Microstructures in planetary_nebulae

THE ASTROPHYSICAL JOURNAL, 487 :304È313, 1997 September 201997. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

FLIERs AND OTHER MICROSTRUCTURES IN PLANETARY NEBULAE. III.

ARSEN R. BRUCE YERVANT AND MARIOHAJIAN,1,2 BALICK,3 TERZIAN,1 PERINOTTO4Received 1997 February 10 ; accepted 1997 April 29

ABSTRACTLong-slit spectroscopic observations along the major axes of four planetary nebulae with interesting

jets and FLIERs (Hb 4, IC 4634, NGC 6369, and NGC 7354) have been conducted with the Palomar5 m telescope. Chemical abundances and physical conditions (n, T ) in microstructures were derivedalong their structural axes. No evidence of conspicuous shock activity or N/O abundance anomalies isseen in most cases, unlike some earlier studies of similar features in other planetary nebulae. Microstruc-tures seem to be a heterogeneous class of structures aside from their low ionization and generally super-sonic motions.Subject headings : ISM: abundances È ISM: structure È planetary nebulae : general

1. INTRODUCTION

Individual planetary nebulae (PNe) have been commonlycharacterized by a single expansion velocity, one set ofchemical abundances, and a unique dynamical age. Thisdescription is not sufficient, however, for even the simplestPNe. Multiple shells, twisted loops, local condensations ofgas and dust and other distinct features are common andsuggest that complex, time-dependent ejection processesshape the nebulae.

In an attempt to understand the nature of distinct com-ponents in the shells of PNe, we have undertaken a long-slitspectroscopic study of PNe, for which this paper is the thirdin a series et al. hereafter et al.(Balick 1993, Paper I ; Balick

hereafter The purpose of this project is to1994, Paper II).investigate the spectroscopic properties of PNe microstruc-tures, which we loosely deÐne to be structures with typicalphysical dimensions of D1016 cm (corresponding to 1A at1 kpc).

Here, we examine the spectral properties of microstruc-tures along the symmetry axes of four heterogeneous PNe.We include in our search some PNe with possible fast, low-ionization emission regions (FLIERs). FLIERs are small(D1A) regions found in pairs located equidistant from, andon either side of, the central nucleus of the PN (Papers andI

The most deÐning qualities ascribed to FLIERs are theirII).highly supersonic velocities and their highly characteristiclow-ionization emission line spectrum. FLIERs are allegedto be the result of discrete and collimated ejection eventswith short dynamical timescales. In we arguedPaper II,that some FLIERs have enhanced N/O ratios compared tothe nebular gas in which they are embedded. However, noextant explanation accurately reproduces all of the bizarreproperties of FLIERs simultaneously.

Our goal is to see if FLIERs and other types of micro-structures have a common origin. To this end we haveobtained optical long-slit spectra at low spectral resolutionof the targets IC 4634 (a PN with two pairs of FLIERs), Hb4 and NGC 7354 (both of which exhibit a pair of faint,

1 Department of Astronomy and NAIC, Cornell University, Ithaca,NY 14853.

2 United States Naval Observatory, 3450 Massachusetts Avenue NW,Washington, DC 20392-5420.

3 Astronomy Department, University of Washington, Box 351580,Seattle, WA 98195.

4 Dipartimento di Astronomia e Scienza dello Spazio, Universita diFirenze, Largo Enrico Fermi 5, 50125, Firenze, Italy.

narrow jets protruding from their bright cores), and NGC6369 (which is noted for its peculiar ““ Ðsh-tail ÏÏÈlike outerloops). Although no kinematic data are available for any ofthe jets in Hb 4 and NGC 7354, we felt that a detailedspectroscopic study of the jets might uncover links toFLIERs, some of which appear jetlike in HST WFPC2images & Borkowsky We shall deter-(Harrington 1994).mine physical and chemical conditions in the emitting gasalong the slit extract limited kinematic information(° 3.1),for some detected structures and comment on the(° 3.3),physical implications suggested by the results (°° and4 5).

2. OBSERVATIONS

Observations were made using the Palomar 200 inchreÑector equipped with the Double Spectrograph on thenight of 1993 July 17 with a 2A slit during seeing.1A.2 Table 1lists the PNe we observed, along with relevant observingparameters. The plate scale was pixel~1 using the 3160A.79lines mm~1 grating for the red chip in the spectrograph,covering wavelengths from 5200 to 7500 with a resolutionA�of 6.1 (2 pixels per resolution element). The plate scale forA�the blue chip was originally pixel~1 ; however, we0A.59rebinned the data to conform to the spatial resolution of thered spectrograph. We used the 300 lines mm~1 grating,permitting wavelength coverage from 3500 to 5150 withA�4.4 resolution. A variety of short and long integrationsA�were made to achieve a good signal-to-noise ratio whileavoiding saturation of the pixels by the bright Balmer,[N II] j6584 and [O III] j5007 lines. The slit orientationsare shown located on an image of each target PN (Schwarzet al. in Figures along with1992a ; Balick 1987) 1aÈ1dspeciÐcally identiÐed features, or microstructures.

He, Ar and Ne lamp standards were used to determinethe wavelength scale. After applying Ñat-Ðeld corrections tothe spectra using di†erential dome Ñats, the spectra wereÑux calibrated using the standard stars BD ]28[4211 andFeige 110 at airmasses of 1.00 and 1.36, respectively.Finally, we removed nonrepeatable events (i.e., cosmic-rayhits) from individual spectra and averaged all exposures oflike integration times for the same source. Standard IRAFroutines were applied to calibrate the data.

