Download The AG Carinae nebula: abundant evidence for a red supergiant

1997MNRAS.290..265S
Mon. Not. R. Astron. Soc. 290, 265-275 (1997)
The AG Carinae nebula: abundant evidence for a red supergiant
progenitor?
L. J. Smith, 1 M. P. Stroud/ C. Esteban2 and J. M. Vf1chez 2
1Department of Physics and Astronomy, University College London, Gower Street, London WCIE 6BT
2Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Spain
Accepted 1997 April 28. Received 1997 March 24; in original fonn 1997 January 7
ABSTRACT
AG Carinae is a massive, evolved supergiant which is thought to be in transition from an 0 star
to a Wolf-Rayet (WR) star and is currently identified as a luminous blue variable (LBV) with
10gLlL0 = 6.0. We present an abundance study of the ejecta nebula surrounding AG Car with
the aim of elucidating the evolutionary history of the central star. Physical parameters and
abundances are derived for five regions across the nebula from high spatial resolution
spectroscopy obtained at the Anglo-Australian Telescope (AAT). We derive an average Te of
6350 ± 400 K, an ne of 820 ± 170 cm-3, and find that nitrogen (N) is enhanced by a factor of
4.5 ± 1.3 and that oxygen (0) is deficient by a factor of 15.1 ± 7.2. The derived abundances
are compared with those determined for ejecta-type nebulae around WR stars and those
predicted by hydrodynamical calculations and stellar evolutionary models. We find that the AG
Car nebula is composed of mildly processed material that has not reached the CNO-equilibrium
abundances predicted for LBV nebulae. The similarity of the AG Car nebular N abundance to
WR nebulae leads us to suggest that the nebulae were ejected at the same evolutionary point,
and have undergone no further chemical modification. For AG Car, this point appears to have
occurred before the LBV phase because of the observed low N enrichment. Comparison of the
observed N abundance with evolutionary model predictions indicates that the AG Car nebula
may represent the hydrogen-rich (H-rich) envelope of a red supergiant (RSG). The problem of
an RSG progenitor for AG Car is discussed and it is found that the LBV model of Stothers &
Chin, incorporating a brief unstable RSG phase, is capable of explaining the observations. We
conclude that despite its high luminosity, AG Car has probably experienced a brief RSG phase
where it ejected its outer layers to form the currently observed nebula.
Key words: circumstellar matter - stars: evolution - stars: individual: AG Car - stars: massloss - stars: variables: other - ISM: abundances.
1
INTRODUCTION
AG Carinae is a member of a small but extraordinary group of stars
that exhibits irregular photometric variations over weeks to decades, and, on occasion, experiences large-scale eruptions. Conti
(1984) introduced the term 'luminous blue variable' (LBV) to
denote such stars - well-known members in the Galaxy include TJ
Car, AG Car, RR Car and P Cyg. LBVs are now recognized as
massive, evolved supergiants that are close to the upper luminosity/
stability boundary in the Hertzsprung-Russell (RR) diagram (the
Humphreys-Davidson limit). They are believed to represent a short
(-25000 yr), heavy mass-loss phase in the evolution of a massive 0
star (Minitiai;;:: 40Mo ) to a Wolf-Rayet (WR) star (cf. the recent
review of Humphreys & Davidson 1994).
AG Car has been regularly monitored for most of this century and
varies between V = 7 and 9 mag over a lO-yr time-scale. At
minimum visual light, the spectrum of AG Car resembles that of
a hot evolved supergiant whereas at maximum the star appears as an
A supergiant, and its position on the RR diagram is above the
luminosity limit for cool evolved supergiants. The mechanism
responsible for these excursions across the RR diagram is currently
unknown but is thought to originate below the photosphere (Humphreys & Davidson 1994; de Koter, Lamers & Schmutz 1996).
AG Car has a prominent elliptical ring nebula 39 x 30 arcsec 2 in
size (Thackeray 1950) with two bright clumps to the north-east and
south-west in the direction of the minor axis. This size translates to
1.1 x 1.0 pc 2 on the sky using the revised distance to AG Car of
6kpc (Humphreys et al. 1989). Paresce & Nota (1989) imaged
the nebula through broad-band optical filters and discovered a
prominent bipolar dust structure aligned with the minor axis. To
the north-east a bright detached clump is seen, whereas in the southwest direction a jet-like feature is observed extending from the star
© 1997RAS
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
266
L. 1. Smith et al.
to the edge of the nebula. Recent images obtained with the Hubble
Space Telescope (HS1) (Nota et al. 1995) resolve the jet-like feature
into a complex series of filaments and bubbles. Spectropolarimetric
observations of the wind of AG Car indicate that it is strongly
asymmetric in a direction perpendicular to the nebular bipolar
structure (Schulte-Ladbeck, Clayton & Meade 1993; Leitherer et
al.1994).
