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1997MNRAS.284...32P Mon. Not. R. Astron. Soc. 284, 32-44 (1997) Imaging and spectroscopy of ejected common envelopes - I D. L. Pollacea 1 and S. A. Bell2 1 2 Isaac Newton Group, Apartado de Correos 368, Santa Cruz de La Palma, 38750 Teneri/e, Canary Islands, Spain Royal Greenwich Observatory, Madingley Road, Cambridge CB30EZ Accepted 1996 July 17. Received 1996 July 8 ABSTRACT Imaging and spectroscopy are presented for four planetary nebulae known to contain close binary central stars resulting from a recent phase of common-envelope evolution. All the objects appear to exhibit axisymmetric nebulae which may be interpreted as inclined bipolar nebulae. In at least one object the bipolar caps can be seen, demonstrating the tightly collimated nature of the PN having an aspect ratio of -7. Determinations of the nebular abundances show that He is significantly enhanced in all of the objects. These results are in agreement with theoretical expectations. Uncertainties in the nebular electron temperature constrain other abundances less well. The line fluxes indicate that N is unexpectedly underabundant. This effect is probably not real and may be an artefact of electron temperature fluctuations within the nebulae. Key words: ISM: abundances - planetary nebulae: general - planetary nebulae: individual: Abell 41 - planetary nebulae: individual: Abell 46 - planetary nebulae: individual: Abell 63 planetary nebulae: individual: Abell 65. 1 INTRODUCTION Binary central stars of planetary nebulae (pNe) are important systems in the comparison of observational data with predictions from stellar evolution theory. For example, Pollacco & Bell (1993, 1994) and Bell, Pollacco & Hilditch (1994) have used the eclipsing central stars of the PNe Abell 63 (l1u Sge) and Abell 46 (V477 Lyr) to derive essentially model-independent masses and radii (with errors generally <10 per cent) for their components. Assuming that the primary components can be considered to be true post-asymptotic giant branch stars and not a product of close binary evolution, these results allow meaningful comparisons to be made with theoretical evolutionary tracks (SchOnbemer 1981, 1983), and the agreement is found to be excellent. These systems are of further interest as they must have recently passed through a common-envelope phase. Then & Tutukov (1989) and Then & Livio (1993) have studied common-envelope ejection in terms of stellar evolution and showed that, depending on the epoch at which the common-envelope phase occurs, a substantially different chemistry from that found in normal PNe may result. They expect He .and N to be enhanced during most phases of ejection, and in extreme cases they suggest that ejected shells composed almost entirely of He and N could be produced. The only objects that are known to display helium enrichment to this extent are the so-called 'hydrogen-deficient' PNe, which constitute a small class of objects of which the most famous are Abell 78 and Abell 30 (Hazard et al. 1980; Jacoby & Ford 1983), and Abell 58 (Seitter 1987; Pollacco et al. 1992). The origin of the abnormal abundances in these objects is usually thought to be a late thermal pulse which forced the central star to return to the AGB and a second PN phase (Then et al. 1983). However, there is no clear evidence to support the contention that any of these objects are binaries, and objects such as those hypothesized by Then & Tutukov remain to be discovered. Bond (1989) has listed the observational parameters for the known binary central stars. Nearly all of these objects exhibit extremely low surface brightness PNe, and were discovered by monitoring campaigns with conventional photoelectric photometers. Indeed, in the original surveys for these objects, candidates were often selected on the grounds that they appeared to have low surface brightness PN e and bright central stars. Hence in this sample there is a lack of objects that reside in 'typical' high surface brightness PNe. The interacting winds model for PN formation was originally proposed by Kwok, Purton & Fitzgerald (1978), and originally applied to spherical nebulae. In this model the PN is illuminated by an interaction of the slow wind of the progenitor AGB star with the fast wind of the central star. The swept-up shell is essentially the visible shell. If the slow wind distribution were axisymmetric, it was realized that aspherical PNe could be produced (e.g. Kahn & West 1985). More detailed physical treatments including photoionization processes and gas dynamics have been able largely to reproduce observed features (e.g. Frank et al. 1993; Frank & Mellema 1994). However, the nature of the process giving rise to the density contrast in the slow wind of the