Morphology and expansion characteristics of the planetary nebula M1-79

Transcription

1 Astron. Astrophys. 326, (1997) ASTRONOMY AND ASTROPHYSICS Morphology and expansion characteristics of the planetary nebula W. Saurer Institut für Astronomie, Leopold-Franzens-Universität, Technikerstr. 25, A-6020 Innsbruck, Austria Received 31 January 1997 / Accepted 22 April 1997 Abstract. In this paper we present deep broad- and narrowband images (Johnson B, Gunn-red, Hα, Hα+[NII], [NII], [OIII]) and high resolution spectroscopy (Fabry-Pérot interferometer (Hα,Hβ, [NII], [OIII]) and Coudé measurements (Hα, [NII])) for the planetary nebula (PN). Systemic and expansion velocities are investigated along with the morphological characteristics of this PN. exhibits an elliptical central region surrounded by a bipolar structure (lobes) which is probably much more extended than previously known. Various symmetry axes of the projected shape of this PN are discussed with respect to a three dimensional model. Two axes cannot be explained by projection effects. They give evidences for ejections along different axes during the formation process of this PN. A possible scenario of the evolutionary process of is discussed. Key words: planetary nebulae: individual: ; general 1. Introduction Recent years have brought a dramatical development and progress in our understanding of the evolution of stars from the asymptotic giant branch to the white dwarf stages. A main contribution to puzzle out this process can be attributed to detailed investigations of the morphology and kinematics of planetary nebulae (PNe). The foundations for our insight in this evolutionary process were laid by a variety of authors, e.g. Kwok et al. (1978) who associated the ideas of the stars development with the origin of the PNe s shells (two-winds model, interactingwinds model), and Balick (1987) who created a classification scheme which put together the evolution of the PNe s shell and the surrounding medium, mainly determined by the former stellar wind. As a very rough summary of the nowadays established scenario the shell of PNe is formed by a fast wind which drives a Send offprint requests to: Walter.Saurer@uibk.ac.at Visiting Astronomer at the Centro Astronomico Hispano-Aleman, Calar Alto, operated by the Max-Planck-Institut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy. shock wave into the non-isotropic remnants of a slow wind and leads to a significant density enhancement. Non-isotropic slow winds can be produced by a rotating single star without (Dorfi &Höfner 1996) and with a binary companion (Livio 1995), or in the presence of a toroidal magnetic field (Chevalier 1995). The basic idea of the two-winds model was taken as a starting point for many subsequent discussions on the origin and evolution of PNe (e.g. Mellema et al. 1991, Icke et al. 1992, Frank et al. 1993, Stanghellini et al. 1993, Corradi & Schwarz 1995, Pascoli 1995). However, there are indications more and more frequently that the shaping of PNe is substantially affected also by additional characteristics of the evolutionary process like multiple shell ejection, binarity of the progenitor star, and precession effects (see, e.g. Miranda & Solf 1992, Corradi & Schwarz 1993, Miranda 1995, Cliffe et al. 1995, Palmer et al. 1996, Manchado et al. 1996a). (PN G , α =21 h 37 m 00. 6, δ = ) was discovered by Minkowski (1946) and included in some papers dealing with statistical properties of PNe. Manchado et al. (1996b) have classified this PN as an elliptical PN with inner structure (their class Es). The first individual treatment of can be found in Saurer (1997, hereafter Paper I). In that paper a three-dimensional model for was proposed. The aim of this paper is to study the morphological and kinematical characteristics of this PN in particular with regard to the model proposed in Paper I, and to discuss a possible evolutionary scenario. 2. Observations 2.1. Direct images The images taken with broad band filters (Johnson B (central wavelength/bandwidth 4360/920 Å) and Gunn-red (6560/840 Å), 300 and 600 sec, respectively) were obtained in May 1986 using the 2.2 m telescope on Calar Alto with a RCA (SID501 EX) detector (pixelsize 30 µm, corresponding to 0. 3 per pixel). Standard reduction was applied using MIDAS. Narrow-band images were obtained with the 2.2 m telescope on