Three of the sources were observed at sufficiently lowairmass and at a slit orientation close enough to the paral-lactic angle for us to neglect misregistration due to atmo-spheric dispersion. The fourth target, NGC 6369, wasobserved with the slit almost normal to the parallactic

304

FLIERs AND MICROSTRUCTURES. III. 305

TABLE 1

PALOMAR OBSERVATIONS

Number qint/Exp.Source Exp. (s) Airmass

Hb 4 . . . . . . . . . . . . 1 15 1.94Hb 4 . . . . . . . . . . . . 4 120 1.95IC 4634 . . . . . . . . . 1 30 1.75IC 4634 . . . . . . . . . 1 120 1.75IC 4634 . . . . . . . . . 1 600 1.75NGC 6369 . . . . . . 1 90 1.84NGC 6369 . . . . . . 3 240 1.84NGC 6537 . . . . . . 1 15 1.71NGC 6537 . . . . . . 5 120 1.72NGC 7354 . . . . . . 1 120 1.13NGC 7354 . . . . . . 7 240 1.13

angle, and we expect signiÐcant dispersion o†sets forj \ 5000 However, only positions 1 and 2 are stronglyA� .a†ected since the gas in the vicinity of positions 3, 4, and 5appears to be smoothly distributed (i.e., few condensations)and probably has homogeneous spectroscopic character-istics.

The contribution of the sky emission was subtracted fromthe co-added two-dimensional spectra as follows. For theless extended PNe (Hb 4, IC 4634 and NGC 7354), the slitcontained swatches of nebula-free sky on either side of thenebular emission. A model sky frame was created for eachcamera image by Ðtting a second-order polynomial acrossthe nebular and stellar emission. In the case of the veryextended nebula NGC 6369, sky frames from the analysis ofthe less extended PNe were scaled by a multiplicative con-stant to compensate for airmass di†erences and to removeall sky emission lines. This procedure was reasonably suc-cessful, but errors remain due to variations in focus alongthe slit, variations in relative line intensities with respect totime and airmass, and sky transparency variations duringthe integration. We further subtracted the stellar andnebular continua near each line from local Ðts using a linearbaseline in the wavelength dimension. The areas of the chipcorresponding to the emission features were then summedin the spectral dimension resulting in ““ slit proÐles ÏÏ (i.e.,plots of line intensity as a function of slit position).

Throughout the rest of this paper, we adopt the followingabbreviations used in Papers and N\ north,I II :S \ south, E \ east, W \ west, [N II]\ [N II] j6584,[O I]\ [O I] j6300, [O II]\ [O II] j3727 \ [O II]jj(3726]3729), [O III]\ [O III] j4959, [S II]\ [S II]jj(6717]6731), [Cl III]\ jj(5518]5538). We generallyignored the [O III] j5007 line (see below).

3. ANALYSIS

3.1. Reddening CorrectionsUnfortunately, there exist no spatially resolved extinction

studies published for the program PNe. Since the PNe inthe present study and all of those discussed in Papers andI

are not found to have signiÐcant variations in theIIobserved Ha/Hb ratios across the extent of the nebulae, theinternal contribution to the reddening is neglected and oneglobal value of (the logarithmic extinction at j4861) wascbadopted for each PN from the literature. We use thereddening curve of and values found in theSeaton (1979)compilation by et al. hereafter to dered-Cahn (1992, CKS)den the Ñuxes at the selected slit positions with cb\ 1.75,

0.50, 2.0, and 1.78 for Hb 4, IC 4634, NGC 6369, and NGC7354, respectively.

3.2. Errors in L ine FluxesAfter correcting for reddening, careful examination of the

lines from the same ionic species (e.g., H`) show no anom-alies except the [O III] j5007 line. Since this line and the[O III] j4959 line arise from transitions with the same upperlevel, the ratio of their intensities is Ðxed by atomic con-stants (i.e., their relative transition probabilities) and isequal to 2.99 However, almost every(Osterbrock 1989).ratio measured in the present data exceeds 3.4. We concludethat the [O III] j5007 line Ñux is corrupted : this is mostlikely the result of the reduced sensitivity near the dichroiccuto† at 5300 Throughout the remainder of this paper,A� .we neglect the j5007 line and use instead the scaled j4959line for all computations.

The most reliable probe of Ñux calibration errors betweenthe red and blue spectra is the Ha/Hb ratio. For reasonablephysical conditions, we would not expect the dereddenedratio to be very di†erent from 2.86 ^ 0.10 (Osterbrock

After examining unsaturated Ha/Hb ratios, we con-1989).cluded that the absolute Ñux calibration is not accurateenough to justify comparisons between lines observed withboth spectrographs. As a result, we only consider Ñux ratiosof lines observed in the same spectrum, and normalize alllines measured with the red spectrograph to Ha \ 280 andall lines measured with the blue spectrograph to Hb \ 100.An exception is NGC 6369 in which the Hb line is verynoisy. In this case, we assume that Hb \ Ha/2.8.

In general, it appears that the ratios of lines from thesame spectrograph were uncertain by approximately 10%,although there are cases of ratios involving weak lineswhere the ratio errors are as high as 40%È50%.

In some individual cases, artifacts are left in the sky-subtracted data due to subtraction stellar continua when (a)the continuum was saturated (e.g., IC 4634) and/or (b) whenstellar emission and absorption features near the nebularline contaminated the interpolation. This a†ects the resultsonly in regions contaminated by bright starlight. The fol-lowing pixels are a†ected : 99È100 and 135È141 for Hb 4,95È97 for IC 4634, and 95È97 and 117È120 for NGC 7354.The associated emission line data are displayed in theÐgures but ignored in all analyses.

3.3. Emission L ine Ratios, Nebular Diagnostics,and Abundances

In this section, we summarize the methods by which tem-peratures, densities, and abundances are measured. Themost reliable quantities are derived from the ratios of lineswith similar wavelengths, since these are likely to su†er leastfrom relative or absolute errors in the reddening correctionand/or Ñux calibration. In any case, all of these Ñuxes rep-resent integrals along the line of sight and may include gasexperiencing a variety of physical conditions, chemicalabundances, and ionization states. The systematic errorsthat result are very difficult to assess.