progenitor star is still largely unknown. Binary central stars have been suggested as a © 1997RAS © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P Ejected common envelopes - I natural way to produce the density contrast (e.g. Soker & Livio 1988). In a study of nebular morphology for 13 PNe with binary central stars, Bond & Livio (1990; hereafter BL) find only two objects that have obvious bipolar structure, while a further six probably have elliptical configurations. Only one object is suspected of being spherical. The rest of the sample are irregular or peculiar in some way. It has already been pointed out that these objects have a low surface brightness, and imaging with larger instruments may reveal further details. Deep Ha imaging of Abell 63 (Walton, Walsh & Pottasch 1993), an object classified by BL as an elliptical PN, clearly demonstrates its strongly bipolar nature. In this paper we present long-slit spectroscopy of four objects (Abell 41, Abell 46, Abell 63 and Abell 65) included in the list of 33 BL. For these objects we also present new, narrow-band imaging obtained with 4-m-class telescopes. These central stars have binary periods of :s; 1.0 d and must therefore have undergone a recent common-envelope phase. These objects are prime candidates in the search for the predicted theoretical abundance anomalies. In addition, a more detailed understanding of the nebular morphology may give a valuable insight into the formation and/or shaping mechanisms for PNe in a more general context. 2 OBSERVATIONS AND REDUCTIONS 2.1 Narrow-band imaging Narrow-band imaging observations were obtained of the target PNe using the European Southern Observatory New Technology Figure 1. Ho< + [N II] }"6584-A image of Abell 41. The large image is scaled to show the faintest material while the insert shows much higher levels. The insert also shows that the bar of material passing through the centre of the nebula is split and appears as an ellipse. If this can be interpreted as an inclined circle of material then this would indicate an orbital inclination of -66°. © 1997 RAS, MNRAS 284,32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P 34 D. L. Pollacco and S. A. Bell Telescope (NIT) on the night of 1995 May 21. The multi-function EMMI spectrograph was employed in imaging mode with a Tektronics 2048 x 2048 pixel CCD detector operated in the red arm. With this configuration the 24-f..Lm pixels of the CCD subtended 0.27 arcsec, giving the instrument an unvignetted field of 9 x 9 arcmin 2 . This pixel size is sufficient to sample the images fully, given that the seeing on each night was in the range 0.81.0 arcsec. All the images were obtained in photometric conditions. The imaging filters in the EMMI red arm are positioned in a converging beam, and as such are not suitable for extremely narrow-bandpass observations. Consequently, we did not attempt any fiux calibration of the objects, and made morphological studies the primary motivation for obtaining these images. The filters used for these observations were selected with reference to the known spectrum of each object (see Section 2.2). The central wavelengths of the two filters employed were close to the [0 III] AS007-A (Abell 46 and 65) and the Ha/[N II] >-.6584-A (Abell 41 and 63) emissIOn lines. Flat-field images using the same instrumental configuration as that used for the science images were obtained in the afternoons and mornings of the observation period using a uniformly illuminated dome patch. Reduction of the CCD frames consisted of bias subtraction and correction for the pixel-to-pixel variations in sensitivity using the fiat-field images. The fully reduced images are shown in the accompanying Figs 1-4. 2.2 Long-slit spectroscopy Spectroscopic observations were obtained with the double-armed spectrograph, ISIS, on the 4.2-m William Herschel Telescope on the nights of 1993 July 16, and 1994 June 13 and 16. TEK and EEV CCD detectors were employed for the blue and red arms respectively. These chips have pixel sizes of 24 and 22 f..Lm, giving spatially projected scales of 0.36 and 0.33 arcsec pixel- 1 Figure 2. [0111] AS007-A image of Abell 46. The large image shows the very lowest levels above the sky background level while the insert shows somewhat higher levels, bringing out details in the inner nebula. The inner nebula can be visualized as an inclined bipolar nebula. © 1997 RAS, MNRAS 284,32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P Ejected common envelopes - I respectively. A series of gratings were employed, giving nominal dispersions of between 120 and 17 Amm -1. This technique allowed accurate flux calibration for the determination of nebular reddening, and also sufficient resolution to enable good sky subtraction. For some redlblue arm