Astron. Astrophys. 354, 674–690 (2000) ASTRONOMY …aa.springer.de/papers/0354002/2300674.pdf · J. Solf: A study of the various bipolar outﬂow components in M2-9 675 HST WFPC2

Astron. Astrophys. 354, 674–690 (2000) ASTRONOMYAND

ASTROPHYSICS

A high-resolution long-slit spectroscopic study of the various bipolaroutflow components in M 2-9 (“Butterfly Nebula”)?

J. Solf

Thuringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany ([email protected])

Received 9 September 1999 / Accepted 12 November 1999

Abstract. High-resolution long-slit spectrograms of the lineemission from the bipolar nebula M 2-9 obtained at various slitpositions are presented. The data are used to study in detail boththe kinematic and morphological properties of the various com-ponents of the bipolar outflow in M 2-9. Three main regions ofoutflow have been distinguished: a compact inner region rep-resented by the central core, an extended intermediate regionrepresented by the bright bipolar lobes, and an outer regionrepresented by the faint outer loops. All three regions show aremarkably high bipolar symmetry with a uniform inclination ofthe bipolar axes (∼73◦), whereas the deduced outflow velocitiesand kinematical ages are largely different from each other.

In the central core region, two physically distinct gas com-ponents, a high-velocity component and a low-velocity com-ponent, have been identified. The fast gas is of relatively highexcitation and represents a highly collimated bipolar outflowsystem (micro-jets) with velocities of up to 195 km s−1. Thekinematical age of the micro-jets is extremely small (<10 years).The observations suggest that the outflow source is surroundedby a dense equatorial disk obscuring the inner portions of thereceding jet. The slow gas is of lower excitation and is suggestedto represent a wind either from the equatorial disk or from theevolved stellar component of the presumed central binary.

In each of the bipolar lobes, a (co-axial) double-shell struc-ture has been identified, consisting of an inner shell of fast hotgas and an outer shell of slow cool gas/dust. The hot gas, tracedby the narrow line component representingin situ emission,shows outflow velocities of∼46 km s−1 indicating a kinemati-cal age of∼1300 years. The highest velocities are found near thebipolar axis. The cool gas/dust, traced by the broad line com-ponent representing dust-scattered emission, exhibits outflowvelocities of∼17 km s−1. The kinematical age of the cool-gasshell is about three times as large as that of the hot-gas shell.

The faint outer loops, traced by dust-scattered Hα line emis-sion, present large redshifts in both loops indicating outflow ve-locities of∼141 km s−1. The deduced kinematical age is∼1300years, comparable to that of the bipolar lobes, suggesting thatboth the outer loops and the bipolar lobes were formed at thesame time.

? Based on observations collected at the German Spanish Astronom-ical Center, Calar Alto, Spain

Our results confirm that M 2-9 exhibits many propertieswhich have little in common with those of planetary nebulae. Inparticular, the detection of fast bipolar jets and of a dense diskin the core region strengthen the hypothesis that M 2-9 probablybelongs to a class of close mass-exchanging binary systems, likesymbiotic novae, which are sources of collimated fast bipolaroutflows.

Key words: ISM: planetary nebulae: individual: M 2-9 – ISM:jets and outflows – ISM: kinematics and dynamics – stars: cir-cumstellar matter – ISM: dust, extinction

1. Introduction

M 2-9 (PK 10 + 18◦2) is one of the most exciting bipolar neb-ulae and has attracted the attention of many researchers in thefield of collimated mass outflows from evolved stars. Basicallythe nebula is composed of three main components, namely twobright, narrow bipolar lobes oriented approximately along thenorth-south direction and extending up to about±25′′ from thecenter; a bright, compact central core (“nucleus”) of about 1′′

extent; and two faint “outer loops” oriented along the bipolaraxis of the lobes and extending up to about±1′ north and southof the center. The general structure at optical wavelengths of thebipolar nebula and of its various components has been describedin detail, e.g., by Kohoutek & Surdej (1980) and Schwarz etal. (1997). Within each lobe several bright condensations havebeen identified which appeared to change their relative positionsin time (Allen & Swings 1972, van den Bergh 1974, Kohoutek& Surdej 1980, Hora & Latter 1994, Goodrich 1991). High-resolution near-infrared images and spectra of M 2-9 by Hora &Latter (1994) revealed a double-shell structure of the lobes, con-sisting of an inner shell of hot (ionized) gas and an outer shell ofcool (molecular) gas. Recently, a high-resolution narrow-bandimage of the core region and the bipolar lobes of M 2-9, obtainedwith the HST WFPC2, has been made available via the Internet(Balick, Icke & Mellema 1997). The HST image of M 2-9 (seeFig. 1) clearly indicates the presence of a double-shell structurein the bipolar lobes and reveals remarkable details of the vari-ous nebular condensations within each lobe. Near-infrared colorimages of M 2-9 by Aspin et al. (1988) suggest that the central

J. Solf: A study of the various bipolar outflow components in M 2-9 675

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Fig. 1. Direct image of the planetary nebula M 2-9 derived from com-bined HST WFPC2 images obtained in the light of [Nii] λ6583,[O i] λ6300, and [Oiii] λ5007 (Balick, Icke & Mellema 1997). Thecentral core region (C) and the bright nebular condensation in thebipolar lobes (N1 to S3) are marked. The various slit positions of theobtained long-slit spectrograms are indicated.

core of M 2-9 is surrounded by a dense extended equatorial diskof luminous dust. Interferometric mapping of the12CO J = 1–0 emission from M 2-9 by Zweigle et al. (1997) indicates theexistence of an expanding equatorial torus of molecular gas ofabout 6′′ diameter around the center of M 2-9.

The evolutionary status of M 2-9 is still controversial. M 2-9has been identified with objects which “bear the characteristicsindicative for very young planetary nebulae”, such as M 1-91,M 1-92 (Solf 1994a), CRL 618 (Schmidt & Cohen 1981) or“proto-planetary nebulae” (Walsh 1981). Because of its mor-phology and nebular spectrum M 2-9prima faciehas been gen-erally considered to be a planetary nebula (PN). Its highly sym-

metric bipolar aspect lead Balick (1987) to consider M 2-9 asthe prototype (“butterfly”) of one of three classes in a series de-scribing a morphological sequence of PNs. As already outlinedby Balick (1989, and references therein), M 2-9 exhibits manyproperties which have little in common with those of typicalPNs. In particular, the central star of M 2-9, designated as B IV(Calvet & Cohen 1978) or late O type (Swings & Andrillat1979), presents (circumstellar) emission lines of very low tohigh excitation, e.g., permitted and forbidden low-ionisationlines of Oi, Fei, Feiii, and high-ionisation lines of [Oiii],[Ne iii], and Nv. As noted by Balick (1989), the Hα emissionline profile in the central nucleus exhibits extremely broad wingsextending over more than 11,000 km s−1 at the base. Theseproperties of the central nucleus of M 2-9, unknown in ordinaryPNs, appear more typical for very close mass-exchanging binarysystems, like other bipolars e.g., MZ-3, CRL 618, M 1-92, orsymbiotic novae e.g., RR Tel, V1016 Cyg, HM Sge, V1329 Cyg.Although direct evidence is still lacking, it is most plausible thatthe central object of M 2-9 is a mass-exchanging close binarysystem.