High-resolution spectroscopy of the AG Car nebula shows that it
is a hollow expanding shell with an expansion velocity of 70 km s-1
(Smith 1991; Nota et al. 1992) and a dynamical age of 8500 yr. Nota
et al. (1992) derive an ionized nebular mass of 4.2M0 from the
intregrated Ha flux. These parameters are consistent with the idea
that the nebula is composed of stellar material ejected during a giant
eruption, similar to those observed in the seventeenth and nineteenth centuries for P Cyg and T/ Car. If this is the case, then the
nebular abundances should reflect the surface composition of the
star at the time of the eruption. Garcia-Segura, Mac Low & Langer
(1996a) have followed the dynamical interaction of a 60-M0 star
with its environment as it evolves through the LBV phase to a WR
star. They predict that the nebula formed during the LBV phase
should have CNO abundances close to CNO-equilibrium values
with N enriched by a factor of 13, and carbon (C) and 0 depleted by
factors of 23 and 18 respectively. Thus the determination of
abundances in LBV nebulae provides a unique insight into the
chemical composition of the star at the time of eruption, and allows
us to test evolutionary theory for massive stars.
There have been two previous chemical analyses of the AG Car
nebula. Mitra & Dufour (1990) found the nebula to be of very low
excitation with very weak [On] and [Om] emission. Their spectra
were not deep enough to detect the electron temperature (Te) [Nn]
diagnostic A5755 line but by adopting Te = 9000 K, they estimated
that the N abundance was normal and that 0 and sulphur were
depleted by at least an order of magnitude. Conversely, de Freitas
Pacheco et al. (1992) obtained Te = l2400K, a surprisingly high
value given the low excitation of the nebula, and found that N was
probably overabundant by one order of magnitude and that 0 was
deficient by at least a factor of 6. Since these two studies are in
conflict, we have sought to clarify the chemical composition of
the AG Car nebula by obtaining deep long-slit spectroscopy at the
3.9-m Anglo-Australian Telescope (AAT). The aim of this paper is
to unravel the evolutionary history of AG Car by deriving accurate
abundances, and comparing them with those predicted for LBV
nebulae from the hydrodynamic models of Garcia-Segura et al.
(1996a) and with those derived by Esteban et al. (1991, 1992) for
WR ring nebulae, since LBV nebulae may well be the precursors of
ejecta-type WR nebulae.
In Section 2 we describe our observations and data reduction
techniques, and in Section 3 we derive the physical conditions and
abundances for five positions in the AG Car nebula. The resulting
abundances are compared with those for WR ejecta nebulae and
theoretical hydrodynamic and stellar evolutionary models in
Section 4.1. We discuss the implications of our results for the
progenitor of AG Car in Section 4.2, and our conclusions on the
evolutionary history of AG Car are presented in Section 5.
2
OBSERVATIONS AND DATA REDUCTION
Observations of the AG Car nebula were obtained during 1992
March 20-21 at the AAT with the ROO spectrograph and 82-cm
camera using a blue Thomson CCD (1024 x 1024, 19-J..Lm pixels) as
the detector. The nebula was observed at two slit positions offset
from the central star using a slit of dimensions 1 arcsec x 3 arcmin
Table 1. Journal of observations.
Date
Wavelength
Range
(A.)
(a) slit position 1 PA= 131
1992 March 20
1992 March 20
1992 March 20
1992 March 20
1992 March 21
Exposure
Time
(sec)
0
3578-4520
4417-5360
5282-6231
6180-7125
8896-9772
2000
2000
1000
500
1000
(b) slit position 2 PA= 30°
1992 March 20
1992 March 20
1992 March 20
1992 March 20
1992 March 21
3578-4520
4417-5360
5282-6231
6180-7125
8896-9772
2000
1000
1000
500
1000
with one pixel = 0.56 arcsec in the spatial direction. The seeing
throughout the observations was typically 2 arcsec. The precise slit
locations are shown in Fig. 1 superimposed on an Ha + [N n] image
of the nebula from Nota et al. (1992). Position 1 (PA = 131°, offset:
da = 7 arc sec west; d<'l = 8 arcsec south) crosses the brightest
portion of the nebula along its south-western edge while position 2
(PA = 30°, offset: da = 6.3 arcsec east; d<'l = 4 arcsec south) is
roughly orthogonal to this and samples both edges of the expanding
shell.
The journal of observations is given in Table 1; four wavelength
ranges were observed using the 250B grating to give complete
wavelength coverage from 3578-7125 A; and a fifth using the 270R
grating to cover the [S m] lines from 8896-9772 A. The spectra
were reduced by first removing cosmic rays, then bias subtracted,
flat-fielded, wavelength calibrated and sky-subtracted using regions
well outside of the nebula emission. Finally, the two-dimensional
spectra were flux calibrated using a water vapour standard CD
- 22~ 7696 which was in tum flux calibrated by LIT 4364 (Stone &
Baldwin 1983). The resulting spectra have a FWHM resolution of
4A. The spectra covering [S m] were relatively flux calibrated and
put on to the same scale as the shorter wavelength observations by
scaling the Paschen lines to give the correct intrinsic flux relative to
H/3.