In the standard treatment used below to computenebular diagnostics and chemical abundances, we assumethat stellar UV photons dominate the ionization andheating of the gas. This situation is not valid in cases inwhich turbulent or coronal ionization and heating areimportant, or where intense magnetic Ðelds, and/or strongshocks crossing the slit. Gas experiencing these conditions

306 HAJIAN ET AL.

FIG. 1.ÈImages of the program nebulae (Hb 4, IC 4634, NGC 6369, and NGC 7354) in Ha ] [N II] or [N II] from the literature. The location of the 2Awide slit is indicated with a white line. Italicized numbers identify the individual features that are deÐned in the text. The two nebulae (IC 4634 and NGC6369) are those in which FLIERs and their low-ionization analogs are suspected, and the other two (Hb 4 and NGC 7354) contain morphological featuresthat we call jets.

also cannot be analyzed using the purely radiative treat-ment below since the temperature and density distributioncan dramatically change over very small (and unresolved)size scales, so that it is not possible to ascribe a single set ofphysical conditions to observed structures. We shall discussany possible e†ects where circumstances warrant.

The line ratios are converted into physical conditionsusing an iterative application of the IRAF v2.10.2 STSDAS/NEBULAR routines. The NEBULAR routines employÐve-level model atoms Dufour, & Hunt(DeRobertis, 1987 ;

& Dufour and, given appropriate emission lineShaw 1994)

ratios, will compute densities and electron temperatures inthe radiating gas. Atomic constants for all ions are usedfrom except for [S II] & PradhanMendoza (1983) (Cai

[Cl III] & Zeippen [Ne III] &1993), (Butler 1989), (ButlerMendoza and permitted He lines1984), (Osterbrock 1989).

Ratios of [S II] j6717/j6731 and [Cl III] j5517/j5537 areused to determine densities in regions of low (typicallyneutral or singly ionized gas) and high (typically regions ofdoubly ionized gas) ionizations, respectively. Temperaturesin each ionization zone are calculated from the [N II]j6584/j5755, and [O III] j4959/j4363 Ñux ratios. Density

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308 HAJIAN ET AL. Vol. 487

measures do not always agree (which is common in suchanalyses), so quality-weighted averages are used in sub-sequent computations. In any case, the densities are toosmall to a†ect the derived abundances signiÐcantly.

Abundances are computed for ions with multiple deter-minations from lines with comparable counts (i.e., [He II]jj5876, 6678), while a single bright line is used for specieswith a dominating emission line (e.g., [N II] j6584 instead ofj5755). In order to correct for the e†ects of missing ioniza-tion states, we include ionization correction factors, foricf,all atomic species to calculate total atomic abundances. Theequations we employ are listed in & BarlowKingsburg

Estimates using techniques in Freitas Pacheco et(1994). deal. were also computed but are not shown due to the(1993)large scatter in the resulting For all elements aside fromicfÏs.O, the uncertainty in the total abundance scales with icf.Therefore, we do not assign a high conÐdence to totalatomic abundances based on large ionization corrections(e.g., the nitrogen abundances for NGC 7354). As was thecase in the helium abundance in this study derivedPaper II,from the j7065 line is consistently a factor of 2È3 higherthan all other determinations and is ignored in the abun-dance computations.

We show selected line ratios, nebular diagnostics, andatomic abundances for the observed PNe in Figures 2aÈ2d.In general, we Ðnd no evidence of signiÐcant spatial tem-perature Ñuctuations and only modest indications of localdensity Ñuctuations. The presence of these Ñuctuationswould require a substantial correction to the derived physi-cal conditions and abundances, as discussed by Peimbert

and Luridiana, & Torres-Peimbert(1967) Peimbert, (1995)(see their discussion of the t2 parameter). As noted in Papersand FLIERs are characterized by velocities of kmI II, Z30

s~1 and much larger in some cases. Since our spectralresolution is so low (about 290 km s~1) we can probe onlythe coarsest kinematic structure of the target nebulae.Contour plots of the two-dimensional spectra will be shownfor the Ha and [N II] lines. All velocities are in km s~1 andare measured with respect to the nebular systemic velocity.These Ðgures are meant only to show relative emission dis-

tributions : Ñuxes are in arbitrary units and unevenly spacedcontour levels have been chosen to highlight emission fea-tures. The velocity structure revealed in these images isuseful for determining the kinematic anomalies betweenindividual features relative to nearby nebular gas.

4. RESULTS AND DISCUSSION

We discuss the Ðndings for each PN separately, followedby a collation of current results for FLIERs. A summary ofdiagnostic and abundance information is displayed in Table

for all positions sampled.2

4.1. Hb 4Hb 4 appears as a small (B4A), elliptical nebular core

(positions 2 and 3) with a pair of detached jets or string ofknots (positions 1 and 4) along the minor axis on either sideof the nebula Distances to this PN have been(Fig. 1a).determined to be 2080 pc and 1300 pc(CKS) (Daub 1982)with the modiÐed and standard Shklovsky methods, respec-tively (these distances are formally uncertain by factors inexcess of 2). The central source has the spectral type of ahydrogen-poor WC 3È4 star et al. and the(Schwarz 1992b),mass based on theoretical evolutionary tracks is 0.66 M

_& Tylenda(Stasinska 1990).Kinematics : The bulk expansion velocity based on high

spectral resolution [O III] observations by Reay,Robinson,& Atherton is 23.0 km s~1. We Ðnd more extreme(1982)motions, as described below. Our slit runs along the linethat joins the jets. For both of the spectral lines probed in

one of the two jets (position 1) in Hb 4 appears asFigure 3,a set of discrete knots detached from the nebula. This isdirectly visible in our slit brightness proÐles as well. Clearly,the jets are not simply a kinematic extension of the nebula :in each case they have a distinct velocity that is in excess ofthe bulk of the nebular gas. The jets in Hb 4 have velocitiesof D^100 km s~1 with respect to the core, as seen in[N II], [O I], and Ha. The [O III] emission from the jetsshow only a small (^50 km s~1) velocity di†erence relativeto the core gas. In any case, the low ionization of the jets orknots, coupled with their small sizes, peculiar velocities, and