The spatio-kinematic properties of the gas and dust withinthe various nebular components of M 2-9 were studied in the pastusing long-slit spectroscopy of the permitted line and forbiddenline emissions (see, e.g., Balick 1989; Icke et al.1989). The de-tailed kinematics of the bipolar lobes was investigated first byCarsenty & Solf (1983) who noted that the Hα line emissionfrom the lobes consists of a narrow component (NC) and a red-shifted broad component (BC). The authors proposed that theNC of the Hα line and the forbidden lines represent intrinsicemission originating from the (hot) gas of the lobes whereas theBC represents reflected line emission originating from the cen-tral core and being scattered by dust particles within the gas ofthe lobes. High-resolution spectropolarimetry of various emis-sion lines from the lobes by Trammell et al. (1995) confirmedthe dust-scattering interpretation. Using a simple outflow modelfor the BC, Carsenty & Solf (1983) derived a flow velocity of∼20 km s−1 for the dust/gas mixture in the bipolar lobes. Solf(1993) suggested that the flow velocity of the (ionised) hot gascomponent of the bipolar lobes can be deduced using the ra-dial velocity and velocity dispersion derived for the observedNC and applying a simple bi-conical outflow model. He ob-tained a (mean) flow velocity of 46 km s−1 for the hot gas.Deep medium-resolution spectrograms of M 2-9 obtained bySchwarz (1990) and Solf (1993) indicated that the faint outerloops present weak Hα emission which (surprisingly) is red-shifted by large amounts in both loops. Solf (1993) suggestedthat the observed Hα line represents reflected emission origi-nating from the central core of M 2-9 and being scattered bydust particles in the loops moving outwards. With this assump-tion, he deduced a velocity of∼150 km s−1 for the dust/gas inthe outer loops. High-resolution spectroscopic observations byBalick (1989) indicated that the [Nii] emission from the centralcore of M 2-9 consists of a single “central” component and twofainter “satellite” components.

In thispaper, we present new high-resolution long-slit spec-troscopic observations of M 2-9 obtained at various slit posi-

676 J. Solf: A study of the various bipolar outflow components in M 2-9

tions and position angles. These data permit us to investigatein considerable detail both the kinematic and the morphologi-cal properties of the complex outflows in each of the differentcomponents of the bipolar nebula. The knowledge of these prop-erties appears to be crucial for any evolutionary and dynamicalmodel the prototype “butterfly” nebular M 2-9.

2. Observations and data reduction

High-resolution long-slit spectroscopic observations of M 2-9 were obtained during August 14–18, 1992, using the f/12camera of the coude spectrograph on the 2.2 m telescope atCalar Alto Observatory (Spain). The f/12 camera (focal length3.60 m) was equipped with an SITe SI-003 CCD detector with1024×1024 pixels, each of 24µm in size. A 632 grooves-per-mm grating was used in the 2nd spectral order providing a re-ciprocal linear dispersion of 2.2A mm−1 and a spectral cover-age of about 54A on the detector. The entrance slit was set to117′′ in length and 0.′′7 in width. The resulting spectral reso-lution (FWHM) is 7 km s−1. The spatial resolution (FWHM)achieved along the slit direction is 0.′′8–1.′′6. The sampling rateon the CCD is 2.3 km s−1 and 0.′′187 per pixel in the directionsof the dispersion and of the slit, respectively. Long-slit spec-tra were obtained in three different spectral regions coveringthe lines of Hα, [N II] λλ6548,6583, [OI] λλ6300,6363, and[O III] λ5007. The position angle (P.A.) of the slit was set bymeans of an image de-rotator.

Hereafter, we will abbreviate north, east, south, and west asN, E, S, W, respectively. [Oiii], [N ii], and [Oi] will be used todesignate the emission lines of [Oiii] λ5007, [Nii] λ6583, and[O i] λ6300, respectively, unless otherwise noted.

Since the bipolar axis of M 2-9 is oriented approximatelyalong the N-S direction, slit positions and P.A.’s related to thatorientation have been selected: namelya) slit positions centredon the central star and oriented along P.A.’s 0◦, 90◦, and±24◦,andb) slit positions offset from the central star by∆δ = ±10′′

and oriented at P.A. = 90◦. Fig. 1 shows these slit positionssuperimposed upon an HST image of M 2-9. Spectra of Hα

and [NII] were obatained at all slit positions mentioned above;spectra of [OIII] at P.A.’s 0◦, 90◦, and±45◦; and spectra of[O I] at P.A. = 0◦ only. In most cases, the exposure time was3600 s. It should be noted that due to the limited slit length theobtained high-resolution long-slit spectrograms do not cover thefaint outer loops of M 2-9.

A single deep, moderate-resolution long-slit spectrogramof M 2-9 at P.A. = 0◦ was secured on May 27, 1988, using theCassegrain Twin Spectrograph on the 3.5 m telescope at CalarAlto during its commissioning. In this case, a slit aperture of1.′′1× 240′′ was used, long enough to reach the faint outer loopsof M 2-9 as well. The spectrogram covers the spectral range of6220–6780A at a spectral resolution of about 45 km s−1.

The evaluation of the CCD data frames (flat-fielding, cali-bration, stellar continuum subtraction, Gaussian line fits) wasperformed using a special long-slit data reduction package de-veloped by the author. The two-dimensional long-slit spectro-scopic data of the observed emission lines are generally pre-

sented as logarithmic isophotic contour diagrams in a position-versus-velocity representation (“PV diagrams”). Throughoutthis paper, all deduced radial velocities are quoted as relativevelocities with respect to the systemic velocityVsys of M 2-9(see Sect. 3.1.1.), and angular positions are quoted as relativepositions with respect to the position of the central star of M 2-9(see Sect. 3.3.).

3. Results

Fig. 2 displays the high-resolution PV diagrams of Hα and [Nii]for the slit position centred on the central star and oriented at P.A.= 0◦. It should be noted that in the central core region of the [Nii]map the contributions from the stellar continuum and from theextended Hα line wing (see below) have been eliminated usinga special subtraction method (for details, see Solf 1994b). ThePV diagrams of [Oi] and [Oiii] for the same slit position arepresented in Fig. 3. Figs. 4 and 5 show the PV diagrams of Hα

and [Nii] for the slit positions centred on the central star andoriented at P.A.’s +24◦ and –24◦, respectively. Fig. 6 shows thePV diagrams of [Nii] and [Oiii] for the slit at P.A. = 90◦ andcentred on the central star. Fig. 7 presents the PV diagrams ofHα and [Nii] for the slit positions at P.A. = 90◦ and offset fromthe central star by∆δ = +10′′ and∆δ = –10′′, respectively. Aportion of the deep medium-resolution spectrogram presentingthe lines of Hα and [NII] λλ6548,6583 is shown in Fig. 8.

As already mentioned in Sect. 1., three main regions havebeen distinguished in the morphology of M 2-9:1) an extended“lobes region” including the two prominent bipolar lobes;2)an “outer loops region” including the two faint nebular loops atabout±1′ N and S of the centre;3) a compact “central core re-gion” of about 1′′ extent. The detailed observational results willbe presented following that distinction of the different regionsof M 2-9.

3.1. Bipolar lobes

The PV diagrams at P.A. = 0◦ (Figs. 2, 3) permit us to study thevariations of the spatio-kinematic properties in both lobes alongthe direction of the main axis of symmetry of the bipolar lobes.The PV diagrams obtained at P.A.’s± 24◦ (Figs. 4, 5) permit usto study those regions which are off the main axis. On the otherhand, the P.A. 90◦ diagrams at∆δ = ±10′′ (Fig. 7) provideinformation on the variations of the spatio-kinematic propertiesalong directions perpendicular to the main axis.

All PV diagrams at P.A.’s 0◦, ±24◦, and 90◦ (Figs. 2, 4, 5,7) show remarkable differences and similarities between boththe kinematics and the morphology of the (permitted) Hα lineemission, on one hand, and those of the forbidden lines, on theother hand. Generally, the forbidden lines present a single, rathernarrow component (NC) only, whereas the Hα line presents twoclearly distinct components: a narrow component (NC) and arather broad component (BC). Both the kinematics (velocity,velocity width) and morphology (spatial extent, intensity distri-bution) presented by the forbidden lines are nearly the same asthose presented by the NC of the Hα emission. In contrast, the


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Fig. 2. PV diagrams of the line emission of Hα (left hand)and [Nii] λ6583 (right hand) of M 2-9 deduced from an f/12 coude long-slitspectrogram obtained with the slit centred on the central star and oriented at P.A. = 0◦ (see Fig. 1). Adjacent isophotic contours represent a factorof 21/2 in the intensity. Relative velocities and position are quoted with respect to the systemic radial velocity of the nebula and to the positionof the central star, respectively. The bright nebular condensations in the northern and southern lobes (N3, S3) intersected by the slit position aswell as the resolved line components in the central core region (C−, C0, C+) are indicated.