AG Car reached maximum brightness with mv = 6.0 in 1981 and
then declined to mv = 8.0 in 1985, and remained at minimum until
it started brightening again in 1991. At the epoch of our observations in 1992 March, AG Car had reached mv = 7.0 with an
equivalent spectral type of early to mid-B (Leitherer et al. 1994).
3
RESULTS
3.1 Line intensities and reddening
To investigate the possibility of spatial variations in electron
temperature, density and abundance, a total of five subregions
were selected to form the spectra for a detailed analysis. The spatial
profiles of various emission lines and their ratios (e.g. [Nn]
A5755/M583 and [Sn] M7171A6731) were examined to identify
regions that could be characterized by a single electron temperature
and density. The resulting regions represent the different morphological aspects of the nebula and are shown in Fig. 1. Three regions
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
AG Car: evidence for a red supergiant progenitor?
267
N
E~
PosiLion 2
200
150
Ul
..........
(])
~
.,...,
P.,
Cl
100
50
Position 1
3"
50
100
150
Pixels
Figure 1. Ha + [N II] image of the AG Car nebula reproduced with pennission from Nota et al. (1992). The image was obtained with the Johns Hopkins Adaptive
Optics Coronograph and has a scale of 0.215 arcsec pixel-I. Superimposed on the image are the two slit positions at which spectra were obtained. The five
subregions (AI-C 1 and A2-B2) selected for the abundance analysis are also indicated.
were chosen for slit position 1: Al (extent 4.5 arcsec) covering the
brightest region of the nebula and coincident with the bipolar dust
feature (Paresce & Nota 1989); Bl (2.8 arcsec) covering a dense
clump near the edge of the nebula; and Cl (3.4 arcsec) covering the
fainter nebular emission adjacent to B 1. For slit position 2, two
regions were chosen for analysis: A2 (3.4 arc sec) and B2 (5.0
arcsec), which cover the fainter, lower density outer nebular
regions.
The extracted nebular spectra are affected by scattered stellar
light due to the dust present in the nebula. In fact, the two slit
positions cover the north-east and south-west quadrants where this
effect is the most pronounced (see fig. 9 of Nota et al. 1992). The
continuum caused by the dust scattering was subtracted from the
data using polynomial fits, and the nebular line intensities were then
measured by fitting Gaussian profiles. The resulting observed
wavelengths, identifications and observed line intensities [F(}')
relative to Hfj = 100], with the errors provided by the fitting
procedure, are given in Tables 2 and 3 for the five regions selected
for study. The line strengths appear to be broadly similar between
the different regions.
The overall nebular spectrum, as originally described by Thackeray (1977), is of low excitation with very strong [NIl] lines
([NII]M583IHu = 0.70), suggesting that N is overabundant. The
low excitation of the nebula can be appreciated from Fig. 2, which
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
268
L. J. Smith et al.
Table 2. Line fluxes for position 1.
Al
Aobs
Alab
3727.1
3835.7
3889.2
3970.3
4101.7
4340.6
4474.7
4861.5
5018.4
5198.9
5754.9
5876.9
6548.3
6563.0
6583.6
6680.0
6716.6
6730.9
7066.5
9014.8
9229.2
9531.1
9546.3
C(H{3)
Fobs(H(3)1
lin
3727.4
3835.4
3889.0
3970.1
4101.8
4340.5
4471.5
4861.3
5015.7
5199.1
5754.6
5875.7
6548.0
6562.8
6583.4
6678.2
6716.5
6730.9
7065.3
9014.9
9229.0
9531.0
9546.0
Ion
k(A)
[011]
0.257
0.237
0.226
0.210
0.182
0.127
0.095
0.000
-0.039
-0.082
-0.195
-0.215
-0.318
-0.320
-0.323
-0.337
-0.342
-0.344
-0.388
-0.599
-0.596
-0.618
-0.619
H9
H8
H€
H6
H-y
HeI
H(3
HeI
[NIl
[NIl]
HeI
[NIl]
Ha
[NIl]
HeI
[Sm
[SII]
HeI
PlO
P9
[Sill]
P8
F(A)
B1
J(A)
F(A)
C1
J(A)
F(A)
4.94±0.53
4.08±2.29
7.32±2.36
11.06±2.31