TABLE 2

NEBULAR DIAGNOSTICS AND ATOMIC ABUNDANCES

n(S`) n(Cl``) T (N`) T (O``)j6717

j6731

j5517

j5537

j6584

j5755

4.0 \ j4959

j4363N/H O/H Ne/H Ar/H S/H Cl``/H

PN,Feature He/H (]105) (]104) (]104) (]106) (]106) (]106)

Hb 4 1 . . . . . . . . . . . . 720 . . . 9800 . . . 0.09 20 5.4 . . . 3.2 1.3 . . .Hb 4 2 . . . . . . . . . . . . 3730 8980 10600 8500 0.09 17 4.8 0.70 2.2 0.58 0.06Hb 4 3 . . . . . . . . . . . . 3800 6350 10700 8600 0.09 18 4.9 0.75 2.7 0.68 0.06Hb 4 4 . . . . . . . . . . . . 850 2250 10000 . . . 0.10 26 5.9 . . . 3.0 1.2 0.18IC 4634 1 . . . . . . . . . 2710 2230 11000 10300 0.09 6.8 3.8 0.55 1.5 0.64 0.04IC 4634 2 . . . . . . . . . 6480 3930 12100 9400 0.10 4.8 2.6 0.35 1.1 0.51 0.06IC 4634 3 . . . . . . . . . 13230 4590 11000 9000 0.09 4.8 3.5 0.43 0.66 0.18 0.05IC 4634 4 . . . . . . . . . 8080 3020 10100 9600 0.08 9.4 5.0 0.78 1.8 0.68 0.05IC 4634 5 . . . . . . . . . 2600 . . . 10500 10600 0.09 5.6 4.3 0.85 2.5 0.87 0.01NGC 6369 1 . . . . . . 580 . . . . . . . . . 0.08 . . . . . . . . . 1.3 . . . 2.7NGC 6369 2 . . . . . . 400 1340 11300 11400 0.08 13 5.6 0.92 1.9 0.38 0.15NGC 6369 3 . . . . . . 2010 3310 10600 9500 0.09 12 5.4 0.90 1.8 0.29 0.07NGC 6369 4 . . . . . . 2030 750 11100 9100 0.09 11 4.6 0.77 1.5 0.26 0.06NGC 6369 5 . . . . . . . . . . . . . . . . . . 0.07 15 10 1.2 2.4 0.34 0.30NGC 7354 1 . . . . . . . . . . . . . . . 9200 0.09 16 11 0.90 1.8 32 . . .NGC 7354 2 . . . . . . 1500 . . . 9800 9900 0.11 13 11 1.7 2.1 12 . . .NGC 7354 3 . . . . . . 2740 . . . 11900 10200 0.10 11 5.6 0.70 1.2 5.6 . . .NGC 7354 4 . . . . . . 2660 . . . 10600 9800 0.11 23 7.7 1.1 1.8 9.8 . . .NGC 7354 5 . . . . . . 2020 . . . 11600 10000 0.09 14 6.1 0.80 1.3 5.2 . . .NGC 7354 6 . . . . . . 970 . . . . . . 11300 0.07 100 8.1 0.94 1.8 25 . . .

No. 1, 1997 FLIERs AND MICROSTRUCTURES. III. 309

FIG. 3.ÈGross kinematic structure of Hb 4 in Ha (upper plot) and [N II] (lower plot). Note that the velocities are in km s~1 with respect to the nebularsystemic velocity (horizontal dashed line). The contour levels are in arbitrary increments and are intended only to highlight the nebular structure. The FWHMof an unresolved night sky line is 285 km s~1.

symmetric placement on opposite sides of the central starsqualify these features as FLIERs.

Physical parameters : Densities derived from [S II] for thecore positions are in agreement with the average densityderived by et al. and et al.Acker (1989) Samland (1992),which are and 4540 cm~3, respectively. However,n

e\ 4520

due to the high excitation class of the object, we have per-formed a quality-weighted average to include the detected[Cl III] density in our adopted values in and inTable 2

for positions 2 and 3. This raises the adoptedFigure 2adensity to B5600 cm~3. As can be seen in the [S II] ratio in

the nebular density drops o† smoothly from theFigure 2a,central core, then becomes discernible again on the jet posi-tions, albeit in the low-density [S II] limit (i.e., [1000cm~3). Between the core and the jets no densities can beestimated. Though the agreement between III]) andn

e([Cl

II]) in the core is reasonable, little [Cl III] is apparentne([S

in the jets.Our derived [N II] temperatures II])D 10,100 K)(T

e([N

show no credible variations with position and agree withthe determination by et al. but are slightlySamland (1992)(10%) higher than the temperature derived by et al.Acker

The [O III] temperature is likewise constant across(1989).the slit but is somewhat lower than the [N II] temperature

III]) D 9700 K).(Te([OAbundances : Our detected nebular abundances (positions

2 and 3) are in reasonable agreement with He, O, S, and Nabundances (slit averaged across the PN) published by

et al. but fall short by a factor of for Ar.Samland (1992) Z2However, Ar abundances measured from the method areicfextremely uncertain. Relative to the nebular core, the jetsdemonstrate D30% depletion of O and S, while showing asimilar enrichment of N. Line emission from the jets is alsobrighter than from the core in the low-ionization speciesN`, O`, and S`. Finally, the ratio of N/O (assumed to beN`/O`) in the jets is typically larger than the core by

D25%È100%. High ionization lines ([O III], [Ne III],[Ar III], and [Cl III]) seem to be approximately constantacross the core but are weak or completely absent in thejets.

