wings of the fore-ground H I layer. The visibility of nebular Lya would be an

indication that the foreground absorption component has asigniÐcantly di†erent velocity than the emission producedby the Orion Nebula, consistent with determinations of therelative velocities of the nebular emission and foregroundcomponents observed at other wavelengths. The absence ofthe Lya dip in the spectra pointed toward the externallyilluminated YSOs implies that the background Lya emis-sion observed toward the SKY positions is absorbed by theexternally illuminated YSO envelope. This may be evidencethat, despite failure to attain perfect tracking with HST , ourobservations were pointed toward the neutral envelope ofthe externally illuminated YSOs.

The C IV feature observed in the FOS spectra is probablyproduced by scattering of the spectrum of h1 Ori C by dustalong the line of sight. The observed full width of thisfeature, 18 implies a stellar wind velocity of about 1600Ó,km s~1 when the Ðnite resolution of FOS is taken intoaccount. This agrees well with previous determinations ofthe h1 Ori C stellar wind terminal velocity (1650 km s~1 ;

& SavageFranco 1982).The FOC images demonstrate that in the UV silhouettes

can be imaged with higher resolution and with lower con-tamination from the stellar continuum of the central starsthan in the optical. The background continuum emissionfrom the nebula is signiÐcantly stronger than at the longerwavelengths. The UV images show circumstellar silhouettesin several objects near h1 Ori C Their symmetric(Fig. 8).and elliptical appearance in the images provides evidencethat these structures are circumstellar disks. The outer radiiof these disks are typically several times smaller than theradii of the emission-line crescents that below are inter-preted to be IFs.

5. DISCUSSION

5.1. Evolutionary State and Mass of the Y SO EnvironmentStar formation in an interstellar gas cloud is not coeval.

The duration of the star formation epoch can range from105 to 106 yr in a given cloud core, to over 107 yr for anentire giant molecular cloud spawning an OB association.

& Terndrup & Walker andHerbig (1986), Jones (1988),have studied the age spread in the Tra-Hillenbrand (1997)

pezium cluster, Ðnding few stars with evolutionary agesover 106 yr, a median stellar age of 3] 105 yr, and somestars with ages as short as 105 yr. Thus, low-mass (0.1È3

star formation has been proceeding for about 106 yr inM_)

the OMC-1 cloud core. We therefore expect an age spreadfor the YSOs embedded in M42. The evolution of moremassive stars proceeds faster than that of the lower massmembers of the same cluster.

We estimate the minimum gas density required to formthe stars present in the Trapezium cluster. If the 700 YSOsfound within a radius of 1 pc of Trapezium have a meanmass of 0.8 the cluster has locked up about 560 inM

_, M

_stars. This corresponds to an average minimum densityneeded to form these stars of cm~3 in aSn(H2)T B 2 ] 103radius of 1 pc. In the cluster core, the minimum gas densityrequired would be about cm~3 basedSn(H2)Tcore B 6 ] 105on the estimated density of 50,000 stars per cubic parsec

& Stau†er in the inner 0.1 pc cluster(McCaughrean 1994)core. Recently & Arthur reanalysed the Tra-Henney (1998)pezium cluster core density, Ðnding a value of 5] 106 starsper cubic parsec. This average minimum density is a littlebelow the current mean density of the ionized core of theOrion Nebula. However, the core average minimum densityis comparable to the mean density of OMC-1, located


behind the nebula, and to the density of cores believed to bein the early phases of gravitational collapse in other regionsof star formation. It is likely that the actual mean density ofthe core that formed the Trapezium cluster was more than 1order of magnitude greater (2 ] 104 to 107 cm~3). Themean separation of stars in the core of the Trapeziumcluster (roughly 0.02 pc, or 5000 AU) is smaller than thetypical radius of star-forming cores in more isolated star-forming regions. These numbers imply that mutual inter-actions between YSOs and the cores from which theyformed had a profound e†ect on their properties.

The cluster crossing time (nearly the same as the free-fallcollapse time) is If we use only the massq

cB (GSoT)~1@2.

present in stars, is about 1.3 Myr for the entire cluster,qcbut under 105 yr for the 0.1 pc cluster core. The cluster

velocity dispersion, is expected to bepcB D

c(GSoT)1@2,

about 1.5 km s~1 for the entire Trapezium cluster and 2.6km s~1 for the core. If we assume that the initial gas densitywas 10 times greater, so that the star formation efficiencywas only 10%, then Myr for the entire cluster andq

cB 0.4

less than 30,000 yr for the core. This results in a velocitydispersion of 5 km s~1 for the entire cluster and about 8 kms~1 for the core.