19.00±2.14
35.86±1.33
7.75±0.82
6.17±3.46
1O.88±3.51
15.97±3.33
26.12±2.95
44.76±1.67
3.1O±0.23
3.56±2.l2
6.09±2.1O
1O.02±2.16
17.51±1.98
35.47±1.l3
5.31±0.38
5.86±3.48
9.79±3.38
15.55±3.36
25.62±2.90
46.26 ±1.48
100.00±0.59
2.25±0.33
3.56±0.31
1.42±0.10
1.65±0.30
145.51 ±3.45
628.81 ±3.46
472.16±3.41
0.70±0.27
29.78±0.62
36.23±0.63
2.41±0.34
5.43±0.83
7.17±0.83
2.65±0.95
7.02±0.87
100.00±0.59
2.1O±0.31
3.08±0.27
1.01±0.07
1.13±0.20
83.40±1.98
359.09± 1.98
268.26±1.94
0.39±0.15
16.38±0.34
20.41±0.35
1.22±0.17
1.99±0.1O
2.56 ± 0.29
0.91±0.33
2.41±0.30
100.00±1.53
0.56±0.18
2.67±O.ll
1.12±0.18
1.37±0.29
138.45±3.61
601.08±3.63
435.21±3.60
100.00±1.53
0.52±0.16
2.24±0.09
0.74±0.12
0.87±0.18
70.92±1.85
307.34±1.86
221.15 ± 1.83
26.80±0.18
32.40±0.18
1.49 ± 0.23
6.56±0.98
9.19±0.98
3.86±1.12
8.80±0.99
13.1O±0.09
15.77±0.09
0.66±0.10
1.94±0.10
2.63±0.28
1.06±0.31
2.41±0.27
0.76
1.35 x 10-13
0.91
1.11 X 10-13
1.67±0.16
2.39±1.75
5.51 ± 1.88
8.94±1.86
14.87±1.67
31.57±3.55
1.16±0.54
100.00±6.36
3.26±0.49
5.66±0.23
2.D4±0.61
3.85±0.59
185.38±4.86
957.45±4.94
570.18±4.79
1.45±0.36
39.92±0.61
45.15±0.69
5.85±0.61
1O.02±1.70
15.06±1.78
5.49±1.68
13.68±1.70
J(A)
4.41±0.43
4.72±3.47
1O.57±3.61
16.37±3.41
25.08±2.82
45.46±5.l1
1.52±0.71
100.00±6.36
2.91±0.44
4.46±0.18
1.17±0.35
2.07±0.32
74.22±1.95
381.03 ± 1.96
225.01 ± 1.89
0.55±0.14
14.94±0.23
16.80±0.26
1.91±0.20
1.88±0.23
2.71±0.32
0.93±0.28
2.31±0.29
1.25
5.27 X 10- 14
ergs-I cm-2 , integrated over the slit area given in the text for each position.
covers the wavelength region from 4900 to 6000 Afor position AI.
It can be seen that we clearly detect the Te diagnostic line [Nu]
A5755 at an observed strength of 1.4 per cent of H,B (Table 2) and
that [0 m] A5oo7 is absent whereas [NI] A5200 is fairly strong. In
addition, HeI A5876 (and to a lesser extent HeI A5015) is observed
to have a more complex profile consisting of the nebular emission
sitting on top of the broad scattered stellar emission line. The
nebular flux (as given in Tables 2 and 3) has been recovered by
fitting a double Gaussian to the overall line profile. We find that the
wavelengths of all the He I lines are redshifted with respect to the
other nebular lines by - + 1.5 A (see Tables 2 and 3). For HeI
M471 and A5015, this can be explained by blending with [Feu]
4474.91 and 5018.43 A. Mitra & Dufour (1990) found that the HeI
line intensities were not in the ratios expected for recombination
and suggested that a significant fraction of the nebular He I emission
could be due to dust scattering of the stellar spectrum. By resolving
the broad and narrow components, we find that the narrow A5876
and M678 components are in roughly the correct ratio (Section 3.2)
but A4471, A5015 and A7065 are not due to blending with [Feu]
lines, and optical depth effects (Clegg 1987) respectively. As
discussed by Mitra & Dufour (1990), the contribution of dust
scattering to the nebular H lines is negligible and thus the only
nebular abundance that will be affected is that of helium (He). The
only 0 line detected is [0 II] A3727 which is unusually weak with an
observed strength of - 5 per cent of H,B, suggesting a low 0
abundance. Regarding other species, we detect moderately strong
[Su] AM717, 6731 and very weak [Sm] A9531 with [Sm] A9069
undetected. [S m] A9531 occurs in a region of the spectrum
particularly affected by water vapour absorption and thus the
measurement error in the line flux given in Tables 2 and 3 may
not be representative of the true uncertainty in the line flux. In
addition, there are numerous [Fe u] and Si u lines present. Overall,
the AG Car nebula spectrum is remarkably similar to the lowexcitation WN8 ring nebula Ml-67 (Esteban et al. 1991), which
has no [Om] and very strong [Nu] lines.