BC of the Hα emission exhibits different kinematic and mor-phological properties compared to those of the forbidden linesand the NC of Hα.

In order to study the spatial distribution and the kinematics ofboth the NC and the BC in greater detail, the (relative) intensityI, the (mean) velocityV , and the velocity dispersion (FWHM)∆V , as a function of the relative position along the directionsparallel as well as perpendicular to the bipolar axis, have beendeduced for each component using a (single-component or two-component) Gaussian line fit of the observed line profiles, rowby row, on the data frames. Fig. 9 presents the distributions ofI,

V , and∆V derived for the NC along the direction of the bipolaraxis using the [NII] data at P.A. = 0◦ (see Fig. 2). Figs. 10 and11 show the results derived for [NII], the NC of Hα, and theBC of Hα along the direction perpendicular to the bipolar axisat offsets∆δ = +10′′ and∆δ = –10′′, respectively, using thecorresponding data of [NII] and Hα at P.A. = 90◦ (see Fig. 7).

The presence of two components (NC and BC) in the Hα

emission of the lobes of M 2-9 was first reported by Carsenty &Solf (1983) who interpreted the observed kinematic differencesas due to different origins of the NC and BC. The NC was con-sidered as emission originatingin situ from the gas in the bright


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Fig. 3. PV diagrams of the line emission of [Oi] λ6300(left hand)and [Oiii] λ5007(right hand). Otherwise as in Fig. 2.

lobes, whereas the BC as emission originating from the centralcore region and being scattered by dust particles associated withthe gas of the lobes. In the following, we will describe in detailthe spatial and kinematic properties deduced for the NC and BCfrom the new observations.

3.1.1. Narrow component (NC) of the line emission

As outlined above, the NC of Hα and the forbidden lines repre-sent intrinsic emission of the (hot) gas within the bipolar lobes.This implies that the radial velocities deduced from the NCof Hα and the forbidden lines represent the (hot) gas flow inthe lobes. The neat bipolar morphology of the lobes of M 2-9strongly suggests that the bipolar outflow velocity is of the sameamount in both lobes. This allows us to derive the systemic ve-

locity of the entire nebulaVsys by averaging the radial velocitiesof the two lobes deduced for the NC of Hα and the forbiddenlines. As already mentioned above, throughout thispaper, theradial velocities deduced for the various nebular componentshave been quoted as relative velocities with respect toVsys.

The spatial extent and intensity distribution of the NC arebest studied on the PV maps of [NII]. The (half) extent alongthe direction of the bipolar axis of the [NII] emission (measuredfrom the central star) (see Figs. 2, 9) is about 27′′, quite com-parable to the extent of the NC of the Hα emission (see Fig. 2)and to the N–S extent of the nebula visible on the HST imageof M 2-9 (Fig. 1). The (half) extent along P.A.’s±24◦ of the NCis about 13′′–14′′ (Figs. 4, 5). The (half) extent perpendicularto the bipolar axis of the NC at∆δ = ±10′′ is about 5′′–6′′

(Figs. 7, 10, 11).


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Fig. 4. PV diagrams of the line emission of Hα (left hand)and [Nii] λ6583(right hand)deduced from a long-slit spectrogram obtained withthe slit centred on the central star and oriented at P.A. = +24◦ (see Fig. 1). The bright nebular condensations in the northern lobe (N1, N2)intersected by the slit position are indicated. Otherwise as in Fig. 2.

Each of the bright nebular condensations (N1 to S3), markedon the HST image (Fig. 1), can be recognised on the various PVmaps, namely N3 and S3 on the maps at P.A. = 0◦ (Figs. 2, 3),N1 and N2 at P.A. = +24◦ Fig. 4), S1 and S2 at P.A. = –24◦ maps(Fig. 5), N2 and S2 at P.A. = 90◦ (Fig. 7).

The velocitiesVNC derived for the NC (see Figs. 9–11) arenearly constant within each lobe, although some characteris-tic variations are apparent (see below). Mean velocities of thenorthern lobeV N

NC = +12 km s−1 and of the southern lobeV SNC

= –12 km s−1 have been deduced for [NII]. The correspondingvalues for the NC of Hα are±10 km s−1, respectively. The ve-locity dispersion∆VNC presented by the [NII] emission in thelobes is generally quite small, typically 18–20 km s−1 or less.

The larger values of∆VNC observed in the NC of Hα are ex-plained as due to the lower atomic weight of H0 compared toN+.

Both (the absolute values of)VNC and∆VNC are somewhatlarger in the nebular condensations N3 and S3 compared to theirneighbourhood (Fig. 9). If one neglects these local “anomalies”,the absolute values of the velocity of the NC present a slightincrease along the bipolar axis as one proceeds toward largerdistances from the centre (Fig. 9). On the other hand, the distri-bution ofVNC perpendicular to the bipolar axis shows a differ-ent behaviour. In both lobes, the absolute values ofVNC exhibittheir maximum near the bipolar axis and decrease in either di-rection toward the lateral “boundaries” of the lobes (Figs. 10,


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Fig. 5. PV diagrams of the line emission of Hα (left hand)and [Nii] λ6583(right hand)deduced from a long-slit spectrogram obtained with theslit centred on the central star and oriented at P.A. = –24◦ (see Fig. 1). The bright nebular condensations in the southern lobe (S1, S2) intersectedby the slit position are indicated. Otherwise as in Fig. 2.

11). The condensations N2 and S2 (near the eastern edge of eachlobe) follow that trend as well (see Figs. 4, 5, 7).

The velocities observed in the condensations N1 and S1,covered by the slit positions at P.A.’s±24◦ (see Figs. 4, 5), showan interesting behaviour. The (absolute) velocity of S1 is unex-pectedly large (larger than that of S2), whereas the velocity ofN1 is unexpectedly small (smaller than that of N2) and presentsthe opposite sign (compared to N1). This implies that either N1and S1 is blueshifted with respect to N2 and S2, respectively. Itshould be noted that “bridges” of weak line emission betweenN1 and N2 and between S1 and S2 are clearly visible on the[NII] maps at P.A.’s±24◦ (Figs. 4, 5).

3.1.2. Broad component (BC) of the Hα emission

The BC of the Hα emission (see Figs. 2, 4, 5, 7) appears tobe considerably broader than the NC of Hα and the forbiddenlines. Typical velocity widths∆VBC of 70–90 km s−1 have beendeduced for the BC, more than a factor of 4 larger than derivedfor the∆VNC (see also Figs. 10, 11). Moreover, in both lobes,the BC is always redshifted by a nearly constant amount withrespect to the NC (see Figs. 10, 11). Mean values ofV N

BC =+30.7 km s−1 andV S

BC = +19.8 km s−1 have been deduced forthe BC of the Hα emission in the northern and southern lobe,respectively.


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Fig. 6. PV diagrams of the line emission of [Nii] λ6583 (top) and[O iii] λ5007(bottom)deduced from a long-slit spectrogram obtainedwith the slit centred on the central star and oriented at P.A. = 90◦ (seeFig. 1). The resolved line components in the central core region (C−,C0, C+) are indicated. Otherwise as in Fig. 2.

Large differences between the BC and the NC are also ap-parent in their spatial extents and intensity distributions alongthe directions of the different P.A.’s. As indicated on the Hα

map at P.A. = 0◦ (Fig. 2), the (half) extent of the BC (from thecentre) along the bipolar axis is about 18′′–20′′, considerablyless than that of the NC (see above). The (half) extent of theBC along P.A.’s±24◦ (Figs. 4, 5) is about 15′′–16′′, somewhatlarger than that of the NC. The (half) lateral extent (perpen-dicular to the bipolar axis) of the BC at∆δ = ±10′′ (Fig. 7)is about 6′′–7′′, somewhat larger than that of NC. In all cases,the intensity distribution of the BC along the various P.A.’s isnearly constant (see also Figs. 10, 11). It should be emphasisedthat none of the bright nebular condensation (N1 to S3), whichare visible on direct images of M 2-9 (see Fig. 1) and recognisedon the PV maps as prominent maxima in the NC of the Hα andthe forbidden line emission, is observable in the BC of the Hα

emission.