grating combinations a dichroic mirror was employed to enable data to be collected through both arms simultaneously. On each night at least two of the spectrophotometric standard stars BD+2So 3941, BD+33° 2642 and 35 BD+28° 4211 (Stone 1977) were observed with each spectrograph configuration. Table 1 shows the log of spectroscopic observations of the target PNe. Observations of copper-neon and copper-argon comparison lamps were obtained before or after each target observation and used for wavelength calibration purposes. In the case of the highest resolution spectra presented here, comparison lamp images were obtained immediately before and after each of the target object observations. Flat-field images for each spectrograph Figure 3. Ha + [N II1M584-A image of Abell 63. The large image shows the lowest levels and the ends of the bipolar caps are revealed for the first time. The degree of collimation is amongst the most extreme for any bipolar PN. As the orbital inclination of this object is nearly 90°, we are confident that we are observing this object in cross-section. © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P 36 D. L. Pollacco and S. A. Bell configuration were obtained using a tungsten lamp and the twilight sky prior to observation on each night. The Starlink package FIGARO (Shortridge 1987) was used in the reduction of the two-dimensional spectra. Each image was corrected for the bias offset voltage, and tungsten flat-fields were used to correct for the pixel-to-pixel variations of each CCD. Twilight flat-fields were used to characterize the vignetting within the instrument in the spatial direction and the derived corrections were applied to the object frames . The wavelength calibration was determined from the images of the arc lamps and applied to the two-dimensional spectra. The target spectra were then corrected for atmospheric extinction using the standard La Palma extinction table (King 1985). The observations of the flux standards were used to derive the spectral sensitivity of the whole instrument and the transformation to the absolute flux scale. This correction was then applied to the target spectra. By checking the flux calibration of one standard star spectrum with another, it was clear that each night was photometric during the observational period and that any variations present were below the 5 per cent level. Subsequent reduction involved the extraction of the one-dimensional spectra. For Abell 46, 63 and 65 the low surface brightness of the nebulae necessitates the summation of as many spatial pixels as possible. For this reason, we are unable to comment on spatial variations within these nebulae. Finally, the line fluxes were measured using the ELF routines available within the DIPSO package (Howarth & Murray 1991). 3 3.1 RESULTS Imaging Figs 1-4 clearly demonstrate the advantage of using the NTT EMMI imaging system with its large chip size and corresponding Figure 4. [0 JIll 1\5007-A image of Abell 65. The gradient in the sky background is caused by the close proximity of the Moon. However, the image does reveal the object to be elliptical in shape with a constriction at the waist. © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P Ejected common envelopes - I 37 Table 1. Log of spectroscopic observations of the target PNe. Object A41 A46 A63 A65 Date Wavelength Range (A) Dispersion (Amm- 1) Slit Width (aresec) PA of Slit (degrees) Total integration Time (secs) 93/07/16 93/07/16 93/07/16 93/07/16 94/06/13 94/06/13 93/07/16 93/07/16 93/07/16 93/07/16 93/07/16 93/07/16 94/06/13 93/07/16 93/07/16 93/07/16 93/07/16 94/06/16 94/06/16 94/06/16 94/06/16 3352-6275 3922-6881 3638-5210 5685-7394 4801-5210 4801-5210 3352-6275 3922-6881 3638-5210 5685-7394 3638-5210 5685-7394 4153-4558 3352-6275 3922-6881 3638-5210 5685-7394 4145-4554 3793-6805 4145-4554 5709-7417 120 120 64 66 17 17 120 120 64 66 64 66 17 120 120 64 66 17 120 17 66 10.0 2.0 1.0 1.0 1.5 1.5 10.0 2.0 1.0 1.0 1.0 1.0 1.5 10.0 2.0 1.0 1.0 1.5 1.5 1.5 1.5 342 342 60 60 60 150 281 281 90 90 0 0 90 294 294 315 315 315 300 135 135 300 300 3000 3000 1000 1000 300 300 3000 3000 1500 1500 1500 300 300 4500 4500 1500 300 1200 1200 wide field of view when compared with systems available elsewhere. (1) Abell 41. A photographic image of this object was presented by Grauer & Bond (1983) and the first impression is that little new information is apparent in the N1T image in Fig. 1. BL classified this nebula as elliptical in Balick's (1987) classification scheme. The nebula is axisymmetric, containing two brighter regions situated roughly east-west (26 x 22 arcsec2). Closer examination shows that the nebular morphology exhibits an 'H' shape with the addition of fainter material forming a continuous loop. Suitable scaling shows