The reddening coefficient C(H,B) was determined for each
spectrum by comparing the observed ratios of H-y, Ho and He to
H,B with the theoretical case B values from Hummer & Storey
(1987) for values of Te= 6000K and ne= 500cm- 3 . The derived
values of C(Hm are given in Tables 2 and 3, and are accurate to
±0.1. C(H,B) varies from 0.76 (AI) to 1.25 (C1) with a mean value
of 0.90 ± 0.20. Region C1 has a stronger continuum relative to the
adjacent region B1, suggesting that the higher extinction found for
C 1 is internal to the nebula and caused by a greater concentration of
dust. Indeed, recent HSTimages (Nota et al. 1995) show that this
region is very bright in continuum-scattered light. The measurements of C(H,B) are in good agreement with the values of 0.72-0.75
determined by Mitra & Dufour (1990) and of 0.81-0.91 by de
Freitas Pacheco et al. (1992). For comparison with the central star,
Humphreys et al. (1989) determined E(B- V)= 0.63 or
C(H,B)= 0.91. The observed nebular line fluxes were extinction
corrected using the individual values of C(H,B) and the formulae of
Howarth (1983) and are given in Tables 2 and 3 [leA) relative to
H,B = 100] together with the reddening coefficients k(A).
3.2 Physical conditions and abundances
Electron densities ne and temperatures Te have been derived for the
five regions in the AG Car nebula from the [S u] AM731/6717 and
[Nu] AM583/5755 line ratios. These values and their associated
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
AG Car: evidence for a red supergiant progenitor?
269
Table 3. Line fluxes for position 2.
B2
A2
Aobs
>-Iab
3727.1
3835.8
3889.5
3970.2
4101.9
4340.3
4861.3
501704
5199.0
5754.6
5877.1
6548.0
6562.8
658304
6679.5
671604
6730.7
7066.6
9014.8
9229.2
9531.1
9546.3
372704
383504
3889.0
3970.1
4101.8
4340.5
4861.3
5015.7
5199.1
5754.6
5875.7
6548.0
6562.8
658304
6678.2
6716.5
6730.9
7065.3
9014.9
9229.0
9531.0
9546.0
Ion
k(>-)
[Oll]
H9
H8
He
Hil
B-y
0.257
0.237
0.226
0.210
0.182
0.127
0.000
-0.039
-0.082
-0.195
-0.215
-0.318
-0.320
-0.323
-0.337
-0.342
-0.344
-0.388
-0.599
-0.596
-0.618
-0.619
H,B
HeI
[NI]
[Nll]
HeI
[NU]
Ha
[Nll]
HeI
[Sll]
[Sll]
HeI
P10
P9
[Sill]
P8
1(>-)
4.00±0.55
2.90±0.24
5.60±0.37
10.70±4.73
19.33±4.69
35.15±4.77
100.00±0.61
1.79±0.26
5.68±0.27
0.82±0.24
0.95±0.38
127.37±3.07
61Oo43±3.05
394.72±3.05
0.60±0.33
27.14±1.07
30.92±1.09
1.45±0.30
4.70±0.44
8.74±1.33
4.81±1.04
9.87±0.92
F(>-)
6.42±0.87
4048±0.38
8.50±0.56
15.75±6.95
27.01±6.56
44.39±6.03
100.00±0.61
1.67±0.24
4.88±0.23
0.57±0.17
0.64±0.25
70.90±1.71
338049 ± 1.69
217.70±1.68
0.32±0.18
14.46±0.57
16042±0.58
0.71±0.15
1.61±0.15
2.91±0.44
1.54±0.33
3.16±0.29
1(>-)
4.38±0.38
2.99±0.65
6.35±0.76
1O.64±0.81
19.26±0.70
36048±0.71
100.00±0.75
1.51±0.39
4.69±O.15
0.98±0.29
0.78±0.29
116.27±2.88
564.45±3.15
370.36±3.15
0.69±0.29
27.19±0.47
29.20±0.50
6.90±0.61
4.55±1.00
9049±1.08
15.44±1.l9
26.58±0.97
45.68±0.88
100.00±0.75
1.41±0.36
4.05±0.13
0.69±0.20
0.53±0.20
66.16±1.80
320.53± 1.78
208.88±1.78
0.38±0.16
14.84±0.25
15.88±0.28
1.50±O.14
7.85±0.98
3.38±1.23
1O.25±1.20
0.52±0.05
2.73±0.34
1.13±Oo41
3042±0.40
0.80
1.55 x 10- 13
c(H,B)
FObs(H{3)1
lIn
F(>-)
0.77
2.08 X 10-13
erg S-I cm-2 integrated over the slit area given in the text for each position.
2.5
[NI]
2.0
I
"i
[NIl]
[OIll]
a.....
"~
HeI
HeI
1. 5
'II
r;;:
1.0
5000
5200
5400
5600
Wavelength (1)
5800
6000
Figure 2. A portion of the nebular spectrum for position Al covering 4900-6000 A. The To diagnostic line [Nu] AS755 is clearly detected while [0 ill] AS007 is
absent. The nebular Hel AS786 line sits on top of broad scattered stellar emission. The units of the ordinate axis are ergs- I cm- 2 A-I integrated over the slit area
of 4.5 arcsec2 •
errors are given in Table 4 for the two slit positions. We find a range
in electron density from 600 (B2: outer nebular shell) to 1050cm-3
(AI: clump in south-west region) in agreement with the range of
630-900cm- 3 found by Mitra & Dufour (1990), and the average
value of 500cm- 3 given by Nota et al. (1992).