The median age of the Trapezium cluster (estimated to be0.3È1.0 Myr) requires that it formed on a timescale compa-rable to or less than 1 free-fall time of the collapsing core.Stars now at the periphery have at most crossed the clustera few times, but those within 0.1 pc of the core may havecrossed several times. Thus, the scenario for star formationin this region is that a massive (a few times 103 coreM

_)

underwent supercritical collapse and fragmented within afree-fall time into about 700 stellar-mass subcores, whichproceeded to form stars. Since the mean stellar separationin the Trapezium cluster is so small, it is likely that anyresidual cloud core material not rapidly accreted onto aprotostar or its accretion disk was tidally or collisionallystripped in a crossing time and became part of lower densitygas that was then dispersed by ionizing radiation.

At the onset of nebular ionization (which presumablyprevents further gravitational collapse and star formationfrom undi†erentiated gas because of the sudden increase oftemperature and pressure by more than a factor of 100),YSOs in a variety of evolutionary stages were exposed. Thestages range from cloud cores on the verge of collapse (Class[II or Class [I in the terminology), to accretingBoss 1995protostars (Class 0), to objects containing YSOs (Classes I,II, and III in the Lada, & Shu terminology).Adams, 1987This sequence represents a progression of decreasing gasmass in the circumstellar environment. In the global col-lapse scenario outlined above, any precollapse cores thatmay have survived are likely to be torn apart or accretedonto protostars, and unless there are YSOs in the regionthat predate this collapse, there has probably not beenenough time for any YSO to evolve beyond the Class IIIstage. The evolutionary state and the mass of the YSOdetermine the amount of gas that will be present in the YSOenvironment when it is exposed to UV radiation and, poss-ibly, stellar winds. The earlier the evolutionary state and thehigher the mass, the more gas likely to be present when theexternal UV source turns on.

5.2. Comparison with Other Star-forming Regionset al. present HST WFPC2 images of theHester (1996)

elephant trunks that lie several arcminutes southeast of thecore of M16. In their discussion of the structure of severalcandidate YSOs in the region, they argue that the YSOsembedded in the Orion Nebula are also elephant trunksthat are seen face-on. Their detached appearance should bea consequence of our point of view: we see the elephanttrunks perfectly end-on. However, since externally illumi-nated YSOs are seen throughout the Orion Nebula, thisinterpretation is completely unlikely.

et al. claim to have identiÐed hundreds ofHester (1996)stars in the M16 elephant trunks. Though claimed to beYSOs, none have been shown to posses any of the signa-tures of youth, such as outÑows, infrared excess emission, orother characteristics of stellar youth. However, their imagesprovide evidence for a few proplyd-like objects in the imme-diate vicinity of the elephant trunks that may containYSOs.

C. Stapelfeldt (1997, private communication) has usedHST to search several nearby H II regions for proplyd-likeobjects. These results have mostly proved negative, thougha few possible candidates have been found in S155. Caulet

reported Ðnding one proplyd-like object in the(1997)Lagoon Nebula (M8) on HST archival images. Yu, &Bally,Devine have searched the young H II region S106(1998b)for proplyds with HST , Ðnding none. The sum total of theseobservations points to the uniqueness of the large collectionof externally illuminated YSOs embedded in the OrionNebula.

A large fraction of the YSOs in the core of the OrionNebula are surrounded by extended circumstellar material.Within 20A of Trapezium, about 80% of stars have retainedtheir circumstellar material and are ““ proplyds.ÏÏ Upperbounds on the masses of the circumstellar material sur-rounding some of these YSOs have been determined bymeans of millimeter-wavelength interferometry by Mundy,Looney, & Lada et al. report the detec-(1995). Lada (1996)tion of 1.3 mm wavelength continuum emission from threeout of 12 externally illuminated YSOs near Trapezium, withmasses in the range 0.007È0.016 Searches for 230 GHzM

_.

13CO and continuum emission toward the large silhouettes182[413 (HST 10), 183[405 (HST 16), and 114[426 withthe Owens Valley Radio Observatory interferometer haveresulted in upper bounds less than about 0.01 towardM

_these and several other objects in the Ðeld of view et(Ballyal. These bounds are consistent with the lower1998a).bounds estimated from extinction & OÏDell(McCaughrean

et al. From the collection of available1996 ; Chen 1998).data, it is possible to conclude that the maximum mass ofdusty circumstellar matter surrounding these YSOs is about10~2 and for a typical mass-loss rate of 10~7 yr~1M

_, M

_the maximum lifetime of externally illuminated YSOsagainst photoablation is about 105 yr.