that the central bar is split and resembles an ellipse. If we assume that this is in fact a circular structure inclined to our line ofsight and that it has been ejected in the orbital plane ofthe binary system, we can derive an orbital inclination of 66°, a value in keeping with the non-eclipsing nature of the binary. Very faint material is visible outside the main disc of the nebula at either end of the 'H'. Compared with the other images presented here, this nebula is of high surface brightness and is most likely density-bounded. Hence we suggest that this envelope is likely to have been recently ejected and that the fast wind of the hot central star has still to break out of the main nebular structure. (2) Abell 46. BL noticed that this object did not fit into the usual classification schemes for normal PNe and concluded that it was likely to have undergone some interaction with the interstellar medium. The N1T image presented in Fig. 2 is significantly deeper than that available to BL, indicating a maximum size of 84 x 97 arcsec2 • Other structures newly detected include the following. (i) A faint bridge connecting the south-eastern and northwestern parts of the nebula. (ii) Various extremely faint nebular extensions extending north-east and south-west of the main body of the nebula. Detached nebulous 'blobs' are visible throughout the field. These objects are likely to be galaxies. (iii) The possibility of an extremely faint bow-shock structure with its apex on the northeastern boundary of the PN, suggesting a possible interaction with the interstellar medium. As the orbital inclination of the central star is _79° , it is likely that we are viewing this object in cross-section, and in some respects there is some resemblance between this nebula and the central parts of Abell 63. In fact, under suitable scaling the object takes on the appearance of a figure eight with the central star situated at the junction of the two halves of the figure. This configuration is to be expected from an inclined bipolar nebula. If this were the case, the ellipticity of the bipolar rings would indicate an orbital inclination of around 65°, although the poor definition of the rings in our images limits the accuracy of this measurement (the light curve solution of Pollacea & Bell 1994 gives an orbital inclination of 80?5). We expect that a deep Ha image will reveal stronger evidence for a bipolar structure. (3) Abell 63. The N1T Ha + [N n] image in Fig. 3 shows a wealth of new information. For example, the ends of the bipolar lobes are clearly visible, while faint material connecting them to the bulk of the nebula demonstrates a remarkable degree of collimation (the ratio of length-to-width is -7), giving the impression of a long tube. The overall dimensions of this tube are 290 x 42 arcsec 2• The brightest parts of the central region of the nebula, covering 48 x 42 arcsec 2, resemble a cylinder with a constriction at the waist. Consequently, this object can be considered to be a 'butterfly' or bipolar nebula in Balick's (1987) classification scheme, although a somewhat extreme example. The central star of this PN is totally eclipsing (i - 87?5), therefore, if the fast wind has broken out through the lowest density regions, i.e. the poles of the system, we can be certain that we are viewing this object in cross-section. (4) Abell 65. The Moon was some 20° distant at the time that this image was obtained, and the quality of the image leaves room for improvement. However, it does warrant some discussion as new details can be seen. BL thought that this object was an elliptical PN and in excellent agreement with the morphology expected for an ejected common envelope. The N1T image in Fig. 4 confirms this conclusion, indicating an object of some 150 x 90 arcsec2 in extent with a constriction at the waist. BL noted a faint wisp of detached material that appeared north-east of the PN. The N1T images reveal © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P 38 D. L. Pollacco and S. A. Bell that this wisp is attached to the nebula at its south-eastern end. There is some indication that a similar feature is visible on the southwestern side of the PN, although better imagery would be required to confirm this. This bow-shock structure may be an indication of an interaction with the interstellar medium. It is not altogether unexpected that large evolved nebular structures such as Abell 46, 63 and 65 should have undergone some level of interaction with the interstellar medium. Assuming radii of 20 arcsec in each case and the distances derived for these objects from solutions of the light curve of the binary central star (pollacco & Bell 1993, 1994; Bell, Pollacco & Hilditch 1994), we can infer ne > 34em-3 for Abell 46, and ne > 14cm-3 for Abell 63 for f = 1. In view of this, we have chosen a representative value of ne = 50 em -3, sinillar to that derived for Abell 65. The adopted values of Te and ne are given in Table 2. 