The derived values of electron temperature from 5900 to 7000 K
differ substantially from previous determinations. Mitra & Dufour
(1990) derived values of 7500 and 9800 K for the two regions they
studied although they note that their [NII] 'A5755 measurement is at
the 20' level of significance. de Freitas Pacheco et al. (1992) detect
[N II] A5755 but at an observed strength a factor of 3 greater than our
measurements given in Table 2. They thus derive a high Te of
12400 ± lOOK, which is not in accord with the observed low
excitation of the nebula.
© 1997 RAS, MNRAS 290,265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
270
L. J. Smith et al.
Table 4. Abundances (12
+ log X/H).
Region
Al
Bl
Cl
A2
B2
NJ/H+
N+/H+
OO/H+
O+/H+
O++/H+
7.23±g:1l
8.27±g:~
<6.89
7.57±g:g
<6.07
6.60±g::rs
>5.10
7.12±g:gg
8.21±g:~
<6.89
7.44±g:?~
<6.07
6.51±g:~
>5.19
7.16±g:~i
8.06±8:1~
<6.89
7.08±g:~~
<6.07
6.38±g:1~
>5.02
7.61±g:~~
8.32±g:1~
<7.06
7.71±g:~~
<6.36
6.64±8:1~
>5.44
7.40±g:I~
8.27±g:~
7.57±g:g
>6.61
0.70::tg:l~
8.21±g:~
7.44::tgS~
>6.53
O.77::tg:li
8.06±g:1~
7.08::tg:~
>6.40
0.98::tg:2
8.32±g:1~
7.71::tg:~~
>6.67
0.61::tg:1i
8.22::tg:18
7.60::t8:~
>6.57
0.62::tg:~~
0.71 ::t 0.12
0.81 ::t 0.31
0.74::t 0.18
0.55 ± 0.11
0.55 ::t 0.11
1.30::t 0.20
1.14::t 0.29
1.25 ::t 0.23
0.40 ± 0.16
0.67::t 0.38
0.49::t 0.23
0.33 ::t 0.13
0.79 ::t 0.33
0.48::t 0.20
6403::tl!~
1049::tm
6306::t~J3
930::t~~
6969::t~~
767::tl~~
5900±~M
736±~~
6155::t!W
S+m+
S++/H+
N/H
O/H
S/H
logN/O
loZHe+ /H+(5876)
102He+/H+ (6678)
102 <He+/H+ >
Te
ne
8.22±g:l8
<7.06
7.60±g:~g
<6.36
6.55±g:1~
>5.25
603::t~2
Table 5. Comparison of abundances.
Object
SpT
Y,tar
Td)'D
logN/H
logO/H
Yneb
N/O
.::IN
lidO
Ref.
8.22::t0.10
8.45±0.15
8.42
8.40±0.35
8.29::t0.27
7.52::t0.20
7.98::t0.27
8.72
8.11::t0.28
8.02::t0.12
0.47
0.48
0.43
0.51
5.7 ± 2.2
3.0 ± 2.3
0.5
1.9 ± 2.4
1.9 ± 1.3
4.5::t 1.3
7.6::t 2.8
7.1
6.8::t 5.2
5.3 ::t 3.2
15.1 ± 7.2
5.3 ::t 3.5
1.0
3.9::t 2.7
4.8::t 1.3
1
2,3
2,4,5
6,7
8,9
8.35±0.25
8.08±0.08
7.57
8.68::t0.15
7.89::t0.08
8.70
0.31
0.44
0.29
0.5 ± 0.3
1.6 ± 0.5
0.1
6.0::t 3.7
6.5 ::t 1.2
1.0
1.0::t 0.3
3.2::t 0.6
1.0
10
11
12
0.7-0.8
0.35-0.4
26
0.5
13
3
18
1.5
13
14
1.8
2.2
1.1
1.7
33
8.4
8.9
5.7
7.2
12
1.6
1.8
1.4
1.8
20
15
15
15
15
15
(xld' yr)
AGCar
MI-67
RCW58
NGC6888
S308
LBV
WN8
WN8
WN6
WN5
(PN)
SN 1987A
(HII regions)
RSG
60M0 model
35M0 model
LBV
RSG
85M0 model
60M0 model
40~ model
BSG
BSG
RSG
RSG
WNL
0.63
0.87
0.84
0.89
1.00
0.85
2.2
2.9
3.4
15
0.34
0.35
0.37
0.43
0.77
1. Smith et al. (1994); 2. Crowther et al. (1995); 3. Esteban et al. (1991); 4. Rosa (1987); 5. Rosa & Mathis (1990); 6. Crowther & Smith (1996);
7. Esteban & VfJ.chez (1992); 8. Hamannet al. (1993); 9. Esteban et al. (1992); 10. Kingsburgh& Barlow (1994); 11. Panagiaetal. (in preparation);
12. Shaver et al. (1983); 13. Garcia-Segura et al. (1996a); 14. Garcia-Segura et al. (1996b); 15. Meynet et al. (1994).
In summary, we find that the electron density varies from 600 to
1050 em-3 over the nebula with a mean value of 820 ± 170 em-3,
whereas the electron temperature appears to be constant within the
errors with a mean value of 6350 ± 400 K. These values are very
similar to the WN8 ring nebula Ml-67 for which Esteban et al.