3.2. Faint outer loops

The faint outer loops, located on the bipolar axis of M 2-9 andextending up to about 1′ from the central core, have been coveredby the slit position of the medium-resolution long-slit spectro-gram obtained at P.A. = 0◦ (Fig. 8). The spectrogram presents

weak Hα emission at the positions of both the northern loop(OLN) and the southern loop (OLS). Schwarz et al. (1997) re-ported the presence of very faint [Nii] λ6583 emission in thesouthern loop which is not present on our spectrogram, probablybecause of a slightly different P.A. of the slit. As already notedby Schwarz (1990) and Solf (1993), both loops present red-shifts of large but different amounts. From our data we deriveradial velocities ofV N

loop = 191 km−1 andV Sloop = 108 km−1

for the northern and southern loop, respectively, in good agree-ment with the results reported by Solf (1993) and Schwarz etal. (1997).

3.3. Central core region of M 2-9

The PV maps of the forbidden lines at P.A. = 0◦ (Figs. 2, 3) andat P.A.’s±24◦ (Figs. 4, 5) clearly demonstrate the differencesbetween the central core region, dominated by a complex broadline emission, and the region of the bipolar lobes, dominatedby a narrow line emission. Compared to the lobes, the [Nii]emission of the core region is weaker (Fig. 2), whereas the [Oiii]and, specifically, the [Oi] emissions are considerably stronger(Fig. 3).

The Hα emission detected in the region of the central coreof M 2-9 (Figs. 2, 4, 5) is dominated by a rather strong “stel-lar” P-Cyg-type line feature. Since that emission is probablyoriginating from the immediate vicinity of the exciting centralstar, the centroid positionX0 of the spatial distribution (alongthe slit direction) of the recorded Hα emission, determined bya Gaussian fit, is likely to represent the centroid position of theexciting central star of M 2-9. As already mentioned above, inthis paper, positions within M 2-9 have been generally quotedas relative positions with respect toX0. (In cases the spectro-grams do not include the Hα line, X0 has been obtained byapplying a Gaussian fit to the spatial distribution of the “stellar”continuum.)

As already noted by Balick (1989) the Hα emissionpresents extremely broad line wings extending up to about±5,500 km s−1 at the base. Using a two-component Gaussianline fit of the P Cyg line profile, the radial velocity of the emis-sion componentVem = –0.4 km s−1 and that of the absorptioncomponentVabs = –11.6 km s−1 with respect to the systemic ve-locity Vsys (see Sect. 3.1.1.) have been derived. By integrationover the entire Hα line profile the “effective” (centroid) radialvelocity of the Hα emissionVeff = +8.9 km s−1 has been ob-tained. (It should be noted that, below in Sects. 4.1.2. and 4.2.,the latter will be adopted as the “rest velocity” of the illumi-nating source as “seen” by the dust particles in the outflows.)Since the strong “stellar” Hα emission engulfs any “nebular”Hα emission from the core region, the distibution and kinemat-ics of the nebular gas in the core region cannot be derived fromthe Hα but from the forbidden line emission only.

The [Nii] line feature, visible in the central core region onthe maps at the various P.A.’s (Figs. 2, 4–6), presents a rathercomplex structure extending over a full width at zero inten-sity (FWZI) of about 175 km s−1. The line feature has beenspectrally resolved into three components: a prominent (cen-


Rel

ativ

e P

ositi

on (

arc

sec)

->

PA

90°

∆δ

-10

10

-10

-5

0

5

M2-9 Hα

M2-9PA 90°

-10"

10

5

0

-5

M2-9PA 90°PA 90°

∆δ +10" ∆δ +10"

[N II]

[N II]M2-9

∆δPA 90°

-10"

Hα

0 50 100 150-50-100-150 -150 -100 -50 0

Relative Velocity (km/s)50 100 150

S2

N2 N2

S2

Fig. 7. PV diagrams of the line emission of Hα (left hand column)and [Nii] λ6583(right hand column)deduced from a long-slit spectrogramobtained with the slit oriented at P.A. = 90◦ and offset by∆δ = +10′′ (upper row)and∆δ = –10′′ (lower row)with respect to the position ofthe central star (see Fig. 1). The bright nebular condensations in the northern and southern lobe (N2, S2) intersected by the slit positions areindicated. Otherwise as in Fig. 2.

tral) “low-velocity component” (LVC) and two faint (satellite)“high-velocity components” (HVCs). The LVC will be desig-nated as C0, the redshifted HVC as C+, and the blueshifted HVCas C−. It can be noticed that C+ is generally somewhat faintercompared to C−. A close inspection of the maps at P.A.’s 0◦ and±24◦ (Figs. 2, 4, 5) reveals that the HVCs have been spatiallyresolved as well. C+ and C− are spatially extended and arepointing (from the centre of the core) into opposite directions.Remarkably, comparable offsets of C+ and C− (toward the E orW) are not visible on the [Nii] map at P.A. = 90◦ (Fig. 6). Thesefindings indicate that the centroids of C+ and C− are offset to-wards the N and S, respectively. The presence of two “satellite”components in the [Nii] line emission from the core region hasbeen noted previously by Balick (1989) and Solf (1993).

For each PV map at P.A.’s 0◦, ±24◦, and 90◦, three-component two-dimensional Gaussian line fits were applied tothe observed [Nii] line features of the core region, in orderto derive, for each component, C+, C0, and C−, the (relative)

peak intensityI, the (centroid) radial velocityV , the velocitydispersion (FWHM)∆V , the angular positionX, and the an-gular width (FWHM)∆X. The results of the fits of the [Nii]line features obtained for P.A.’s 0◦ and 90◦ are listed in Table 1.V has been quoted relatively to the systemic velocityVsys, andX relatively to the position of the central starX0. ∆V and∆X have been corrected for the spectral and spatial resolution,respectively. It should be noted that the (absolute) values ofXderived for C+ and C− at P.A.’s±24◦ (not shown in Table 1) areabout 30 percent smaller than those quoted for P.A. = 0◦. Theseresults clearly indicate that the maximum and minimum offsetsX of the HVCs occur along the N-S direction and the E-W di-rection, respectively. Hence the elongation of the core region ofM 2-9 is oriented along the direction (N-S) of the main axis ofsymmetry of the bipolar lobes. More specifically, the resolvedredshifted C+ and blueshifted C− are pointing into opposite di-rections and hence are likely to represent the two components abipolar outflow system (see Sect. 4.3.1. below).


0


M 2-9PA 0°

60

40

20

0

-20

-40

-60

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ativ

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ositi

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arc

sec)

-

> P

A 0

°

+108

+12

-400 400

N[II]6583N[II]6548

-12

+191αH

OLN

OLS

Fig. 8. A section of a deep moderate-resolution long-slit Cassegrain spectrogramof M 2-9 obtained with the slit centered onthe central star and oriented at P.A. = 0◦. Thesection presents the spectral range of the Hα

and [Nii] lines. Adjacent isophotic contoursrepresent a factor of 2 in the intensity. TheHα emission from the faint outer loops ismarked (OLN, OLS). Deduced radial veloc-ities are quoted in km−1. Otherwise as inFig. 2.

It is noteworthy that the [Nii] results at P.A. = 0◦ listed inTable 1 permit us a comparison with previous results from ahigh-resolution spectrogram of M 2-9 at P.A. = 0◦ obtained bythe author in 1983 using the same spectrograph then equippedwith an image-tube detector system. The old image-tube spec-trogram was re-evaluated and a two-dimensional Gaussian fitapplied to the recorded [Nii] line feature, in order to obtain the(centroid) positionsX of the components C−, C0, and C+. Acomparison of the 1992 and 1983 results indicates that C− andC+ have moved away from the centre into opposite directions ata rate of about 0.′′012 yr−1 and 0.′′027 yr−1, respectively. These“proper motions” imply a total “flowing time” from the centre(up to the epoch of the 1992 observations) of 27 years and 22years for C+ and C−, respectively.