4 ABUNDANCES DERIVED FROM RECOMBINATION LINES 3.2 Spectroscopy The extracted one-dimensional spectra are shown in Fig. 5. The low-resolution, narrow-slit observations were used to place the relative line intensities on an absolute scale. The H~ and Ha fluxes determined from the higher resolution spectra were then scaled to those measured on the low-resolution spectra. The scaling factors for each arm were used to place all the lines on a common absolute scale. The difference between the fluxes measured on the low- and high-resolution spectra was always <10 per cent, and for most spectra these differences were usually of the order of a couple of per cent. In the case of Abell 41 where Ha is partially blended with [NII] M584 A, several other isolated lines were used to determine the scale factor for the red ~ectra (e.g. He r A5876 A). During the 1994 observations, the 64 Amm- 1 blue arm grating was unavailable and line fluxes for Abell 65 in this region of the spectrum were measured from the lower resolution spectra. The low-resolution, narrow-slit observations were also used to determine the galactic extinction towards the PNe. This was determined from a comparison of the measured Balmer line ratios with those predicted for an optically thick plasma as calculated by Brocklehurst (1971). Table 2 shows the dereddened line fluxes for the PNe. For some lines such as [0 m] M363 A the flux was measured from the highest resolution spectra available to avoid contamination with atmospheric emission. For the strongest unblended lines the measured fluxes are probably accurate to ±5 per cent, whereas the errors for severely blended or faint lines may be substantially larger, probably 50-100 per cent. In the case of Abell 46, there was no apparent variation in line fluxes/ratios between the two slit position angles. The electron temperature, Te, and density, ne, were calculated using standard techniques. In general, Te was obtained from the [Om] line ratio 1(5007}/1(4363), but, in the case of the lower ionization PN Abell 41, the [N n] line ratio 1(6584)/1(5755) could be measured with greater certainty. It is worth noting that a 50 per cent error in the measurement of the [0 m] M363-A or [Nn] A5755-A line leads to significant errors in the determination of Te(-± 1000 K). With the data currently available, ne could only be derived from the [S II] line ratio 1(6719)//(6731). In the case of Abell 46 and 63, these lines were not detected but, as these objects appear to be old PNe, we would expect a low value of ne. A lower limit can be derived from the following: n e = 406 I(W) aHPf(PD eff cm- 3 a: where I(W) is the total dereddened flux, is the effective recombination coefficient for the H~ transition at Te = 104 K, f is the fraction of the nebula IDled by hydrogen, () is the angular radius of the nebula in arcseconds and D is the distance expressed in kiloparsecs. Acker et al. (1991) and Kaler (1983) have determined the observed flux, F(H~}, to be 1.41 x 10-12 and 1.59 x 10- 13 ergcm-2 S-1 for Abell 63 and Abell 46 respectively. In view of the uncertainties associated with the determination of the PN physical parameters, we start by considering the abundances that can be derived from recombination lines, as these have only a weak dependence on the assumed values of Te and ne. 4.1 Helium In general, the helium abundance can be derived from the relations N(Her) _ l(HeI) d N(Hen) _ l(Hen) N(H) - Xl I(W) an N(H) - X I(H~)' where Xl and X2 are ratios formed from the total recombination coefficient for the observed He r or He n line and H~. Xl andX2 are only weakly dependent on the assumed values of Te and ne. Adopting Te = 104 K and ne = 100 cm -3 gives essentially the same results as using the derived values of Te and ne. Helium abundances for the target PNe are given in Table 3. For most of our objects the helium abundance can be determined with some accuracy, as both stages of ionization are represented by lines in the optical spectral region. This is not true for Abell 41, and it is likely that the nebula contains neutral material. Consequently, our derived abundance is a lower limit. Using the empirical formula given by Clegg (1987), the contribution of collisional excitations to the He I line strengths was determined. Intuitively, we may expect the low electron density to limit the necessary correction, and for the 4471-, 5876- and 6678-A lines this is indeed the case. The correction is <1 per cent, resulting in these terms being ignored. However, for the 7065-A line, the corrections were found to be at the 3-8 per cent level. As this line was usually the weakest of the helium lines measured (and hence has the largest error in the line flux), and to avoid the correction for collisional excitation, this line was omitted from the abundance calculations. 