(1991) find ne= 1000cm-3 and Te= 62ooK.
Ionic abundances for ~, N+, 0+, S+ and S++ have been
calculated from the measured line intensities at the appropriate
values of Te and ne by solving the equations of statistical
equilibrium. The resulting abundances are given in Table 4 for
the five regions together with the associated errors derived from the
uncertainties on the line fluxes and Te and ne. Since the [S m] }"9531
line flux is very uncertain due to water vapour absorption, the
S++ /H+ abundance is given in Table 4 as a lower limit. Upper limits
to the abundances of 0° and 0++ are also given; these have been
calculated from the 3/1 upper limits to the line fluxes using the
observed signal-to-noise ratio of the continuum in the region of
[01] A6300 and [Om] }"s007. From Table 4, it is apparent that the
dominant ionization stages are Wand 0+. Thus to derive elemental
abundances, we have made the reasonable assumption that no
corrections are necessary for unseen ionization stages since the
derived upper limit on 0++ gives 0+/0++ > 100. Therefore, we
assume that all the 0 and N in the ionized nebula are in the form of
0+ and N+. The resulting total abundances are given in Table 4.
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
AG Car: evidence for a red supergiant progenitor?
Within the errors, there appears to be no variation in abundances
across the nebula.
He +IH+ ratios have been derived using the strengths of the He I
"10876 and 6678 lines and the recombination coefficients of
Brocklehurst (1972) interpolated to the appropriate Te. The resulting ratios are listed in Table 4. The He+lH+ ratios for AI, Bl, A2
and B2 agree well with a mean ratio of 0.57 ± 0.12 x 10- 2 . For
position Cl, which contains the greatest concentration of dust
(Section 3.1), the He+lH+ ratio of 1.25 ± 0.23 x 10-2 is a factor
of 2 higher than the mean ratio. These ratios overall are very low,
suggesting that the bulk of He is in neutral form. if we assume that
the true He abundance is similar to the value of HelH = 0.22
determined for MI-67 by Esteban et al. (1991), then we derive a
large ionization correction factor of 40.
4
4.1
DISCUSSION
Comparison of abundances
From Table 4, the mean abundances (12+10gXlH) derived for the
AG Car nebula are N = 8.22 ± 0.10 and 0 = 7.52 ± 0.20 with
N/O = 5.7 ± 2.2 (by number). For comparison, Mitra & Dufour
(1990) obtained N = 7.5 and 0 = 7.2 with N/O = 2.0. These
much lower abundances arise because they assumed a too
high a value for Te of 9000 K. It is thus crucial to obtain an
accurate flux for the weak Te diagnostic line [N II] 10755 in order
to derive reliable abundances. AG Car is located at a galactocentric
distance Rgc = 8.6kpc (assuming the solar galactocentric distance
Ro = 8.5 kpc) and thus it is appropriate to compare the derived
mean abundances with those tabulated by Shaver et al. (1983) for
solar neighbourhood Hn regions of N = 7.57 ± 0.04 and
o = 8.70 ± 0.04. Using these values, we find that the AG Car
nebula is enriched in N by a factor of 4.5 ± 1.3, and depleted in 0
by a factor of 15.1 ± 7.2. Atmospheric abundance analyses of
unevolved B stars for 6 < Rgc < 10 kpc (Fitzsimmons, Dufton &
Rolleston 1992; Kilian 1992) give 0 and N abundances in agreement with the Shaver et al. (1983) values.
Abundance studies of ejecta-type nebulae around other massive,
evolved stars have shown that the gas is always enriched in N and
usually depleted in O. The AG Car nebula clearly belongs to this
class of object. In Table 5, we compare the derived abundances for
the AG Car nebula with those determined for nebulae that represent
processed stellar material from a variety of objects. In drawing up
this table, we have restricted the sample to those objects for which
reliable abundances have been determined, i.e. Te is well measured.
Other nebulae associated with LBVs [e.g. 'Y/ Car (Davidson et al.
1986), P Cyg (Johnson et al. 1992) and HR Car (Nota et al. 1997)]
have apparent N enrichments and 0 depletions. Conversely, some
WR ring nebulae have abundances that are consistent with swept-up
interstellar gas (Esteban et al. 1992). In Table 5, we list abundances
for the WR ejecta nebulae Ml-67 (Esteban et al. 1991); RCW 58
(Rosa 1987; Rosa & Mathis 1990); NGC 6888 (Esteban & Vflchez
1992); and S308 (Esteban et al. 1992). We also list the average
abundances for a large sample of Galactic planetary nebulae (pNe)
(Kingsburgh & Barlow 1994), the abundances determined for the
inner ring of SN 1987A (believed to be composed of RSG material)
based on HSTobservations (Panagia et al. 1994, and in preparation),
and the solar neighbourhood H II region abundances from Shaver et
al. (1983).