No evidence of reddening variations are seen in theHa/Hb ratio. The ionization structure of Hb 4 is containedin the He I j4471/Hb and the He II j4686/Hb proÐles. NosigniÐcant variations are seen in He I j4471/Hb, but there isclear evidence for an ionization bounded He`` region inthe core of the nebula. The overall helium abundance showschanges between the FLIERs and the core, where thebrightness changes rapidly. However, these are most likelyto be artifacts of a small spatial misalignment of the He I

j5876 and Ha slit proÐles.

4.2. IC 4634A point-symmetric nebula consisting of a central core

with two detached blobs IC 4634 has Shklovsky(Fig. 1b),distances of 2770 pc and 1750 pc The(CKS) (Daub 1982).low-mass and metal-deÐcient central star is located approx-imately 500 pc above the galactic plane (Maciel 1993 ;

de Freitas Pacheco, & Codina-LandaberryMaciel, 1990).Our abundance analysis of IC 4634 is hampered by the

saturation of the Ha line, even for the shortest exposures,which renders the line Ñux for position 3 suspect. We havecorrected for this by using a scaled version of the Hb line tocompute the Ha Ñux where the latter is saturated.

Kinematics : A nebular expansion velocity of 14.4 km s~1based on high ionization lines ([O III] and [Ne III]) wasmeasured by however, we Ðnd that the inter-Wilson (1950) ;nal motions seen in other lines are much larger. A casualglance at the kinematic structure of the PN indicates(Fig. 4)that the [O III], [N II], and Ha intensity distributions andkinematics are generally similar. The data suggest that fea-tures 1 and 5 form one pair of FLIERs at ^100 km s~1,while features 2 and 4 form another pair at roughly reversed


FIG. 4.ÈSame as except for the [O III] (upper plot) and [N II] (lower plot) contours from IC 4634. Note that the [O III] gas in position 1 is redshiftedFig. 3,relative to position 2. However, the corresponding [N II] velocity shifts have the opposite sign.

velocities (with respect to features 1 and 5). Both the innerand outer pairs seem to be interconnected by a ““ bridge ÏÏ ofHa and [N II] emission. For FLIERs 1 and 5, the [O III]peaks are located closer to the central star than the centersof the [O I] and [N II] peaks, which is characteristic ofFLIERs (Papers andI II).

On closer inspection, a very peculiar trend is seen in thekinematics of the outer FLIERs. In proceeding eastwardfrom the central star, one encounters feature 4 at a velocityof D]100 km s~1 as seen in all lines, then the [N II] linedisappears and the [O III] line is seen faintly at D[100 kms~1. Then, at the inner edge of the FLIERs in position 5, the[O III] line peaks and the sense of the velocity reverses,becoming D]100 km s~1 again. On the far edge of theFLIER, [O III] becomes faint and [N II] peaks, but at avelocity of D[100 km s~1 ! On the western side of thenebula the situation is the same, but with the velocitieshaving the opposite signs. This odd behavior is not an arti-fact of errors in the data.

Physical parameters : Both [S II] and [Cl III] line ratiosyield cm~3 for position 1. However, even thoughn

eB 3000

both pairs of lines are bright, the densities that each suggestare not in agreement across the remainder of the slit posi-tions. Despite some scatter, II])B 104 cm~3 through-n

e([S

out, while III]) averages only B40% of that value.ne([Cl

Our [S II] density results in agree with thoseFigure 2bby Freitas Pacheco, Maciel, & Costa who listde (1992),

II]) \ 104 cm~3. Temperatures deduced from [O III]ne([S

line ratios are mostly constant, with at most 10% excur-sions from 10,000 K. Freitas Pacheco et al. derivede (1992)

III])\ 9450 K, which agrees well with our slit-averageTe([O

of III]) \ 9300 K in Line ratios of [N II]Te([O Figure 2b.

yield generally larger temperatures across the PN, averag-ing B12,000 K with position 3 showing possible real excur-sions to 13,000 K.

Abundances : The core is bright in high ionization linessuch as He`, [O III], and [Cl III], with weaker emissionfrom [N II], [S II], and [O I]. Moving outward from thecore to the inner pair of knots in positions 2 and 4,[O III]/Hb remains constant, while He`/H` curiously

increases. This result is conÐrmed by the reddening insensi-tive ratios He` j4471/Hb and He` j6678/Ha. Further-more, [N II]/Ha, [S II]/Ha, and [O I]/Ha also increase fromthe core to the inner knots. Spectroscopic data obtainedthrough a large aperture exist for IC 4634, but spectra atvarious positions are not available. & CzyzakAller (1983)and Freitas Pacheco et al. compute similar slit-de (1992)averaged abundances that are in general agreement withour results. Positions 2 and 4 appear virtually the sameexcept for a B80% increase in N/H in position 4, whichappears to be real.

The outer pair of knots are quite similar to the inner pairof knots except that the outer knots are D2È4 times lessdense, and the signs of their velocities (with respect to thecentral star) are opposite. The outer knots are kinematicallyolder than the inner knots since both have comparablevelocity magnitudes (with respect to the central star). Thesedata argue in favor of the inner knots being a less matureversion of the outer knots. We shall elaborate in ° 6.

4.3. NGC 6369The distance estimates to this B15A nebula span a large

range. Statistical distances give small values, including theestimates of 660 pc and 420 pc Other(CKS) (Daub 1982).methods give larger distances. For example, an H I absorp-tion distance of 2.0 ^ 0.7 kpc is computed by et al.Gathier

Hydrogen Zanstra temperatures of the central star(1986).yield T \ 67,600 K & Pottasch and the(Gathier 1989)remnant mass is M \ 0.65 & TylendaM

_(Stasinska 1990).

The nebula is expanding at 41.5 km s~1 (Meatheringham,Wood, & Faulkner The PN (see consists of a1988). Fig. 1c)bright ring (positions 3 and 4), which is the dominantnebular feature, and fainter, looped and arclike conden-sations (positions 1, 2, and 5) at larger radii from the centralstar.