4.2 Carbon For lines that are thought to arise through recombination, the carbon abundance can be derived from similar relations to those used in the determination of the helium abundance (Table 3). In Abell 63 and 46 it is somewhat surprising to find that the Cn M267-A line is easily measurable. Although the formation of this line is usually considered to be the result of recombination, abundances derived from it are often significantly higher, sometimes unreasonably so, than those determined by other means. In these cases other mechanisms are thought to contribute to the formation of the line. However, in this case, we have no other means of determining a C abundance and so the results of this analysis are presented here. 5 ABUNDANCES DERIVED FROM FORBIDDEN LINES The determination of abundances from forbidden lines is more problematical, where even a small error in Te can have a significant © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P Ejected common envelopes - I effect on the derived ionic abundance. The relatively high value of Te for Abell 46, 63 and 65 warrants some discussion. Generally speaking, most PNe have Te- 10 000 K. While our estimates for Te are high, they are not anomalously so. However, the effect of Te on the abundances is critical, and even small variations in Te can produce considerable variations in the derived abundances. In order to produce a typical 0 abundance Te would have to take a value of close to 9000 K, a value that would require a considerable change in the line ratio, being beyond the observational error. One solution to this discrepancy maybe that there are significant Te fluctuations within the nebulae themselves. Spatially resolved two-dimensional spectroscopy would be required to investigate this possibility. 39 Furthermore, as only optical data are currently available, corrections must be applied for unobserved ionization states (ionization correction factors or icfs). Aller & Keyes (1987) use a combination of an analysis of optical and UV lines with icfs derived from photoionization models. As our data are limited to optical lines, we will apply the less elaborate icf scheme of Barker (1983). Here the icfS can be derived from the following relations: (0+ 0++) H+ + H+ H++)-1 ; where = (1 - ~e o= II x icf(O), icf(O) I ..--. 0 ,....; I < X (')-.::1< .-< I I 0 [/J ,....; tlD h X C'J (1)(') -.....,...-< I >< 0 ;:::1 ,....; X -< Ii.. Abell 41 C\1 2:3I til ~ .0 0 ,....; h < o 5000 4000 6000 7000 Wavelength (Angstroms) I I o < ,....; X li.l Abell 46 o 4000 5000 6000 7000 Wavelength (Angstroms) Figure 5. Observed low-resolution spectra of Abell 41, 46, 63 and 65. Identifications of the emission lines are given in Table 2. For each panel the bottom spectrum shows an overall view of both the red and blue arm spectra. For Abell 65 a lower resolution blue arm spectrum is shown. © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P D. L. Pollacco and S. A. Bell 40 N N+ S +S++) II = (S+ H+ II = H+ X icf(N), where icf(N) Ne II = =~; where icf(S) Ne++ H+ x icf(Ne), . = 0++; Ar _ (Ar++ H - +M+ + H+ . where Icf(Ar) = Ar4+) [ . Icf(S), 1- (1- ~ )3] + -113 Using this scheme, highly populated atomic states that have representative lines in the optical region are expected to have icfs that are close to unity, implying a small correction for unobserved states. As a result, the abundances for these states will be more precisely determined. States with a minor population in the optical region will have the largest icfs. Table 4 shows the ionic abundances, the derived icfs and the corrected abundances. It should be noted that, in the case of Abell 41, To, derived from the [N n] ratio, 0 where Icf(Ne) = X . x Icf(Ar), S+ +S++ S++ ,,-... en .... .... 1 1 0 <t: ...... .... 1 x C\1 Ul tlJ) h Q) >< en ;::i .... ...... 1 "" 0...... >, h Abell 63 t1:l ~ ..0 h <t: 0 4000 5000 6000 7000 Wavelength (Angstroms) ,,-...