Download Imaging and spectroscopy of ejected common envelopes

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. D., 1987, ApJS, 65, 405
BaJick B., 1987, AJ, 94, 671
Barker T., 1983, ApJ, 267, 630
Bell S. A., Pollacco D. L., Hilditch R. W., 1994, MNRAS, 270, 449
Bond H. E., 1989, in Torres-Peimbert S., ed., Proc. IAU Symp. 131,
Planetary Nebulae. Kluwer, Dordrecht, p. 251
Bond H. E., Livio M., 1990, ApJ, 355, 568 (BL)
Brocklehurst M., 1971, MNRAS, 153, 471
Clegg R. E. S., 1987, MNRAS, 229, 31p
aegg R. E. S., 1989, in Torres-Peimbert S., ed., Proc. IAU Symp. 131,
Planetary Nebulae. Kluwer, Dordrecht, p. 139
Frank A., Mellema G., 1994, A&A, 289, 937
Frank A., BaJick B., Icke V., Mellema G., 1993, ApJ, 404, L25
Grauer A. D., Bond H. E., 1983, ApI, 271, 259
Hazard C., Terlevich R. J., Morton D. C., Sargent W. L. W., Ferland G.,
1980, Nat, 285, 463
Howarth I. D., Murray J., 1991, Starlink User Note, No. 50.13
Then I., Jr, Tutukov A. V., 1989, in Torres-Peimbert S., ed., Proc. IAU Symp.
131, Planetary Nebulae. Kluwer, Dordrecht, p.505
Then I., Jr, Livio M., 1993, PASP, 105, 1373
Then I., Jr, Kaler J. B., Truran J. W., Renzini A., 1983, ApJ, 264, 605
Jacoby G. H., Ford H. C., 1983, ApJ, 266, 298
Kahn F. D., West K A., 1985, MNRAS, 212, 837
Kaler J. B., 1983, ApJ, 264, 594
King D. L., 1985, La Palma technical note No.31, Royal Greenwich
Observatory
Kwok S., Purton C. R., Fitzgerald P. M., 1978, ApJ, 219, L125
Pollacco D. L., Bell S. A., 1993, MNRAS, 262, 377
Pollacco D. L., Bell S. A., 1994, MNRAS, 267, 452
Pollacco D. L., Lawson W. A., Clegg R. E. S., Hill P. W., 1992, MNRAS,
257,33p
Schtinbemer D., 1981, A&A, 103, 119
Schonbemer D., 1983, ApJ, 272, 708
Seitter W. c., 1987, ESO Messenger, 50, 14
Shortridge K, 1987, FIGARO User Guide, Starlink Node Documentation
Soker N., 1994, PASP, 106, 59
Soker N., Livio M., 1988, ApJ, 339, 268
Stone R. P. S., 1977, ApJ, 218, 767
Walton N. A., Walsh J. R., Pottasch S. R., 1993, A&A, 275, 256
We would like to thank the Panel for the Allocation of Telescope
This paper has been typeset from aTE X/L~EX file prepared by the author.
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