2 1188 W. Saurer: Morphology and expansion characteristics of the planetary nebula Calar Alto, too, with a RCA (SID 006 EX) detector in October The chip used has a pixelsize of 15 µm, which corresponds to a spatial scale of per pixel. The interference filters used for these observations (Hα+[NII] (1 800 sec), [NII] (900 sec), and [OIII] (1 800 sec)) were centered at 6574 Å, 6590 Å, and 5005 Å with pass bands of 104 Å, 20 Å, and 90 Å, respectively. After applying standard reduction procedures the images were corrected for bad columns using an intensity dependent mask which was derived from flatfields. For all direct images the seeing was 1. 5 ± High resolution spectroscopy Fabry-Pérot spectroscopy These observations were carried out in October 1984 and May 1986 with a pressure scanned Fabry-Pérot spectrometer (Hippelein & Münch 1981) attached to the 1.23 m telescope on Calar Alto. The nebula was observed using the interference filters Hβ 4866/11, [OIII] 5011/18, [OI] 6309/12, Hα 6570/15, and [NII] 6590/20. The integration time was 10 sec (20 sec for Hβ and [OI]) for each data point with a velocity separation of 3.1 km/s and a spectral resolution of 10 km/s. The scans were taken at the center of the nebula with apertures of 10,20,or 60. They were sky subtracted by means of measurements at positions nearby the nebula and corrected for atmospheric extinction. The wavelength calibration was done using laboratory lamps. Absolute flux calibration was carried out using scans of the standard nebulae NGC 6543 and NGC 7027 and the line fluxes as tabulated by Hippelein (1984). To interprete the line profiles qualitatively we have approximated the observed data by least square gaussian fits with one or more underlying gaussian functions. To keep authenticity we have not smoothed the data before fitting Coudé observations These observations were carried out in November 1990 with the coudé spectrograph at the 2.2 m telescope on Calar Alto. We used the f/3 camera and as detector a RCA (SID 006 EX) CCD-chip ( pixel, 15µ per pixel) with the long side chosen as wavelength axis. The reciprocal linear dispersion is 8.7 Å/mm which corresponds to 0.13 Å/pixel. The spectral resolution is 0.4 Å (FWHM). The spatial scale is 0. 50/pixel. Two coudé observations were made with position angles (P.A.) 80 (slitwidth 1, sec) and 90 (2, sec). For all observations an image-derotator was used. The calibration was done using a Th-Ar laboratory lamp. To guarantee an exact guiding the telescope was centered at the only star visible within the eyepiece. In Fig. 1 this star is located at the very western border halfway in vertical direction peeping into the image from the right (its spectrum can be seen in Fig. 5 b). Unfortunately, due to the limited length of the slit it was only possible to get a spectrogram of the western part of. The P.A. used for the observations are in respect to this star. 3. Results 3.1. Morphology In Fig. 1 a f we present a gallery of direct CCD-images of M1-79 in the light of Johnson B, [OIII], Hα+[NII], [NII], and Gunnred. When investigating the morphology of it turned out that this can best be done by considering three distinct regions: the inner (elliptical) part and an inner and outer emission region. All these different regions exhibit point-symmetry along their outer edges. In the following we will discuss these regions of the nebula The central region The images taken with the Johnson B and [OIII] filters (Fig. 1 a, b) suggest a classical torus-like shell seen almost edge on. However, the geometrical minor axis of the projected ellipse (P.A. 175 ) and the axis defined by the two intensity enhancements (P.A. 14 ) are tilted. When changing the dynamical range of the presentation this global appearance will not change. In Paper I we have shown that this phenomenon can be explained geometrically by assuming three-axial ellipsoidal shapes of the inner and outer boundaries of the PNs shell seen under a specific viewing angle (α, β). The projected appearance of the central part of thus motivates to introduce two axes of symmetry for this PN. On the other hand, when investigating the Hα+[NII] and [NII] images, the appearance of is strongly dependent on the dynamical range of the representation (see Fig. 1 c and d). By lowering the dynamical range the nebula shows its microstructure which is mainly caused by low-ionization regions. exhibits a most complex structure and looses pointsymmetry when considering details. The two intensity enhancements exhibit inner structure by breaking into two (three) knots in the southern (northern) part. A single knot without pointsymmetrical counterpart can be seen to the east of the northern density enhancement. In the western part of the nebula an almost vertical line can be identified which connects the bright edges of the shell. Also this feature cannot be seen in the eastern part of. In Fig. 2 a cut through the intensity axis of the central part of is shown which gives the relative brightness distribution (normalized to 1 for the peak values) in the lines of Hα+N[II] (full line) and [OIII] (broken line). It can be recognized that the outer edges of appear sharper in Hα+N[II] than in [OIII] and the size of the nebula is larger in Hα+N[II]. Both characteristics indicate that the edges can be regarded as well defined ionization fronts together with the presence of ionization stratification. The fact that the intensity ratio between the peak and the central trough is higher in Hα+N[II] than in [OIII] can be explained by projection effects (see Paper I). The size of the central region is 30 along the intensity axis, 24 along the minor geometrical axis, and 46 along the major geometrical axis measured in Hα+N[II].