Kinematics : As sampled through our slit, the bright ringof emission in NGC 6369 exhibits broad lines (D1 pixelbroader than an unresolved line) but no resolved variationsin velocity. An exception may be the inner portion of feature3, across which a velocity gradient of D1 pixel (D150 km


s~1) is evident. Outside the bright inner gas is emission fromfainter material, all of which is systematically blueshifted byD1 pixel with respect to the bright ring. The [N II] emissionfrom feature 4 is comprised of two distinct blobs that Ñankthe single clump seen in other lines. The velocity gradient infeature 4 is about 50 km s~1 per arcsecond, suggestinghighly bipolar supersonic motions or gradients across thefeature.

Physical parameters : Strong lines such as [S II], [N II],[O III], and Hc suggest constant density, temperature, andreddening across the nebula. We Ðnd that varies betweenn

e1200 and 3500 cm~3 along the slit as shown in Figure 2c,except in positions 1, 2, and 5, where the [S II] line ratios arenear the low-density limit and the [Cl III] lines are too noisyto yield a meaningful density estimate. Based on the radioand Hb Ñux of NGC 6369, Ðnds thatGathier (1987) n

e\

3200 cm~3. This technique favors the brightest (i.e., densest)portions of the nebula. Other determinations are somewhathigher. [S II] densities determined in the spectroscopicstudies of et al. and et al. giveAcker (1989) Samland (1992)densities nearer to 4500 cm~3 based on observed andmodeled spectra, respectively. Our temperature measure-ments yield II]) \ 11,800 K and III])\ 9900 KT

e([N T

e([O

at all slit positions in conÐrming the determi-Figure 2c,nations by et al.Acker (1989).

Abundances : The abundances in NGC 6369 found byet al. (N/H\ 1.55] 10~4, O/H \ 4.68Samland (1992)

] 10~4, S/H \ 1.32] 10 [ 5, and Ar/H \ 2.46] 10~6)are broadly consistent with many of our results, exceptpossibly in the case of S/H for which we derive D2 ^ 1] 10~6. As noted above, positions 1, 2, and 5 are kinemati-cally distinct from the rest of the nebula. Chemically, thesepositions are more similar, exhibiting possibly signiÐcantdepletion in N/O by 25%È50% and otherwise only smalldeviations from the average chemistry of the nebular ring.Unfortunately, the chemical analysis of position 1 ishampered by faint line Ñuxes, resulting in especially uncer-tain Ne``, O`, and for this position.icf-values

4.4. NGC 7354Distances to this B25A sized PN range from the

reddening distance of 3430 pc to 1270 pc(Higgs 1971)to 650 pc The e†ective temperature of(CKS) (Daub 1982).

the central star is 64,600 K & Pottasch(Gathier 1989).Kinematics : NGC 7354 is one of the few PNe in the

literature with a spatiokinematic model Bian-(Sabbadin,chini, & Hamzaoglu The [O III] and [N II] expansion1983).velocities (corrected for inclination e†ects) are close (24.5and 27.0 km s~1, respectively). Ha and [N II] echelleobservations of Preston, & Icke show that theBalick, (1987)motions in the low-ionization lines are complex, showinglarge velocity gradients in the various [N II] knots insidethe body of the object. The present observations do nothave adequate spectral resolution to investigate internalmotions.

Physical parameters : We Ðnd densities of 1500È2600cm~3 in the inner regions of the nebula (positions 2È5) in

Electron densities based on the 5 GHz radio ÑuxFigure 2d.are about a factor of 2 higher than our slit-(Gathier 1987)

averaged density of 2500 cm~3. In the jets (positions 1 and6), the predicted density is low cm~3) and insensi-([1000tive to the [S II] ratio. Temperatures are somewhat scat-tered in due to noise in [N II] j5755 and [O III]Figure 2dj4363 and are clustered near III]) \T

e([N II])\ T

e([O

10,300 K throughout the core. Line emission from [O III]j4363 is absent and III]) is not measurable in the jets.T

e([O

We Ðnd no evidence for meaningful [N II] temperature Ñuc-tuations.

Abundances : Our ratios for the abundance of He``/Hare consistent with the results of and &OÏDell (1963) KalerLutz Although there are no signiÐcant changes in(1985).the total helium abundance in the nebula, an ionizationbounded He`` region is visible as a pair of peaks inHe II j4686/Hb bounded by small He I j4471/Hb enhance-ments at larger radii. This region is only marginally visiblein the [N II] image in Figure 1d.

Most of the gas in the center of the nebula seems to behighly ionized. There is very little O` or S` in the nebula.Consequently, are huge for N` and are thereforeicfÏssuspect. The N/H and S/H ratios in the jets are also anom-alous owing to very uncertain ionization corrections. Sincelittle Ne``, He`, He``, Ar`` or O`, and abundant N` isseen from the jets, we conclude that the jets are tenuousregions of low ionization. The chemical structure in the jetsof NGC 7354 may be similar to those in Hb 4.

5. SHOCK ACTIVITY

Computer simulations of shock emitted spectra havebeen derived in the literature Raymond, & Hart-(Hartigan,mann hereafter in order to1987, HRH; Hartigan 1989)compare with observations of HH objects. Since shockedgas can display line ratios that depart signiÐcantly fromthose expected from a thermally equilibrated gas, a purelyradiative treatment can yield anomalous abundances. Theresults of these studies can also be compared to our emis-sion line ratios to qualitatively assess the likelihood ofshock activity.