The [Oiii] line features observed in the core region at P.A.’s0◦ and 90◦ (Figs. 3, 6) have been spectrally and spatially re-solved as well. They exhibit a similar structure as the [Nii]feature consisting of a LVC (C0) and two HVCs (C+ and C−).In this case, however, C+ is rather faint, whereas C− is ratherstrong, even stronger than C0. Compared to the situation in[N ii], the brightness ratio of C+/C− in [O iii] is significantlysmaller. Similarly to the case of [Nii], components C+ and C−

observed in [Oiii] at P.A. = 0◦ (Fig. 3) exhibit small spatialoffsets toward the N and S, respectively, whereas at P.A. = 90◦

(Fig. 6), such offsets (toward the E or W) are not present. As thecase of [Nii], two-dimensional multi-component Gaussian linefits were applied to the observed [Oiii] line features of the coreregion at P.A.’s 0◦ and 90◦. The results of the fits, except for

∆X, are also listed in Table 1. (∆X has been omitted, becausea correction for the actual spatial resolution was not feasible inthis case.)

The [Oi] line feature observed in the core region at P.A.= 0◦ (Fig. 3) exhibits a different structure compared to [Nii]and [Oiii]. In this case, the line feature is dominated by a sin-gle bright, rather narrow LVC (C0). Extremely faint extendedline wings on either side of the LVC are barely visible on thePV diagram, suggesting that rather weak HVCs (C+ and C−)might be present, though a definite claim cannot be made. Acentroid velocityV of –7.7 km s−1 and a velocity width∆V of∼32 km s−1 have been deduced for C0.

The PV diagrams of [Nii] and [Oiii] obtained at P.A. = 90◦

with the slit centred on the central star (Fig. 6) present line emis-sion in the core region only. Outside that region (perpendicularto the bipolar axis of M 2-9) extended nebular emission in theforbidden lines or in the Hα line (diagram not shown) has notbeen detected.

4. Discussion

4.1. Double-shell structure of the bipolar lobes

Optical polarisation maps of M 2-9 by King et al. (1981) andAspin & McLean (1984) revealed the existence of a dust shellassociated with the lobes. The polarisation is produced by scat-tering dust grains illuminated by light from the central core re-gion. Spectropolarimetry of M 2-9 by Schmidt & Cohen (1981)showed that the polarisation of the forbidden lines in the bipolar


Table 1.Peak intensityI, centroid radial velocityV , velocity dispersion (FWHM)∆V , centroid angular positionX, and angular width (FWHM)∆X of the resolved line components C−, C0, C+ in the central core region.

Component Line P.A. = 0◦ P.A. = 90◦

I V ∆V X ∆X I V ∆V X ∆Xkm s−1 km s−1 arcsec arcsec km s−1 km s−1 arcsec arcsec

C− [N II] 33 –57 49 –0.32 0.8 44 –51 47 –0.01 0.9C0 [N II] 100 –6 54 –0.11 1.1 100 –4 54 +0.03 1.2C+ [N II] 21 +57 40 +0.60 1.5 16 +56 37 +0.05 2.2C− [O III] 135 –41 30 –0.20 ... 164 –41 31 +0.08 ...C0 [O III] 100 –5 33 +0.04 ... 100 –4 32 +0.00 ...C+ [O III] 26: +34: 40: +0.11: ... 26: +34: 40: -0.03: ...

10-10-20-30 30

0

1

2

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30

-30

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nsity

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Relative Velocity

Velocity Width (FWHM)

Vel

ocity

Wid

th (

km/s

) M 2-9

0 20

0

20

40

60

80

100

120

S3

CN3

Relative Intensity

Relative Position (arc sec) -> PA 0°

Fig. 9. Relative intensityI (bottom), radial velocityV (centre), andvelocity dispersion (FWHM)∆V (top) derived for the [Nii] λ6583line, as a function of the relative position with respect to the centralstar, along the direction of the bipolar axis at P.A. = 0◦ (see Fig. 1). Thebright nebular condensations in the northern and southern lobes (N3,S3) and the central core (C) are indicated.

lobes is much lower than that of the permitted lines and the stel-lar continuum. This result indicates that (most of) the observedforbidden line emission arisesin situfrom the hot gas within thelobes, whereas a major fraction of the permitted line emissionand continuum radiation represents dust-scattered light origi-

nally produced near the central star. This implies that two gascomponents in each lobe must be present, namely a compo-nent of hot (ionised) gas and a component of cool (molecular)gas and dust. Imaging polarimetry in the Hα line by Scarrott etal. (1993) and high-resolution near-infrared imaging by Hora &Latter (1994) revealed a double-shell structure in each lobe,consisting of an elongated cavity-like inner shell of hot gasand a co-axial envelope-like outer shell of molecular gas anddust. High-resolution spectroscopy of M 2-9 by Carsenty & Solf(1983) showed that the permitted lines and the forbidden lines inthe lobes exhibit different kinematical properties as well. Theseresults indicate that the two gas components of the double-shellstructure can be studied by an analysis of the spectral propertiesof the permitted lines and the forbidden lines observed in thelobes.

As outlined in Sect. 3.1., the Hα line emission in the bipo-lar lobes presents a complex structure, consisting of a narrowcomponent (NC) and a broad component (BC), whereas theforbidden lines present a single NC only. The NC and BC ofHα exhibit different kinematic properties (centroid velocities,velocity widths) which have been extracted by means of a two-component Gaussian fit. As already shown above, the derivedspatial distribution and the kinematical properties of the NCof Hα are quite similar to those of the forbidden lines. On theother hand, the BC of Hα presents a much larger velocity widthcompared to and is redshifted with respect to the NC and theforbidden lines. These results suggest that the NC representsline emission originating in the hot (ionised) gas component,whereas the BC represents line emission originating in the coreregion of M 2-9 and being scattered by dust particles associatedwith the cool (molecular) gas component of the lobes. In thefollowing we will discuss in detail the different morphologicaland kinematical properties of the two gas components based onan analysis of the observed BC and NC. These properties allowus to apply bipolar outflow models in order to derive the “true”bipolar outflow velocities for each of the two gas components.

4.1.1. Morphology of the cool-gas/dust component and thehot-gas component of the double-shell structure

The long-slit spectroscopic results indicate that the cool gas/dustcomponent in the lobes exhibits a spatial distribution which


∆δ +10

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[N II] 6583

Relative Intensity

Relative Velocity

H NC

Velocity Dispersion

Inte

nsity

N2 N2

α

Hα BC

Fig. 10. Relative intensityI (lower row), ra-dial velocity V (central row), and velocitydispersion (FWHM)∆V (upper row) de-rived for the [Nii] line (left hand column),the narrow component (NC) of the Hα line(central column), and the broad component(BC) of the Hα line (right hand column), as afunction of the relative position with respectto the position of the bipolar axis, along thedirection of the slit oriented at P.A. = 90◦

and offset by∆δ = +10′′ (see Fig. 1). Thebright nebular condensation in the northernlobe (N2) intersected by the slit position isindicated.