3 W. Saurer: Morphology and expansion characteristics of the planetary nebula 1189 a) Johnson B b) [OIII] c) Hα + [NII] d) Hα + [NII] e) [NII] f) Gunn-red Fig. 1. Direct image gallery for. The size of all images is 80 60, North is to the top and East to the left 1,2 1,0 Hα [OIII] 0,8 relative intensity 0,6 0,4 0,2 0, position (arcsec) Fig. 2. The relative brightness distribution of in the light of Hα+N[II] and [OIII] along a cut through the intensity axis. The brightness was normalized in both cases to let the peak values The inner halo emission In Fig. 3 we present a high contrast image in Hα+[NII]. It reveals the presence of a very faint, diffuse and aspherical emission. The boundaries of this emission are defined by a sharp decrease in intensity which can be described geometrically by an elliptical part (P.A. 100 ) and a pair of pincer-like lobes (P.A. 140 ) put on in the north-west and south-east (see also Fig. 1). This Fig. 3. High contrast direct image of in the light of Hα+[NII]. The size of the image is 77 86, North is to the top and East to the left emission exhibits point-symmetry, too, but it cannot be associated with any axis of symmetry of the inner region. Whereas the tilt of the elliptical part could be caused and/or influenced by superposition of the central region and the lobes, the lobes

4 1190 W. Saurer: Morphology and expansion characteristics of the planetary nebula are clearly defining an additional axis of symmetry. They seem to originate directly from the intensity enhancements The outer bipolar lobes Fig. 3 indicates that the two lobes extend to a much larger distance. In the lower left corner of this figure a very faint bowshaped feature can be noticed. In addition, a similar feature, but even fainter, can be recognized in the upper right corner. When following the optical appearance of these two bows it seems that they continue the south-eastern and north-western pincerlike lobes and also maintain the symmetry. These features are positioned correctly when assuming point-symmetry with the central star of as a reference. If these features are part of the shell, the largest size of this PN would be Kinematics Fabry-Pérot measurements Systemic radial velocities. In Fig. 4 we present the measurements with the Fabry-Pérot interferometer in the light of Hα, Hβ, [NII], and [OIII]. The measurements in [OI] are not shown here because we could not detect any signal. The maximum of the Hα measurements entirely covering the nebula (Fig. 4 c) will give the systemic velocity to a high degree of reliability, without any further assumptions on geometry. Our result is V hel = 29.8 km/s and the symmetry of the data also implies an approximate symmetrical behaviour of the expansion. In agreement are the results for the [NII] line with a small aperture (Fig. 4 d, 27.6 km/s) and for [OIII] (Fig. 4 e, 29.5 km/s). The data achieved in Hα with smaller apertures (Fig. 4 a and b) need to be discussed in more detail. Both measurements exhibit an asymmetrical gradient concerning the wings. The slope of the approaching wing is significantly steeper than that of the receding part, indicating kinematical components in addition to the main component which represents the whole expanding shell. In Fig. 4 b a two-component gaussian was used to fit this shape. It is a common feature that in the light of Hα the line splitting caused by the expansion of the shell is less pronounced, even when using small apertures (see, e.g. Gieseking et al. 1986; Hippelein & Weinberger 1990). Fig. 4 a gives the measurements with a very small aperture. We have used a two- (solid lines) and a three-component (dotted lines) gaussian to fit these data. Although both fits can account for the observed shape, we prefer the latter one, because it leads to a more symmetrical alignment of the two main components in respect to the bulk velocity assumed. The measurement in Hβ (Fig. 4 f) exhibits too much scatter to fit two gaussians. However, the least square fit of one gaussian leads to a proper result. In short, all measurements with the Fabry-Pérot interferometer presented here are consistent with the assumption of one systemic radial velocity common to all emission lines (V hel (mean) = 28.5±5 km/s). This is a considerable improvement compared to earlier measurements, since the only value found in the literature is 24±25 km/s (Schneider et al. 1983). The results of all measurements together with the errors are summarized in Table 1. The errors given in this table are reflecting only the errors of the mathematical process of fitting. The total errors including wavelength calibration can be estimated to be smaller than 5 km/s (Hippelein & Weinberger 1990). We will accept this value for all our velocity data. Expansion velocities. If a pronounced double peaked structure is obvious, the expansion velocity can be deduced