Star formation in the Orion Nebula may be representa-tive of the typical environment in which most stars are born.We have a fairly complete census of molecular clouds, star-forming regions, and young stars within 500 pc of the Sun.At least 90% of all stars younger than 10 or 20 million yearswere formed within the boundaries of four major OBassociations : Scorpius-Centaurus, Perseus OB2, OrionOB1, and Lacerta OB1 associations (cf. Blaauw 1991).While typical OB associations such as Sco-Cen and Orionproduce about (1È3) ] 104 stars each during a 5È15 Myrperiod of star formation, isolated dark clouds such asTaurus produce fewer than 103 stars in a comparable time


interval. Most star formation in the OB associations occursin short-duration ““microbursts ÏÏ during which the gravita-tional collapse of 103È104 cloud cores produce clustersM

_of 50È1000 stars in less than 1 million years in a regionsmaller than 1 pc Alves, & Lada(Lada 1992 ; Lada, 1996 ;

Devine, & Reipurth The birth of one or moreBally, 1996).massive stars, which heats, dissociates, and ionizes theremaining gas, may terminate star formation in such aregion. If the fraction of mass converted into stars (the starformation efficiency) is less than about 30% of the initialmass, the resulting cluster of stars will be gravitationallyunbound. Low-mass stars formed in OB associations arelikely to be exposed to the harsh UV radiation and, in somecases, the stellar winds produced by massive O and early Bstars, for a time ranging from 0.1 to 10 million years.

The detection of a large number of YSOs surrounded bywhat appear to be circumstellar disks may provide someconstraints on the formation frequency of planets. If theexternally illuminated YSOs are shown to be losing mass atthe rates estimated above, and if these disks contain lessthan 0.01 of material, then they may be destroyedM

_before forming planets. Furthermore, if most stars form inenvironments similar to the Orion Nebula, then planet for-mation may be relatively rare. On the other hand, it ispossible that most YSOs that are now externally illumi-nated are sufficiently old that their disks have condensed orcoagulated a large fraction of their solid matter intocentimeter- or meter-sized objects that can resist photoeva-poration and which may be hard to detect with presentmethods. In this case, our disk mass estimates may be severelower limits, and the formation of protoplanets mayproceed in environments like the Orion Nebula. Are theOrion Nebula YSOs surrounded by protoplanetary disksthat have, or soon will, form new planets? Or will photo-ablation destroy the disks before they have had a chance toform new planets? Future studies of YSOs in the OrionNebula and in other star-forming regions will determinewhether planet formation is very rapid or rare.

6. CONCLUSIONS

We have presented 9È23 AU resolution HST FaintObject Camera and Planetary Camera images of severaldozen YSOs embedded within the Orion Nebula that aresurrounded by externally illuminated circumstellarmaterial. We have presented peak surface brightness mea-surements in Ðve Ðlters centered on the [O III], [O I], Ha,[N II], and [S II] emission lines. We also have presented theÐrst UV spectra and images of several objects. We havereviewed the observed properties of the YSO environmentsand presented an interpretation of the observed sourcestructures and morphologies in terms of a dynamical modelof external radiation-induced mass loss. The main results ofthis study are as follows :

1. Most YSOs within a 1@ diameter region centered onthe Trapezium cluster are surrounded by bright emission-line crescents facing the massive stars in the region. In mostobjects, the crescents are hemispherical. The peak surfacebrightness of the crescents declines rapidly with increasingprojected distance from h1 Ori C. We interpret the crescentsas quasi-static ionization fronts with peak electron densitiesjust outside these fronts ranging from to 106 cm~3.n

e\ 105

The emission Ñux ratios of the observed lines are consistentwith photoionization models. Assuming that the thermal

pressure in the IFs dominates the dynamics,the freshly ionized plasma is likely to expand away from thedense neutral condensation at about the sound speed of theionized gas as an isothermal IF wind. The resulting r~2density proÐles are expected to produce r~3 emission-lineintensity proÐles that are in good agreement with the azi-muthally averaged and background-subtracted Haemmision-line gradients observed toward most objects.Estimated mass-loss rates are around yr~1.M0 \ 10~7 M

_2. The Orion Nebula contains several types of Ðlamen-tary structures bright in [O III] and Ha. About 50% of theexternally illuminated YSOs within a radius of about 30A ofh1 Ori C are surrounded by arcs of [O III] and Ha emissionon the side facing h1 Ori C. These arcs have typical radiiranging from 1A to about 5A. We argue that the arcs concen-tric with externally illuminated YSOs trace the shock struc-ture formed by the interaction of the isothermal Ñow fromthe crescent-shaped IFs surrounding the externally illumi-nated YSOs with the fast stellar wind ejected by h1 Ori C.Neither the reverse shock propagating into the low-velocityphotoablation Ñow nor the stationary bow shock throughwhich the h1 Ori C stellar wind must pass in moving pastthe obstacle presented by the YSO can excite the observedemission. However, a dense layer forms between these twoshocks, and gas in this region can be excited either by UVradiation or by turbulent mixing with the hot postshockstellar wind. The [O III] and Ha Ðlaments that are notconcentric with IF crescents tend to have random orienta-tions and are located far from stellar objects. This secondclass of Ðlaments are likely to trace HH Ñows, which areshocks powered by high-speed stellar outÑows or jets.