100

80

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pers

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Vel

ocity

(km

/s)

Relative Position (arc sec) -> PA 90°

∆δ −10

Relative Velocity

Inte

nsity

Velocity Dispersion

Relative IntensityS2 S2

Fig. 11. Relative intensityI (lower row), ra-dial velocity V (central row), and velocitydispersion (FWHM)∆V (upper row) de-rived for the [Nii] line (left hand column),the narrow component (NC) of the Hα line(central column), and the broad component(BC) of the Hα line (right hand column)as afunction of the relative position with respectto the position of the bipolar axis, along thedirection of the slit oriented at P.A. = 90◦

and offset by∆δ = –10′′ (see Fig. 1). Thebright nebular condensation in the southernlobe (S2) intersected by the slit position isindicated.

in general resembles the bipolar morphology presented by thehot gas component, although some significant differences areapparent. Compared to the hot gas component, the cool gas/dustcomponenta) appears to be less extended along the directionof bipolar axis,b) exhibits a somewhat larger opening angle,andc) does not present regions of enhanced brightness. Thesefindings can be best understood if the cool gas/dust is forming a(relatively thin) envelope-like outer shell which is embedding acavity-like inner shell filled up with ionised hot gas. Only alongdirections near the bipolar axis, the hot gas of the inner shell isreaching much beyond the boundary of outer shell. The bright

nebular condensations visible in the lobes clearly belong to the(inner) hot-gas component. Some of them, N2 and S2, appearto be located at the “boundary layer” of the inner shell; others,N3 and S3, appear the be located whithin the inner shell closeto the position of the bipolar axis.

4.1.2. Outflow model of the bipolar cool-gas/dust shell

As outlined above, the cool-gas/dust component of the bipolarlobes is traced by the BC of Hα. The BC represents emissionoriginating from the central core region and being scattered by


dust particles contained in the outflowing cool gas of the outershell. Using a simple bipolar outflow model for the scatteringparticles in the lobes we can relate the outflow velocityV exp

dust

of the cool gas/dust and the inclination angleΘdust of the bipo-lar outflow axis (with respect to the line-of sight) to the radialvelocitiesV N

dust andV Sdust of the dust particles observed in the

BC of Hα in the northern and southern lobe, respectively:

V Ndust = Veff + V exp

dust × (1 + cos Θdust), (1)

V Sdust = Veff + V exp

dust × (1 − cos Θdust), (2)

whereVeff denotes the “effective” velocity (with respect tothe systemic velocityVsyst) of the central illuminating Hα linesource caused by the P Cyg line profile “seen” by the scatteringdust particles. From Eqs. (1) and (2) we obtain:

V expdust = 0.5 × (V N

dust + V Sdust) − Veff (3)

and

Θdust = arccos(V N

dust − V Sdust

2 × V expdust

). (4)

Inserting our resultsV Ndust = V N

BC ≈ +31 km s−1 andV S

dust = V SBC ≈ +20 km s−1 (see Sect. 3.1.2.), and assuming

that the dust particles “see” virtually the same P Cyg profile asthe terrestrial observer,i.e., Veff ≈ +9 km s−1 (see Sect. 3.3.),we obtain an outflow velocity of the cool-gas/dust shellV exp

dust ≈17 km s−1 and an inclination angle of the bipolar outflowΘdust

≈ 71◦. It should be noted that the deduced value ofV expdust is

somewhat lower than that reported by Carsenty & Solf (1983),who did not take into account the “effective ” velocityVeff ofthe central illuminating source.

Adopting a distance of 650 pc for M 2-9 (Schwarz et.al. 1997) and using the deducedV exp

dust and the (half) extent(along the bipolar axis) of about 20′′ observed in the BC (seeSect. 3.1.2.), we obtain a kinematic age of about 3800 years forthe cool-gas/dust shell.

4.1.3. Outflow model of the bipolar hot-gas shell

The hot (ionized) gas of the bipolar lobes is traced by the NCof Hα and the forbidden lines. In order to derive the outflow ve-locity V exp

ion of the ionized gas and the inclination angleΘion ofthe bipolar axis, we adopt a bi-conical outflow model (see Solf& Bohm 1999) assuming that the velocity dispersion (FWHM)∆Vion, presented by the NC of Hα and the forbidden lines,is caused by the divergence of the velocity vectors of the out-flowing gas, filling up the conical volume of each lobe. IfΦion

denotes the (half) aperture angle of the bi-conical shell andVion

the mean (absolute) value of the radial velocity of the ionizedgas in the lobes we obtain:

Θion = arctan(∆Vion

2 × Vion × tan Φion) (5)

and

V expion =

Vion

cos Φion × cos Θion. (6)

Because of the (expected) rather large inclination of thebipolar outflow axis, the aperture angleΦion of the bi-conicalshell can, in principle, be derived from direct images of M 2-9in the light of the forbidden lines. However, due to the specificmorphology of M 2-9, the (effective) aperture angle is not con-stant if one proceeds along the bipolar axis. Hence it is usefulto select a restricted zone in each lobe, where the aperture angleis well determined. Selecting, e.g., the zone between 15′′ and20′′ (from the central core), we derive a (mean)Φion of about13◦ (see Fig. 1), a (mean)Vion = |VNC | of about 13 km s−1,and a (mean)∆Vion = ∆VNC of about 20 km s−1 for [N II](see Fig. 9). Using these results we obtain a (mean) outflow ve-locity V exp

ion ≈ 46 km s−1 for the hot (ionized) gas of the lobesand an inclination angleΘion ≈ 73◦ for the bipolar axis. Forthe adopted distance of M 2-9 the deduced outflow velocity indi-cates a (mean) kinematic age of about 1300 years for the hot-gasshell (in the considered zone) of the bipolar lobes.

As mentioned in Sect. 3.1.1., the (absolute) radial velocitiesin the regions near the (projected) bipolar axis are systematicallyhigher than in those near the lateral “edges” of the lobes. More-over, the radial velocities of the bright nebular condensationsN3 and S3 (observed near the bipolar axis) are systematicallyhigher than those of the neighboring regions on the axis. Theseobservations suggest that the outflow velocity of the hot gas ofthe lobes is largest along the polar axis and is decreasing at lowerlatitude angles. Furthermore, if N3 and S3 are indeed locatedon the bipolar axis, their true flow velocity must be even higherthan the (mean) polar flow velocity of the surrounding materialin their neighborhood. On the other hand, the fact (mentionedabove) that the line emission from condensation N1 and S1 isblueshifted with respect to that of N2 and S2, respectively, in-dicates that both N1 and S1 are probably located on the front“boundary layer” (pointing toward the observer) of the hot-gasshell.

Our results indicate that the orientation in space of the bipo-lar shell of the hot (ionized) gas is approximately the same asthat of the shell of the cool gas/dust,i.e., Θdust ≈ Θion, sug-gesting that both shells have the same bipolar axis. On the otherhand, the (polar) outflow velocity of the hot-gas shell is muchlarger (by a factor of∼3) than that of the cool-gas/dust shell.This implies that the two shells must be spatially separated inthe sense that the (younger) shell of the (fast) hot gas is embed-ded within the (older) shell of the (slow) cool gas. Only alongdirections near the bipolar axis, the fast gas is breaking throughthe slow-gas shell, leading to a much more elongated structureof the hot-gas shell compared to the cool-gas shell. It should benoted that this cocoon-like structure is well compatible with theresults from the analysis of the morphology of the double-shell(see Sect. 4.1.1.).

4.2. Spatio-kinematic structure of the faint outer loops

It has been suggested by Solf (1993) and confirmed by Schwarzet al. (1997) that the Hα line emission detected in faint outerloops represents dust-scattered light originating from the centralcore region of M 2-9. Hence we can apply Eqs. (3) and (4) in


order to deduce the bipolar outflow velocity of the scatteringdust/gas in the outer loopsV exp

loops (instead ofV expdust) and the

inclination angleΘloops (instead ofΘdust) of the bipolar axis.Inserting the radial velocitiesV N

loop andV Sloop (instead ofV N

dust

andV Sdust) derived for northern and southern loop, respectively,

(see Sect. 3.2.) and taking into account the effective velocityVeff of the illuminating source (see Sect. 3.3.) we obtainV exp

loops

≈ 141 km s−1 and Θloops ≈ 73◦. These results are in goodagreement with those reported by Solf (1993) and Schwarz etal. (1997).