by means of the separation of the two peaks (assuming a symmetrical expansion in the line of sight). When no splitting is indicated in the observed line profile, the measured value of the FWHM can be approximated by a quadradic sum of the expansion velocity, the FWHM of the instrumental broadening, and the thermal broadening (Robinson et al. 1982, Gieseking et al. 1986, Holzmüller et. al 1987, Banerjee et al. 1990): V exp = 1 2 (FWHM2 obs FWHM 2 instr 8(ln 2)kT e /m) 1/2 (1) The electron temperature T e can be approximated to be 10 4 K, k is the Boltzman constant, and m the atomic mass of the element considered. The transition zone between these two extremes is more complicated to treat and has to be discussed separately. This was done by Holzmüller et al. (1987) by calculating the expected theoretical line profile of an expanding shell and will be shortly summarized in the following (for a visualization see their Fig. 1). When making the aperture smaller the one peak will double and a central trough will form in the measurements. With smaller apertures the separation of the two peaks will grow larger and the central trough will become deeper. At stages where the central trough is larger than at least 50% of the peak value the separation of the two peaks is much smaller than 2 V exp and can only be regarded as a (very low) lower limit for 2 V exp. The determination of the expansion velocities with the FWHM-method will give an upper limit in those cases (Fig. 4 a and e). The expansion velocities, together with the errors are given in Table 1. Because both the methods used to determine the expansion velocities do not depend on wavelength calibration, the fitting errors can be regarded as a good estimate for the total errors here. The FWHM instr was determined using calibration lamps: 12 km/s. The mean expansion velocity of according to our measurements is 22.7 km/s. However, the results obtained for different ions may not agree necessarily, as was pointed out by Weinberger (1989) and Gussie & Taylor (1994). The expansion velocities measured from [OIII] tend to be higher than those measured from hydrogen. This trend can also be recognized in Table 1. Only one measurement of the expansion velocity could be found in the literature: 19 km/s, measured in the light of [OIII] (Sabbadin et al. 1985).

5 W. Saurer: Morphology and expansion characteristics of the planetary nebula 1191 flux (10-14 erg cm -2 (km/s) -1 s -1 ) 2,5 1,5 1,0 0,5 a) ap. 10" Hα v exp (km/s) ,0 b) 8,0 ap. 20" 6,0 Hα 4,0 0,0 v (km/s) exp ,0 c) 15,0 ap. 60" Hα 10,0 5,0 0,0 v (km/s) exp flux (10-14 erg cm -2 (km/s) -1 s -1 ) , ,5 d) 4,0 e) f) 3,0 4,5 3,5 ap. 10" 4,0 2,5 ap. 10" ap. 20" [NII] 3,0 [OIII] 3,5 Hβ 2,5 3,0 1,5 2,5 1,0 1,5 0, , (km/s) v (km/s) hel v (km/s) hel v hel Fig. 4. Fabry-Pérot measurements. The abscissa at the bottom of each figure is the heliocentric velocity. The vertical line corresponds to a heliocentric velocity of 30 km/s. The abscissa at the top of each figure gives the expansion velocity Absolute fluxes. In Table 1 we also present absolute fluxes for. As above, the errors given are only due to the fitting procedure. We have no tools to really calculate the total errors. However, a comparison of the data for objects which were observed more than once shows that the errors can be estimated to be 30% Coudé observations In Fig. 5 we present the measurements made with the coudé spectrograph. The open symbols and the filled symbols (circles for Hα, squares for [NII] 6583) represent the measurements with P.A. 90 (horizontal cut southern to the center of ) and 80 (along the major geometrical axis of ), respectively. Each data point was obtained by averaging two lines in Fig. 5 a (b) and fitting a gaussian to get the central wavelength. As it was for the Fabry-Pérot measurements no line-splitting can be seen in Hα. Moreover, the heliocentric systemic velocity and the expansion velocities achieved are in good agreement with the Fabry-Pérot results. However, in the line of [NII] 6583 an asymmetry between the blue- and red-shifted component can be recognized which is more pronounced in the measurement with P.A. 80. A comparison of these results with the proposed model of Paper I (α =20,β=20 ) can be found in Fig. 5 c. The curves in this figure give the synthetic observations due to the outer model ellipsoid expanding with a velocity proportional to the distance to the center of this ellipsoid. The expansion velocity at a normalized distance of 1 was set to be 23 km/s. According to the model the minimum and maximum expansion velocity of the shell would be 18 km/s and 35 km/s (along the directions of the normalized semi-diameters 0.8 and 1.5). The asymmetries in the expansion velocities could be properly explained by projection effects. These measurements also allow to eliminate the ambiguity concerning the viewing angle (α, β). According to the result of the fit in Fig. 5 c the ellipsoids of the model are seen from below as shown in Fig. 1 of Paper I. This can also be deduced from Fig. 5 a and b. In the light of [NII] (especially in [NII] 6583) the blue- and redshifted parts both exhibit a maximum in intensity. However, the maximum of the redshifted part is located more westerly than its counterpart. With the assumption that these maxima arise from the equatorial plane this is only possible when the model is seen from below. 