Since it was found in Papers and and in this paperI IIthat FLIERs possess small angular sizes and are movingaway from the PN nucleus more rapidly than the surround-ing nebular gas, it is conceivable that shock mechanismsplay an important role in the emitted line spectrum. Fur-thermore, FLIERs could exhibit strong hydrodynamicinteractions with their local surroundings since FLIERspossess ram pressures much larger than the thermal pres-sure conÐning them. In the authors compared thePaper II,results of model spectra from shocked gas in the literatureto detected FLIERs spectra and concluded that shocks playlittle role in the line ratios. However, the basis for this con-clusion was that the observed [N II]/Ha º 2 could not beduplicated by any shock model. In our cases, the largestvalues of [N II]/Ha are found in Hb 4-position 1 (0.91), Hb4-position 4 (0.75), NGC 6369-position 1 (1.5), and NGC6369-position 2 (0.63). The remaining [N II]/Ha ratios aresmaller than 0.36. In light of these new values, the shockscenario deserves a second look.

We compared our observations of lines that are stronglya†ected by shocks ([N II] j6584, [S II] j6717, [S II] j6731,and [O I] j6300) with the models of which cover aHRH,wide range of shock parameters do not consider(HRH[S III] j6312, otherwise it would have been included in ouranalysis). Because shock excited line ratios are a strongfunction of shock velocity, and because we have access tokinematic data from the FLIERs, we consider only modelswith km s~1. While most of the individual linevshock\ 100ratios are reproduced by one or more of the shock models,we Ðnd that no planar shock or bow shock model accu-rately duplicates the detected spectrum for most of the PN


positions considered. We examine this in more detail below.Here, we only consider models of preionized gas entering

a shock (I models). This type of model is realistic in our casesince it is likely that much of the FLIERs gas has alreadybeen ionized by the intense radiation Ðeld from the hotcentral nucleus. A few positions in Hb 4 and in NGC 6369possess extremely high values of [N II]/Ha, spanning 0.63È1.5, and lie outside the domain of virtually all shock models.The I models predict [N and hence cannotII]/Ha [ 0.66explain the line ratios from Hb 4-position 1 and 4, or NGC6369-position 2. In addition, the shock models predict[O I]/Ha and [S II]/Ha line ratios that are factors of 2È4and 2È10 larger than observed for these positions, respec-tively. For NGC 6369-position 1, despite its demonstratingthe huge observed ratio [N II]/Ha \ 1.5, the [O I] and [S II]Ñuxes can be well represented by the I models.

The sole PN position surveyed that is consistent withshock model predictions is IC 4634-position 5 ([N II]/Ha \ 0.34), which generally reproduces the predictions of

I models well. Although the [N II]/Ha ratios for thisHRHÏsposition present an easy match to a variety of models, theobserved [O I]/Ha ratios are factors of 5È10 too small to Ðtany model, and the [S II]/Ha ratios are often factors of 2È10too small as well.

6. SUMMARY AND CONCLUSIONS

Several of the positions that we probed in the shells of asample of PNe show deÐnite spectroscopic evidence forlocalized, low-ionization emission. The jetlike extensions(positions 1 and 4) of Hb 4 and (positions 1 and 6) of NGC7354, and positions 1, 2, and 5 in NGC 6369 are examples ofthis phenomenon, and they may be similar to FLIERs inthis regard. Positions 1 and 5 and positions 2 and 4 in IC4634 reproduce the morphological and spectroscopic qual-ities of several PNe probed in Papers and The knots atI II.these positions are FLIERs. The line ratios from IC 4634position 5 can be explained by either photoionization orshock excitation models. Otherwise, all measured locationsof intense low, ionization emission possess [N II]/Ha ratiosthat are too large and/or [O I]/Ha and [S II]/Ha ratios thatare too small to agree with any shock models, suggestingthat photoionization dominates in these regions. In general,we do not conÐrm the nitrogen enrichment claimed in pre-vious spectroscopic studies of FLIERs (Paper II).

The jets in Hb 4 and NGC 7354 share several commoncharacteristics with jets found in other PNe. These charac-teristics include large velocities (relative to the central star),a large ratio of [N II]/Ha, low-ionization emission, and theexistence of pairs of jets with one on each side of the centralstar & Goodrich et al.(Trammel 1996 ; Bobrowsky 1995 ;

Vazquez, & Rodriguez & SolfLopez, 1995 ; Miranda 1990).Several of the qualities are exhibited by FLIERs as well,suggesting that similar physical mechanisms are at work inboth phenomena.

Originally detected as localized enhancements of the[N II] j6584 in narrowband images of planetary nebulae by

FLIERs have proven to be a tantalizingBalick (1987),astrophysical enigma. Their characteristics, which were Ðrstexplored and deÐned in Papers and are difficult toI II,reproduce with any coherent model. For instance, there isno known outÑow mechanism from an evolved remnant (assuggested by the high nitrogen abundance of FLIERs rela-tive to the nebular shell), which can eject collimated pairs of

small (B1A) sized blobs that are symmetric about thenebular major axis and which travel at supersonic speeds.Furthermore, there is no signiÐcant enhancement of gas ordust density in the FLIERs as suggested by constant [S II]j6717 to j6731 and Hc to Hb ratios in Papers and It isI II.also difficult to imagine how the FLIERs can emit a spec-trum of low-ionization emission lines without containing anadditional dust component. The PNe shell gas surroundingthe FLIERs emits lines with a predominantly high ioniza-tion character, and since this gas is bathed in the sameradiation Ðeld as the adjacent FLIER, there is no obviousreason for FLIERs to distinguish themselves from thenebular gas !

A critical clue to the nature of FLIERs is found in theHST observations of NGC 6543 by & Bor-Harringtonkowsky In these images, the FLIERs are clearly(1994).resolved. Numerous Ðlaments and wispy trailing structuresare seen in the images, and a Ðlamentary feature is seen onthe leading (i.e., directed away from the central star) edge ofeach FLIER. Although we often do observe an ionizationgradient across the FLIERs, suggesting that it is possible toapproximately resolve the relative position of the shock andthe downstream recombination zone, the bow shocks arenot directly observable from the ground due to their faint-ness and narrow widths. This is a signiÐcant di†erencebetween FLIERs and Herbig-Haro (HH) objects. Althoughsimilar in that both phenomena consist of discrete blobs ofgas located along a line passing through a central star, HH-objects emit bright lines that are dominated by shock exci-tation (HRH).