~ -<t: 1 1 1 0 ...... x ""<!' Ul Abell 65 o 4000 5000 6000 7000 Wavelength (Angstroms) Figure 5 - continued © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P Ejected common envelopes - I 41 Table 2. The dereddened line fluxes and physical parameters for the target PNe. The reddening is determined from a comparison of the observed line ratios with those expected for Case B recombination (Brocklehurst 1971). For Abell 46 and 63, no density diagnostic lines were detected. As these PNe are likely to be old objects we have assumed ne =50 em -3, a value typical of old PNe and similar to that observed in Abell 65. The Abell 65 emission lines with >. <5600A were measured from the lowresolution spectra, with the exception of the [0 m] 4363-A line which was also .observed at high resolution. For Abell 65 the lower limit for the n., as derived from the [Srr] 6719/31-A line ratio, lies in the low-density limit (LDL) for these lines. E B- [Orr] h3727/9 229.8 2.6 HI2 3.1 Hu H IO 3.5 4.9 H9 [Nem] h3869 3.7 15.8 He I h3888 + Hs [Nem] h3968 + HE 12.7 HelM024 2.7 [Srr] >'4069 0.9 [Srr] M076 0.6 H6 20.0 M267 40.0 H'Y [Om] M362 0.8 Hel >'4471 4.4 Herr M542 Herr M686 HeIM713 H,8 100.0 HelM921 1.3 [Om] M959 52.5 [Om] hS007 167.9 Hel hS016 3.8 [Nrr] hS756 0.6 Hel hS876 12.7 [01] >.6300 2.8 [Sm] >.6314 0.5 [01] >.6363 1.1 32.8 [Nrr] >.6548 Ha 290.0 [Nrr] >.6583 99.6 Hel>.6678 4.2 [Srr] >.6719 20.9 18.2 [Srr] >.6731 Hel >'7065 2.5 [Am] >'7136 11.0 3.6 [Orr] >'7319 [Orr] >'7330 2.9 (6.8) (28.4) (30.5) (17.6) (20.2) (33.6) (9.7) (18.1) (30.2) (45.3) (52.1) (7.2) err Te (x103 K); [Om] Te (x103 K); A46 0.21 A41 0.40 V [Nrr] n. (cm- 3 ) [Srr] (4.7) (61.0) (10.1) (4.1) (30.1) (5.7) (5.7) (43.0) (32.1) (19.5) (6.2) (35.3) (20.1) (16.9) (2.0) (3.0) (7.4) (5.4) (6.5) (9.4) (3.2) (14.7) (19.2) A65 0.12 29.0 (29.9) 7.7 (91.4) 5.1 30.1 17.0 21.0 (33.6) (11.3) (38.5) (29.9) 12.6 16.7 13.4 (28.4) (40.0) (36.6) 24.2 7.4 43.2 5.1 7.8 1.7 25.8 2.0 100.0 (12.0) (22.9) (6.0) (31.0) (31.2) (131.2) (6.9) (82.6) (4.4) 20.1 5.4 38.6 4.0 6.0 (15.3) (65.7) (7.0) (25.3) (25.0) 24.6 (90.1) 48.1 7.2 6.0 (34.2) (25.0) (26.1) 7.7 (18.5) 47.1 (16.7) 100.0 (4.6) 100.0 (18.3) 117.9 356.6 (4.3) (4.8) 88.7 268.0 (5.1) (3.0) 152.7 437.7 (12.0) (10.9) 19.6 (8.9) 17.4 6.6 (11.8) (88.8) 18.2 (15.5) (123.2) (101.2) (3.6) (85.3) (21.4) 13.5 287.1 45.2 4.5 10.1 7.2 (18.2) (2.3) (3.3) (8.7) (5.8) (7.1) (39.1) (12.5) 11.0 6: 3: (4.5) (45.6) (97.2) 290.0 3.0 5.3 (14.8) 2.1 1.3 290.0 3.1 4.8 2.6 6.4 (62.5) (11.2) 1.6 9.0 (2.5) (46) (35) (36) 78t 38 39 8.0:::~:~ 7.2:::g:~ 13.4:::g 13.4:::t1 13.8:::U 300:::m 50 50 50:::lli was used to calculate the abundances of the lower ionization species ([Orr], [01], [Srr] and [Nrr]), while T. derived from the [Om] line ratio was used for the remaining states. 6 A63 0.44 DISCUSSION 6.1 Are they bipolar nebulae? Models of common-envelope ejection suggest that, while systems may have different orbital and physical parameters, the morphology of the nebula is expected to be broadly similar, i.e. bipolar. Even theoretical computations of the future evolution of Sun through a red giant phase show that a body as massive as Jupiter could impart sufficient orbital angular momentum to spin up the extended solar atmosphere and eventually produce an axisymmetric nebula (Soker 1994). In order to help us visualize the varying morphologies produced by inclining a bipolar-shaped nebula, we have produced a geometric model based on the scaled dimensions of Abell 63 to serve as a demonstration. The basic model is shown at a range of © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P 42 D. L. Pollacco and S. A. Bell Table 3. Abundances for helium and carbon derived from recombination lines. In the case of the He I abundance, the final value was derived by weighting the line fractional abundance by the fitting error. PNe A41 A46 A63 A65 Line HeI HeI HeI HeI HeI HeI HeI Hen Cn HeI HeI HeI Hen Cn HeI HeI HeI Hen 4471 4921 5876 6678 4471 5876 6678 4686 4267 4471 5876 6678 4686 4267 4471 5876 6678 4686 Fractional Abundance 0.090 0.097 0.094 0.108 0.159 0.145 0.137 0.021 0.0078 0.123 0.127 0.124 0.006 0.0057 0.123 0.135 0.116 0.039 HeI Abundance N([Om]) N([On]) N([OI]) N([Nn]) N([Sm]) N([Sn]) N([Nem]) N([Arm]) Abell 41 Abell 46 Abell 63 Abell 65 22.64 47.20 2.20 5.13 0.68 0.25 1.47 0.25 5.46 0.35 4.06 0.10 0.51: 0.04 6.13 <0.03 0.04: 0.88 0.03 0.36 1.14 16.60 1.05 41.60 1.06 1.03 icf(Ar) 1.00 1.53 1.01 3.18 1.37 N(O) N(N) N(S) N(Ne) N(Ar) 72.04 7.83 0.94 4.68 0.34 6.70 <0.50 4.38 1.25 0.94 0.37 icf(O) icf(N) icf(S) icf(Ne) ? 0.40 0.02 2.06: 0.05 1.29 2.80 1.11 1.56 12.32 1.15 0.06 3.20 Total He Abundance 0.146 Cn Abundance >0.097 0.097 0.021 0.167 0.0078 0.124 0.006 0.130 0.0057 0.125 inclinations in Fig. 6. Comparison of Figs 1-4 with Fig. 6 shows that the morphology of each of the PNe can be produced by this or a similar model. Of course, for these objects the nebulae are unlikely to have the same relative dimensions as Abell 63, if only because of evolutionary effects, but the observed morphologies are at least reasonably consistent with the known or likely orbital inclinations. Table 4. Abundances for 0, N, S, Ar and Ne derived from forbidden lines. The icfs are calculated for each ionic state using the interpolative formulae of Barker (1983). A colon next to the ionic abundance indicates an umeliable result, and the corresponding abundance is ignored in the final total elemental abundance. The abundances