4. Discussion The most striking feature of can be extracted from the direct images as the existence of several axes of symmetry which are defined in the plane perpendicular to the line of sight by the geometrically projected inner ellipse (P.A. 85, major axis), the intensity enhancements (P.A. 14 ), and the two pairs of lobes (+ outer lobes, P.A. 140 ). In the following we will discuss a possible evolutionary scenario for using the information we got from the direct images and the threedimensional model of Paper I.

6 1192 W. Saurer: Morphology and expansion characteristics of the planetary nebula Table 1. Results of the Fabry-Pérot measurements. The second column gives the number of gaussian components used for fitting. Diameters of apertures are given in arcseconds; the component according to Fig. 4 is denoted by main (main component), sec (secondary component), left (blueshifted component), and right (redshifted component); values with errors 0.0 have been fixed when fitting multiple gaussians. Expansion velocities were calculated according to Eq. (1) except those denoted by an asterisk which were determined using the separation of the two intensity peaks. line # comp. ap. Fig. 4 comp. V hel (comp.) FWHM V hel V exp flux flux (total) used ( ) (km/s) (km/s) (km/s) (km/s) (10 14 ergs cm 2 s 1 ) Hα 3 10 a left 45.0± ± > ± a right 14.1± ±4.3 < ± a sec 47.5± ± ± 3.3 Hα 2 20 b main 27.4± ± ± b sec 52.5± ± ± 4.9 Hα 1 60 c main 29.8± ± ± [NII] 2 10 d left 51.7± ± ± d right 3.4± ± ± 3.1 [OIII] 2 e left 43.6± ± > ± e right 15.3± ±5.4 < ± 8.3 Hβ 1 20 f main 27.3± ± ± We can immediately reduce the number of axes of symmetry to be investigated by means of the proposed model for. The tilt between the intensity axis and the inner ellipse is due to the specific viewing angle. Moreover note that the elliptical part of the inner halo emission (P.A. 100 ), although less confirmed, could be obtained by projecting the envelope of the ellipsoidal model onto the sky (see Fig. 3 and Fig. 1 c in Paper I). This observational result could be expected when a small fraction of the ionizing photons can penetrate the ionization front which are responsible for this light. Morphological asymmetries in the shapes of PNe can be due to the interstellar environment into which a PN is growing. For there are strong indications that the surrounding medium has influenced the shell. The borders of the emission are surprisingly sharp-edged, even in the light of [OIII] in which the overall appearance of this object seems to be rather smooth. This can be interpreted in terms of an ionization front that penetrates into a very dense surrounding material. As would be expected, the diameter of the PN is smaller in the light of [OIII] than in Hα+[NII] and [NII] (Fig. 2). is thus expected to be ionization bounded. Mainly because of the overall point-symmetry of the different regions of we rule out the possibility that the interstellar medium itself can be responsible for the remaining two axes of symmetry which should be discussed. It also would be difficult to explain the two pincer-like lobes and the outer bipolar lobes. A second possible reason for pronounced individual shapes of PNe can be seen in different initial conditions of their evolution or in changing conditions during their evolutionary process. It is evident from Fig. 1 that the central elliptical part of exhibits a polar to equatorial density contrast (see also Paper I). From this we can deduce that this PN has developed at least during a certain time according to the two-winds model. However, the tilt ( 50 ) between the inner ellipse and the two pairs of lobes (and the outer bipolar lobes) cannot be explained by projection effects. We now proceed in estimating the directions of the axes of symmetry in three-dimensional space. The projected polar axis of the model of Paper I would give a P.A. of 110.Onthe other hand, the geometrical appearance of the two pairs of lobes in Fig. 1 give rise to the assumption that they are representing the limb brightened part of a three-dimensional cylindrical- or barrel-shaped structure. In this case the three-dimensional tilt between the polar axis of the model and that of the two pairs of lobes cannot differ very much from 50, disregarding the inclination of the lobes to the line of sight (as far as this