The two pairs of FLIERs in IC 4634 represent a rareopportunity to study the evolution of FLIERs. Thoughejection mechanisms for FLIERs are difficult to constrain,point-symmetric PNe with detached blobs have been verysuccessfully modeled by et al. as the result of anCli†e (1995)episodic, precessing jet. Such a jet can give rise to pairs ofknots oriented along di†erent trajectories provided that thejet rotated through a sufficiently large angle between suc-cessive ejection events. Furthermore, if the jet rotatedthrough the normal to the line of sight to the PN, then thesign of the radial velocity would be di†erent from one pairto the other. Thus, projection e†ects can cause the signdi†erences between similar lines in di†erent (i.e., the innervs. the outer) knots.

If we assume that the velocity sign reversals can be areexplained by projection e†ects, and that there are onlyslight spectroscopic deviations between the two pairs, itappears that the only signiÐcant di†erence remainingbetween the inner and outer pair of FLIERs in IC 4634 isthe density contrast of D2È4, which is anticorrelated withtheir apparent dynamical ages (i.e., uncorrected for projec-tion e†ects). Suppose that the FLIERs are tiny, high-density(n [ 104 cm~3), neutral clumps surrounded by extendedionized sheaths. The large Doppler shifts of the emissionlines from the FLIERs as compared to the surrounding gaswould give rise to leading edge bow shocks similar to thoseseen in NGC 6543. Since II]) probes the density of then

e([S

ionized sheath, and since there do not seem to be anychanges in the volume occupied by the inner versus theouter FLIERs, we conclude that the FLIERsÏ ionizedmasses scale with their densities for IC 4634. Neglectingprojection e†ects, the outer FLIERs are D500 yr olderthan the inner FLIERs. Assuming that the two sets ofFLIERs were formed with the same initial mass [M \


which for r \ 2.2] 1016 cm,(4/3)nR3nemH, n

eB 4000

cm~3 is M B 10~4 and that the ionized gas has theM_

]same density throughout the volume of the FLIERs impliesthat the outer FLIER have lost grams of(4/3)nR3*n

emHionized material relative to the inner FLIER (i.e., is the*n

edi†erence between the electron densities of the inner andout FLIERs). The agent causing the removal of ionizedmaterial from the FLIERs is most likely the ionized, fastwind from the central star. This could happen if the fastwind entrains ionized FLIER material via shear-Ñow pro-cesses. Adopting cm~1 yields an average mass*n

e\ 2000

entrainment rate of yr~1. The actualM0 \ 1.1 ] 10~7 M_value of may be larger if, for example, high energyM0

photons continually ionize fresh material.Is the hot wind from the central star energetic enough to

ablate the ionized sheath, creating a wind-blown tail ? Arough analysis indicates that the answer is yes. &PatriarchiPerinotto derive km s~1 using IUE(1991) v

w\ 1600

spectra from IC 4634, and we adopt yr~1M0w

\ 10~8 M_as a value typical of PNe central stars Peri-(Maciel 1993 ;

notto 1989 ; & Perinotto resulting in aCerruti-Sola 1989),total mechanical luminosity in the fast wind of L

w\

Now, since the inner FLIERs in IC 46340.5M0w

vw2 B 2 L

_.

subtend and angle of B20¡ with respect to the central star,each knot intercepts B0.075 from the total wind lumi-L

_nosity. If the fast wind imparts all of its energy to theionized sheath, then 10~7 yr~1 can be accelerated to asM

_high as 100 km s~1 relative to the FLIER and away fromthe central star if the wind sheath coupling is good. We notethat even if we assume that the ionized gas characterized bythe physical conditions in this paper entirely Ðlls theobserved volume of the FLIERs, the expected Ha lumi-nosity from photoionization can account for less than 10%of the observed Ha luminosity, indicating that anothermechanism may dominate Balmer emission. As pointed outby et al. most of the interaction luminosityRaymond (1994),

in an entrained viscous Ñow emerges as La and two-photoncontinuum emission, which are clearly related to the Haluminosity. These arguments suggest that the fast wind isenergetic enough to ablate FLIERs and can generate a tailof ionized gas pointing away from the central star. Thissituation is also reminiscent of the cometary knots in therecent HST image of the Helix Nebula & Handron(OÏDell

and of neutral globules in H II regions1996), (Spitzer 1978).This preliminary treatment also suggests that the di†erentline velocities for the same FLIER seen in areFigure 4indicative of FLIER gas that has (i.e., [O III]) and has notbeen (i.e., [N II]) swept up by the fast wind. The interactionbetween the nuclear wind, nuclear photoionization Ñux andneutral FLIER will be further investigated in a future paper

& Balick(Hajian 1997).The HST observation of NGC 6543 and the computa-

tions above favor a model describing FLIERs as denseblobs of photoevaporating gas. A crucial datum that canverify this scenario is the outÑow velocity of any wind-blown tail material from the FLIERs. HST is currently theonly instrument that can spatially resolve these Ñows, andthe STIS instrument can provide spectra with high spectralresolution to probe the kinematic structure. It may also beuseful to search for CO (based on its large abundance andEinstein A coefficient) and HCN gas with the OVROand/or BIMA arrays. Detections of neutral gas cores (whichis possible with the resolution available at these[1Afrequencies) would conÐrm the nature of FLIERs as par-tially neutral photoevaporating clumps.

A. R. H. and Y. T. were supported in part by NAIC,which is operated by Cornell University under a coopera-tive agreement with the National Science Foundation.Observations at Palomar Observatory were made as part ofa continuing collaborative agreement between the Califor-nia Institute of Technology and Cornell University.

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