are given relative to hydrogen [N(H) = 1] and in units of 10-5 . Hen Abundance 0.039 0.164 6.2 Are the nebular abundances reasonable? Mean logarithmic abundances for the target objects are given in Table 5. There seems little doubt that He is enriched in all of the objects. Furthermore, in the case of Abell 41 and possibly Abell 63, the lack or extreme weakness of the He II 4686-A line suggests that some Heo may also be present but remains unobservable. The situation with the other abundances is less clear. However, as the excitation potential of the [0 II] is similar to that of [N II], images of PNe in these ions are often similar. It is unusual to see strong [0 II]3728/30 A and not detect [N II] 6584 A at all. This weak argument implies that N is probably deficient in all the objects except Abell 41. However, a calculation of the expected line strength of the [N II] 6584-A line as a function of abundance and Te, summarized in Fig. 7, shows that for reasonable values the line flux should be detectable in our spectra, lending more support to our tentative conclusion of N deficiency (but note that the To derived here is from the [0 ill] lines which may not be representative of the Te in the [Nil] region). This may well be a further indication of Te fluctuations in the nebula. In fact, these abundance patterns are similar to those observed in some halo PNe (Clegg 1989). It is not our intention to suggest that these binaries share a common evolutionary history: we merely wish to point out that such abundances, although unusual, are not unique. If the C II M267-A line Table 5. Mean logarithmic abundances for target objects (relative to 10gH= 12.00) and also a typical PN (designated 'PN' in the table). PN He A41 A46 A63 A65 PN 11.16 11.23 11.12 11.21 11.04 C 9.89 9.75 8.85 0 8.86 7.83 7.64 8.09 8.62 N 7.89 <6.69 7.10 7.05 8.11 S Ne Ar 6.97 7.67 6.97 6.57 6.53 8.02 6.40 5.35 6.99 © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P Ejected common envelopes - I 43 /I ~ ~ " Inclination = O· Inclination = 40· Inclination = 70· Inclination = 20· Inclination = 50· Inclination = 80· Inclination = 60· Inclination = 90· Inclination = 30· Figure 6. A simulation based on a geometric model (assuming axial symmetry) of Abell 63 and displayed at various inclinations. As this model takes no account ofthe observed density and brightness distribution of the object, its use is limited to a comparison with the PN images. The images of Abell 41 and 46 show close similarities to the Abell 63 model, while that of Abell 65 is weaker, but still apparent. © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 1997MNRAS.284...32P 44 D. L. Pollacco and S. A. Bell Time for the generous award of time on the William Herschel Telescope, which is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. We would also like to thank the time allocation committee for the ESO New Technology Telescope which performed perfectly while being operated from Garching and was a joy to use. These data were reduced and analysed using Starlink software at RGO and La Palma and also on Dan Pentium machines. REFERENCES ACKNOWLEDGMENTS Acker A., Stenholm B., Tylenda R., Raytchev B., 1991, A&AS, 90, 89 Aller L. H., Keyes C. 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Log(N) = 6.0 Detection uniit 4000 6000 Te (K) Figure 7. A calculation of the theoretical line strength of [N ul >.6584 A as a function of T. and nitrogen abundance. A consideration of the emission-line detection limit (as indicated by the dashed line) shows that unless the T. is much lower then the [0 Ill] diagnostic suggests then N must be deficient. strength can be interpreted as a straightforward recombination line then the derived C abundance is very high compared with a normal PN (almost one dex). Again, this is not a unique situation (Oegg 1989). However, comparison of the total CNO abundance for Abell 46 and 63 with that of the Sun and PNe in general casts some doubt on these conclusions, as it is expected that this abundance will exhibit little variation. The PNe studied here are known to be ejected common envelopes and, although these results support He enrichment, the possible N deficiency is not expected and deserves further observational and theoretical study. As the abundances determined for Abell 41 appear to be reasonably normal, this PN can probably be understood in terms of common-envelope ejection during the ascent of the AGB, and hence resembles a post-AGB single-star PN. All four objects studied here exhibit axisymmetric PNe and can be understood in terms of inclined bipolar nebulae. The sternest test of this conclusion will come when kinematical data for these nebulae become available. © 1997 RAS, MNRAS 284, 32-44 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System