inclination is not too large). The two polar axes of strongly indicate that the asymmetrical shape of this PN can be attributed to multiple shell ejection of the progenitor star. However, the axis of symmetry has to change between the two subsequent ejections. This idea was introduced by Manchado et al. (1996) when investigating PNe with two pairs of lobes (quadrupolar PNe). Adopting a distance of 2.2 kpc (Kaler 1983, Tajitsu & Tamura 1996) the linear extent of the semi-diameter of the major geometrical axis would be 0.23 pc and that of the outer emission 0.43 pc. With a mean expansion velocity of 23 km/s the time between two subsequent ejections can be estimated to be years. If the velocity of the first ejection would be somewhat higher, say 40 km/s, this time difference would be lowered to years. When adopting a higher distance of 2.6 kpc (Cahn et al. 1992) these two time differences would amount to and years. Due to the results of Manchado et al. (1996) the precession period for a very similar case (precession angle 53 ) is less than years. They also have discussed how to realize this scenario. Although not quadrupolar, also for this idea seems to be a most natural and feasible explanation for its shape. In order to make the progenitor star or a density enhanced disk precessing an external torque is needed. A possible rea-

7 W. Saurer: Morphology and expansion characteristics of the planetary nebula expansion velocity (km/s) a) Coudé observation with P.A. = 80 relative position (arcsec) heliocentric velocity (km/s) b) Coudé observations with P.A. = 90 c) Velocity-position diagram for both P.A. 80 and 90 Fig. 5. High resolution long-slit coudé spectrograms of obtained with P.A. 80 and 90. Part a) and b) shows a linear grey scale representation of the Hα-line and the two [NII]-lines. Part c) of this figure is the corresponding velocity-position diagram of Hα (cirles) and [NII] 6583 (squares) for P.A. 90 (open symbols) and 80 (filled symbols). The vertical line represents a heliocentric velocity of 30 km/s, hence the abscissa at the top gives the approximate expansion velocity. West is at the bottom, East at the top. Note that the P.A. refer to a field star (see section 2.2.2). This was taken into account also for the model son for this torque could be a companion of the progenitor star (Manchado et al. 1996). Because there is an ongoing discussion on a possible link between binarity and bipolar PNe we have investigated if satisfies (+) the criteria of classical bipolar PNe. We refer to Corradi & Schwarz (1995) who have investigated a sample of more than 400 objects and have derived six properties in which elliptical and bipolar PNe differ statistically: a) (+) has a distance of 90 pc from the galactic plane. b) (±) The systemic velocity corrected for the Local Standard of Rest leads to 21 km/s. However, no conclusions can be drawn from this information. c) (+) Jacoby & Kaler (1989) have measured the visual brightness of the central star. Their results are V = mag and T Z = K (hydrogen Zanstra temperature); log(t Z ) = 5.1 is more typical for bipolar than for elliptical PNe. d) (±) There are only few data available on the abundances of elements in. The ratios He/H = 0.18 and O/H = would be in favour of a bipolar PN whereas N/O = 0.45 is typical for elliptical PNe. All values are from Kaler (1983). e) ( ) We have no indications of high outflow velocities in our data. f) ( ) is not really of giant size, even not when adopting a diameter of 0.86 pc. This comparison shows that also shares several physical properties with an average bipolar PN in addition to its morphological shape. This gives further indications that the formation process of this PN should be treated within a binary framework. Altogether we may conclude that the PN can be regarded as a candidate for having a noticeable asymmetrical evolutionary history. 5. Conclusions In summary, seems to have two different axes of polar symmetry in three-dimensional space. This fact can be regarded as a strong observational evidence that it has not maturated in a simple way. It surely has experienced an agitated evolutionary history. appears as a PN with an elliptical central part and a bipolarity in the outer regions. It exhibits an overall point-symmetry along the edges but not when considering details of its inner structure. There are strong indications to make a companion of its central star responsible for the shape of this PN. On the one hand the coudé measurements agree with the proposed three-dimensional model of Paper I. This also would imply a non-circular distribution of the mass within the equatorial plane. On the other hand exhibits characteristics of classical bipolar PNe. However, both the size and the expansion velocities are not typical for bipolar PNe. Probably has not yet reached the phase of a classical bipolar PN.