It is remarkable that the deduced orientation in space of thebipolar outflow observed in the outer loops is the same as thoseof the outflows in double-shell structure of the bipolar lobes,i.e.,Θloops ≈ Θdust ≈ Θion, whereas the outflow velocityV exp

loops

of the loops is larger by a factor of∼3 thanV expion of the hot

(ionized) gas and larger by a factor of∼8 thanV expdust of the cool

gas/dust in the double-shell structure of the lobes.For the adopted distance of M 2-9, the derived outflow ve-

locity of the faint loops and their (angular) separation from thecentral core indicate a kinematic age of about 1300 years forthe outer loops. It is most remarkable that this age is much lessthan that of the cool-gas/dust shell but is about the same as thatof the hot-gas shell of the bipolar lobes. These findings suggestthat the outer loops and the shell of (fast) hot gas of the bipolarlobes are generically related to each other, whereas the shell of(slow) cool gas and dust has been formed in a different processat an earlier stage.

4.3. Structure of the central core region

4.3.1. High-velocity bipolar micro-jets

As shown in Sect. 3.3., the line features of [Nii] and [Oiii] ob-served in the core region of M 2-9 present a complex structurewhich has been spectrally and spatially resolved into three com-ponents, namely two HVCs (C+, C−) and a single LVC (C0).The results of a two-dimensional Gaussian fit (see Table 1) indi-cate that the redshifted C+ and the blueshifted C− are elongatedalong the N–S direction and that their position centroidsX areoffset (from the position of the central star) toward the N and S,respectively. This result suggests that C+ and C− are likely torepresent the two components of a high-velocity bipolar outflowwhich is oriented along the N–S direction and originating fromthe center of the core. The deduced spatial characteristics of C+

and C− (see Table 1) indicate that the outflow is collimated orjet-like. Moreover, the small angular offsetsX and small widths∆X deduced for C+ and C− suggest that the bipolar outflowsystem in the core region is rather compact (“micro-jets”).

Remarkably, the (projected) orientation (N–S) of the micro-jets is the same as that of the prominent bipolar lobes and thatof the faint outer loops. Moreover, the deduced (centroid) radialvelocitiesV of C+ and C− (see Table 1) indicate that the bipolaraxis of the jetsΘjets is inclined in the sense that the southernjet and the northern counter-jet are pointing towards and awayfrom the observer, respectively. A similar orientation of the flowaxis is observed in the bipolar lobes and the faint outer loops.

Hence it is most likely that the inclination angle of the jetsΘjets

is about the same as that derived for the bipolar lobes and theouter loops,i.e., Θjets ≈ 73◦.

Adopting the latter value and using the results of the Gaus-sian fit (V , ∆V ) obtained for C+ and C− (see Table 1), we canapply Eqs. (5) and (6) in order to deduce the flow velocity ofthe jetsV exp

jets and the (half) opening angle of their collimationΦjets. In the case of [Oiii], we deriveV exp

jets ≈ 137 km s−1 andΦjets ≈ 7◦; the corresponding values for [Nii] are∼195 km s−1

and∼7◦, respectively. Hence the flow velocity of the micro-jetsis higher by at least a factor of 3 compared to that of the hot-gas component of the bipolar lobes, but is of approximately thesame magnitude as that of the faint outer loops.

The deduced opening angleΦjets is remarkably small, muchsmaller than that of the bipolar lobes, suggesting a higher col-limation of the high-velocity flow in the core region. It is note-worthy that the opening angle subtended by the bright nebularcondensations N3 and S3 on the bipolar axis (see Fig. 1) appearsto be of approximately the same magnitude as that deduced forthe micro-jets. Therefore it is tempting to suggest that N3 andS3 represent regions of interaction of a highly collimated fastbipolar outflow from the center with slower material within thelobes. The fact that the (absolute) radial velocities deduced forN3 and S3 are somewhat higher than those observed in theirimmediate neighborhood supports that suggestion.

Surprisingly, the flow velocity of the micro-jets deduced forthe [Nii] emission is higher than that deduced for the [Oiii]emission. It should be noted that the higher and lower velocitiesof the jets observed in the [Nii] and [Oiii] emission, respec-tively, correspond to the larger and smaller offsets of the centroidpositionsX of the jets, derived for the respective lines. Thesefindings suggest that some acceleration process may be presentin the core region leading to a downstream stratification of thejet material. It seems that the material in the vicinity the source,which is characterized by higher excitation and lower velocity,is headed by the material at larger distances, which is of lowerexcitation and of higher velocity. The stratification along theoutflow direction of the jets resembles a situation generally ob-served in the shell structure of PNs presenting both an increaseof the expansion velocity and a decrease of the excitation if oneproceeds to larger distances from the central star.

As noted in Sect. 3.3., the HVCs in the core of M 2-9 presentrather weak [Oi] line emission, if any, but a rather strong [Oiii]emission, thus indicating relatively high excitation of the micro-jet gas. This result and the stratification effect mentioned aboveare more typical for photo-ionized gas regions, like PNs, butless typical for regions dominated by shock excitation, like jetsfrom young stellar objects. Thus it is likely that the main sourceof excitation of the jet gas in the core region of M 2-9 is photo-ionization rather than shock interaction.

The high velocity and the compactness of the micro-jetsindicate an extremely small kinematic age. For the adopteddistance of 650 pc, the resulting kinematic age of the visiblemicro-jets is∼5 years only. This age is significantly less thanthe “flowing time” of 22–27 years obtained from the deduced


“proper motions” of C+ and C− (see Sect. 3.3.). Unless the dis-tance of M 2-9 is much larger than adopted, the deduced kine-matic age cannot represent the dynamical age of the micro-jets.It might well be that the observed micro-jets are tracing a sta-tionary outflow process in the vicinity of the outflow source.This would be the case if the outflowing material is observableonly in a certain region of limited length and at a certain distancefrom the exciting source. Downstream, outside that region, thephysical conditions may be different such that the flow is notlonger traceable by particular emission lines. If this is the case,the dynamical age of the micro-jets could be larger than the de-duced “flowing time”. On the other hand, it cannot be excluded,that we are witnessing a very recent (and highly episodic) “out-burst” in the core region of M 2-9. Follow-up high-resolutionobservations of the core region are required in order to decidebetween the different propositions.

4.3.2. Dense disk in the equatorial plane of the bipolar jets

As already mentioned above, the strength of the observed [Nii]emission of the (redshifted) counter-jet (C+) is significantlylower compared to that of the (blueshifted) jet (C−). A similarsituation, even more pronounced, has been found in the [Oiii]emission. It is unlikely that these findings are caused by intrin-sically different strengths of the line emissions from the jet andcounter-jet. Hence it is probable, that the different line strengthsobserved are caused by differences in the foreground extinction.We propose that, similarly to the situation known from bipolaroutflows from young stellar objects, the central outflow sourceof M 2-9 is surrounded by an extended dense equatorial disk ortorus. The symmetry axis of the disk is considered to coincidewith the jet axis. Hence the “front half” of the inclined disk(i.e. its northern half) is likely to obstruct the inner parts of the(receding) counter-jet (C+) but will leave the (approaching) jet(C−) unaffected. Moreover, the presence of an extended densedisk also explains the different strengths of the [Oiii] and [Nii]lines observed in the counter-jet (C+) only. It is obvious that theobstruction of the (receding) counter-jet by an extended disk willbe more effective for the less extended [Oiii] counter-jet andless effective for the more extended [Nii] counter-jet.