3. Externally illuminated YSOs located at a projecteddistance of more than 30A from Trapezium tend to be largerthan YSOs located closer to h1 Ori C. They lack concentric[O III] and Ha emission-line arcs. In a few objects, theradial intensity proÐles can be well Ðtted by an exponentialfunction. We argue that these objects are embedded in thehot bubble formed by the h1 Ori C stellar wind after itpassed through a strong reverse shock. Thermal conductioncan alter the structure of the photoablation Ñow at dis-tances larger than about 200 AU from an embedded YSOby heating the outer parts of the expanding ionizedenvelope. The radial temperature gradient is expected toaccelerate the Ñow, producing a steeper than r~2 densityproÐle that may result in large deviations from r~3 intensityproÐles.

4. Most externally illuminated YSOs have tails rangingin length from to over 4A that point radially away from0A.3the source of illumination. There is no systematic depen-dence of tail length on projected distance from h1 Ori C.However, as the forward-facing IFs increase in radius, theapproximately constant tail lengths lead to progressivelystubbier looking objects. Tails trailing behind YSOs near h1Ori C are long and skinny, while those behind sourceslocated at large projected distances are short and stubby.Many tails are dusty and opaque and seen in silhouetteagainst the nebular emission. Tail formation is not under-stood.

5. Many externally illuminated YSOs contain darkregions at their centers seen in silhouette in [O III], [N II],[S II], Ha, and the continuum. These silhouetted regions areconcentric and symmetric about the central stars when thelatter are visible in our images. The silhouetted regions maytrace high-opacity circumstellar disks. The radius of the


silhouetted region is typically at least a factor of 2 smallerthan the radius of the IF. In several of the larger objects,where the light of the central star does not interfere, thesilhouetted regions are seen in emission in the [O I] line.This component of the [O I] emission is distinct from thatarising from the IFs, whose intensities agree with the predic-tions of models of the photodissociation region. Theabsence of [S II] emission is used to derive a density ofabout 1010 cm~3 in the [O I] emission region at the disksurface. The excitation of the 6300 [O I] line requires 2 eVÓcollisions, or temperatures of at least 4000 K. Soft UV radi-ation that can penetrate to the disk surface is not likely tobe capable of producing temperatures sufficiently high toexcite the [O I] emission. We argue that a nonthermalprocess, such as collisional excitation by a high-velocity tailof particles that are not in thermodynamic equilibrium, maybe responsible for the [O I] excitation. Examples of possibleexcitation mechanisms include 2È6 eV electrons ejectedfrom grains by the photoelectric e†ect, or suprathermal H2molecules ejected from grain surfaces where reformationhas taken place.

In a companion paper et al. we show(Johnstone 1998)that photoheating of the embedded disk surface by the inci-dent soft UV radiation Ðeld can drive a low-velocity andmostly neutral disk wind at a velocity of about 3 km s~1

with a mass-loss rate that is approximately independent ofthe distance from h1 Ori C. This wind is expected to drivethe IF to several disk radii and has a mass-loss rate thatclosely matches that needed to explain the properties of thephotoionized portion of the externally illuminated YSOenvironment.

We thank the sta† at STScI for their excellent support.We thank C. Robert OÏDell for many thought-provokingdiscussions and the use of his ERO data. We also thankHans Zinnecker, Dave Hollenbach, Mark McCaughrean,and John Dyson for useful discussions. We thank DavidTheil for extensive help with data reduction, Ðgure pro-duction, and many interesting discussions. This work wasconceived over a decade ago at a Santa Cruz star formationworkshop during a lunchtime conversation between HansZinnecker and J. B. This work was partially supported byNASA grant GO-5469 from the Space Telescope ScienceInstitute, which is operated by the Association of Uni-versities for Research in Astronomy, Inc. under NASA con-tract NAS5-26555. J. B. was also partially supported byNASA grant NAGW-3192. R. S. S. would like to acknowl-edge the support of the NASA Astrophysical TheoryProgram at the University of Colorado (grants NAGW-766and NAGW-1479).

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T A J ,116:293 1998.TheAmericanAstronomicalSociety ...migall.fastmail.fm/astronomy/stars_and_nebulae/orion/objects/orion_s_m...THE ASTRONOMICAL JOURNAL,116:293È321,1998July (1998.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A