8 1194 W. Saurer: Morphology and expansion characteristics of the planetary nebula Acknowledgements. The author is grateful to Prof. R. Weinberger for helpful discussions and carefully reading the manuscript. We also would like to thank for observing time at the 1.2 m and the 2.2 m telescopes of the German-Spanish Calar Alto Observatory. For their helpfulness special thanks go to the staff of this observatory. We also would like to thank U. Boorgest, J. Schramm, H. Hippelein, and R. Weinberger who have made some observations used in this paper during their observing runs. This work was supported financially by the Fonds zur Förderung der wissenschaftlichen Forschung, project P 5708 (travel costs), by the Österreichische Forschungsgemeinschaft, project 06/1006 (travel costs), and by the Jubiläumsfonds der österreichischen Nationalbank, project 4713 (computer facilities). References Balick B., 1987, AJ 94, 671 Banerjee D.P.K., Anandarao B.G., Jain S.K., Mallik D.C.V., 1990, A&A 240, 137 Cahn J.H., Kaler J.B., Stanghellini L., 1992, A&AS 94, 399 Chevalier R.A., in Asymmetrical Planetary Nebulae, ed. A. Harpaz & N. Soker, Ann. Israel Phys. Soc., 11, 240 Cliffe J.A., Frank A., Livio M., Jones T.W., 1995, ApJ 447, L49 Corradi R.L.M., Schwarz H.E., 1993, A&A 268, 714 Corradi R.L.M., Schwarz H.E., 1995, A&A 293, 871 Dorfi E.A., Höfner S, 1996, A&A 313, 605 Frank A., Balick B., Icke V., Mellema G., 1993, ApJ 404, L25 Gieseking F., Hippelein H., Weinberger R., 1986, A&A 156, 101 Gussie G.T., Taylor A.R., 1994, PASP 106, 500 Hippelein H., Münch G., 1981, Mitt. Astron. Ges. 54, 193 Hippelein H., 1984, Fabry-Pérot-Interferometer, Benutzeranleitung, Max-Planck-Institut für Astronomie, Heidelberg, Germany Hippelein H., Weinberger R., 1990, A&A 232, 129 Holzmüller G., Huemer G., Saurer, W., 1987, Mitt. Astron. Ges. 70, 339 Icke V., Balick B., Frank A., 1992, A&A 253, 224 Jacoby G.H., Kaler J.B., 1989, AJ 98, 1662 Kaler J.B., 1983, ApJ 264, 594 Kwok S., Purton C.R., Fitzgerald M.P., 1978, ApJ 219, L125 Livio M., 1995, in Asymmetrical Planetary Nebulae, ed. A. Harpaz & N. Soker, Ann. Israel Phys. Soc., 11, 51 Manchado A., Stanghellini L., Guerrero M.A., 1996a, ApJ 466, L95 Manchado A., Guerrero M.A., Stanghellini L., Serra-Ricart M., 1996b, The IAC Morphological Catalog of Northern Galactic Planetary Nebulae, Instituto de Astrofísica de Canarias Mellema G., Eulderink F., Icke V., 1991, A&A 252, 718 Minkowski R., 1946, PASP 58, 305 Miranda L.F., 1995, A&A 304, 531 Miranda L.F., Solf J., 1992, A&A 260, 397 Palmer J.W., López J.A., Meaburn J., Lloyd H.M., 1996, A&A 307, 225 Pascoli G., 1995, Ap&SS 234, 281 Robinson G.J., Reay N.K., Atherton P.D., 1982, MNRAS 199, 649 Sabbadin F., Ortolani S., Bianchini A., 1985, MNRAS 213, 563 Saurer W., 1997, A&A in press (Paper I) Schneider S.E., Terzian Y., Purgathofer A., Perinotto M., 1983, ApJS 52, 399 Stanghellini L., Corradi R.L.M., Schwarz H.E., 1993, A&A 276, 463 Tajitsu A., Tamura S., 1996, preprint Weinberger R., 1989, A&AS 78, 301 This article was processed by the author using Springer-Verlag LaT E X A&A style file L-AA version 3.