The results from the Gaussian fit (I, X, ∆X) obtained forC− and C+ (see Table 1) permit us to derive a rough estimatefor the diameter of the obscuring equatorial disk (or torus). Forthat purpose it is assumed that the (intrinsic) line strength and(true) length of the jet are approximately the same as thoseof the counter-jet. The fact that the actually observed [Oiii]intensity of C+ is smaller by at least a factor of 5 than that ofC− suggests that a major fraction of the [Oiii] counter-jet isbeing obstructed by the disk. Hence the (projected) disk radius(along the direction of the projected axis) is probably largerthan the deduced offset of the [Oiii] centroid (X = 0.′′2) of the(un-obscured) C− (see Table 1). On the other hand, the fact thatthe observed [Nii] intensity of C+ is not much less than that ofC− suggests than only a minor fraction of the [Nii] counter-jetis being obscured by the disk. This implies that the (projected)disk radius is probably somewhat smaller than the offset of the

deduced [Nii] centroid (X = 0.′′6) of C+. Taking these resultstogether and assuming that the inclination angle of the disk axisis the same as that of the bipolar lobes (≈ 73◦), we derive arough estimate of 2′′–3′′ for the (de-projected) diameter of theequatorial disk (or torus) surrounding the central outflow sourceof M 2-9. It is noteworthy that interferometric radio observationsof the 12CO J = 1–2 emission by Zweigle et al. (1997) haverevealed the existence of a toroidal CO envelope encircling thenucleus of M 2-9. These observations indicate that the torus hasa diameter of about 6′′, and its symmetry axis is inclined by73◦ with respect to the line-of-sight. These results from radioobservation are compatible with our results from the forbiddenline observations in the core region. The toroidal envelope tracedby the CO line emission probably represents outer regions ofthe equatorial dust disk inferred from the observed obscurationof the (receding) counter-jet flow.

4.3.3. Slow wind from the core center

Besides the two HVCs (C−, C+), a single LVC (C0), hasbeen identified in the line emission from the core region whichpresents velocities of about –4 to –7 km s−1, much lower thanthose observed in the HVCs. This implies that C0 cannot bepart of the high-velocity jet flows, but is likely to represent a(physically) distinct low-velocity gas component (slow wind)originating from the central core region of M 2-9. Moreover, therelatively large strength of the [Oi] line (and the lower strengthof [O iii]) observed in C0 suggest a lower excitation (and prob-ably a higher density) of the slow wind compared to the fastmicro-jets.

In this context it is useful to refer to an analogous situationknown from mass outflows of young stellar objects (e.g. T Tauristars). In the spectra of many of these objects, a HVC and a LVChave been detected in the lines of [Oi], [N ii], and [Sii]. In thesecases the HVC has been attributed to a collimated fast stellarwind from the central star, whereas the LVC has been consideredto represent a slow wind probably originating from the accretiondisk which is surrounding the star (see, e.g., Solf & Bohm 1993,Kwan & Tademaru 1988). As already noted above, the existenceof a circumstellar (or circumbinary) disk or torus in M 2-9 hasbeen inferred from interferometric radio observations (Zweigleet al. 1997) and our observations of the micro-jets. It might wellbe that LVC (C0) represents a slow disk wind emanating fromthat disk. An other possible explanation could be, that the LVCrepresents a slow stellar wind from the evolved companion ofthe exciting star in the central binary system.

The (small) negative velocity deduced for C0 can be ex-plained if some portion of the receding part of the slow wind ishidden by the extended disk. Taking the inclination of the diskinto account we derive a rough estimate of about 20 km s−1

for the (mean) velocity of the slow-wind in the core region.The fact that the velocity dispersion∆V deduced for C0 (seeTable 1) is larger than the estimated flow velocity suggests arather larger opening angle of the slow-wind flow. It is note-worthy that the deduced velocity of the slow wind in the core


Table 2. Key results and empirical conclusions for the various bipolar outflow components in M 2-9

Outflow Basic structure Observed line component Outflow Inclination Opening Typical Kinematiccomponent velocity angle angle extent age

km s−1 degr degr arcs yrs

Central core Micro-jets HVC of [Nii], [O iii] 195, 137 (73) ∼7 ∼0.5 <10Central core Disk/stellar wind LVC of [Oi], [N ii], [O iii] ∼20 ∼1

Bright lobes Hot-gas shell NC of Hα, forbidden lines 46 73 ∼13 ∼27 1300Bright lobes Cool-gas/dust shell BC of Hα (dust scattered) 17 71 >40 ∼20 3800

Outer loops Cool gas and dust Hα (dust scattered) 141 73 <10 ∼60 1300

region is of the same magnitude as the flow velocity derived forthe cool-gas/dust shell of the bipolar lobes.

5. Conclusions

Long-slit spectrograms of high spectral and spatial resolution ofthe line emission obtained at various slit positions have enabledus to study in great detail both the kinematic and morphologicalproperties of the various components in the bipolar outflow ofM 2-9. The key results and empirical conclusions are summa-rized in Table 2.

Three main region of bipolar outflow have been distin-guished in the nebula: the central core region, the region of thebright bipolar lobes, and the region of the faint outer loops. In allthree regions, the identified outflow systems present high bipolarsymmetry with uniform orientation of the bipolar axes, whereasoutflow velocities, opening angles, and kinematical ages arelargely different from each other.

In the region of the central core, the existence of two phys-ically distinct gas components has been inferred for the firsttime from an analysis of the observed line emission, namely ahigh-velocity gas component, attributed to a system of bipolarjets (micro-jets), and a low-velocity gas component, attributedto a slow wind. The micro-jets present relatively high excitationand high collimation. Their kinematical age is extremely smallindicating that the central outflow source is presently still active.The observations suggest the existence of a dense equatorial diskwhich is obscuring the inner portions of the receding counter-jet. The slow wind shows lower excitation and (probably) higherdensities. It is suggested that the slow wind is originating eitherfrom the disk or from the evolved stellar component of the pre-sumed binary system in the center. Our results confirm interfer-ometric radio observations (Zweigle et al. 1997) indicating theexistence of a circumstellar disk or torus in M 2-9.

In each the bright bipolar lobes, a co-axial double-shellstructure has been inferred from the distinction between thenarrow line component (tracing emission formedin situ withinthe lobes) and the broad line component (tracing dust-scatteredemission). Our results represent refinements and clarificationsof previous results derived from near-infrared imaging and spec-troscopy (Hora & Latter 1994), spectropolarimetry (Trammellet al. 1995), and long-slit spectroscopic studies (Carsenty & Solf1983; Solf (1993). The outer shell consists of cool gas and dust

presenting low outflow velocities and a wide opening angle.The kinematical age of the outer shell is the largest of all neb-ular components of M 2-9. The inner shell, partially embeddedwithin the outer shell, consists of hot gas of relatively high ve-locity and high collimation. The highest velocities have beenfound on the bipolar axis at the positions of the bright nebu-lar condensation N3 and S3. The kinematics of N1, S1, N2, S2suggest that these condensations are located near the “boundarylayer” separating the inner and the outer shell.

In the region of the faint outer loops, the observations indi-cate very high velocities and high collimation of the outflowinggas and dust, thus confirming previous long-slit spectroscopicresults (Solf 1993; Schwarz et al. 1997). Remarkably, the kine-matical age of the faint outer loops is comparable with that ofthe bright bipolar lobes, suggesting that both the faint loops andthe bipolar lobes were formed at the same time.

The observations strengthen the hypothesis that the bipolarnebula M 2-9 has little in common with ordinary planetary neb-ulae. More likely, M 2-9 represents an evolved interacting (sym-biotic) binary system which is a (continuously active) source ofcollimated high-velocity bipolar outflow. Similarly to the case ofsymbiotic novae (e.g., V1016 Cyg, HM Sge, RR Tel), mass ex-change between the binary components seems to be the drivingagent leading to the formation of a circumstellar (or circumbi-nary) disk and to the ejection of fast bipolar jets. The hot gas ofthe jets is boring through a (circumbinary) envelope of cool gasand dust, previously deposited by the slow stellar wind from theevolved component of the binary. In the case of M 2-9, the bipo-lar ejection of the fast hot gas started about 1300 years ago, asindicated by the age of both the faint outer loops and the brightbipolar lobes. The ejection process is presently still active, asindicated by the bipolar micro-jets detected in the central core.

Acknowledgements.I am grateful to the staff at the Calar Alto observa-tory for assistance during the observation. I would like to thank BruceBalick for valuable discussions and suggestions on the subject.

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Astron. Astrophys. 354, 674–690 (2000) ASTRONOMY …aa.springer.de/papers/0354002/2300674.pdf · J. Solf: A study of the various bipolar outﬂow components in M2-9 675 HST WFPC2