In Table 5, we list for each nebula the spectral type (SpT) of the
central star and its surface He mass fraction Ystar ; the dynamical age
Tdyn of the nebula (as given by the observed radius divided by the
271
measured nebular expansion velocity); the observed N and 0
abundances (12+10g XIH) with errors, where available; the
measured nebular He mass fraction Yneb ; the observed N/O ratio;
the N enrichment factor AN, and the inverse of the 0 depletion
factor 1/A O. The four WR nebulae are situated at galactocentric
distances of 7 -10 kpc or ± 1.5 kpc relative to AG Car and the Sun.
We have therefore used the solar neighbourhood H II region
abundances of Shaver et al. (1983) to calculate the enrichments
and depletions, without any correction for abundance gradient
effects because of the similar galactocentric distances. Recent
studies of the N and 0 abundance gradients in the Galaxy (Vflchez
& Esteban 1996) find that they are substantially flat with log N/O
= - 1.0 irrespective of galactocentric distance. For SN 1987A, we
have assumed that the Large Magellanic Cloud (LMC) N and 0
abundances are 0.3 dex below the Galactic values (Rolleston et al.
1996). In the final column, the references are given for Y star and the
abundance determinations.
From Table 5, it is apparent that within the errors, the observed
N enrichment in the AG Car nebula is similar to or slightly less
than that measured in the WR nebulae, which have a mean
AN = 6.7 ± 1.0. The AG Car nebular 0 depletion is higher than
that observed for the four WR nebulae, which have a mean
l/AO = 3.8 ± 1.9. The AG Car measurement has a 50 per cent
error associated with it which may be an underestimate because at
the blue wavelength of the weak [0 II] 1-3727 line, there are
observational errors (which are difficult to quantify) in correcting
for atmospheric and interstellar extinction, and in applying the
absolute flux calibration. In addition to these uncertainties, it is
possible that there is some depletion of 0 on to dust in the nebula.
The strong [NII] 1-6584 line does not suffer from the same
uncertainties since it occurs at much longer wavelengths and is
close to the Ha line. With these reservations about the degree of 0
depletion in the AG Car nebula, we will concentrate on the N
enrichment when comparing abundances predicted by evolutionary
models.
The fact that the N abundances are so similar for the AG Car and
the WR ejecta nebulae, yet the dynamical ages (Table 5) vary by
large factors, suggests that the nebulae were ejected at the same
point in the evolution of the central stars, and have undergone no
chemical modification since their formation. The mean N enhancement determined for PNe (Kingsburgh & Barlow 1994) is similar to
the mean value found for AG Car and the WR nebulae but PNe are
not observed to be 0 deficient and are only moderately enhanced in
He in contrast to massive star ejecta. Finally, the N enhancement in
the AG Car nebula is similar to that derived for the inner ring of SN
1987A, which is believed to be composed of RSG material (Panagia
et al. in preparation).
We next compare the observed abundances with theoretical
predictions. Garcia-Segura et al. (1996a) have computed the
dynamical interaction of a 60-M0 star with its circumstellar
medium from its 0 star stage through the LBV phase to the end
of the WR stage, by combining the evolutionary model of Langer et
al. (1994) with a hydrodynamic code. They predict that LBV
nebulae will have CNO-equilibrium abundances as a consequence
of the heavy mass loss which is swept up to form the nebula during
the LBV phase. Their predicted abundances are given in Table 5.
Garcia-Segura, Langer & Mac Low (1996b) have also investigated
the evolving circumstellar structure of a 35-M0 star that has an RSG
phase prior to the WR stage. They predict that the WR nebula will
consist of the H-rich RSG envelope and will only be slightly
enriched in He and CNO-processed material. The predicted abundances for this model are also given in Table 5; the N enhancements
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
272
L. 1. Smith et al.
and 0 deficiencies have been calculated using the starting values of
their models. It can be seen that the observed N enrichment of the
AG Car nebula is almost a factor of 3 below the CNO-equilibrium
values whereas the agreement is much better for the 35-M0 model.
Finally, for comparison in Table 5, we also list the predicted
surface abundances from the evolutionary tracks of Meynet et al.
(1994) for models with initial masses of 85, 60 and 40M0
corresponding to the evolutionary points that show closest agreement with the observed N abundance of the AG Car nebula. For the
85- and 60-M0 models, best agreement is found between the
tabulated N surface abundance and the observed nebular N abundance towards the end of the main-sequence phase when the products
of nuclear burning first appear on the surface and the star is a normal
blue supergiant (BSG). Conversely for the 40-M0 case, best
agreement is found for the RSG stage as shown in Table 5 where
the last two points of the RSG phase and the late WN (WNL) phase
are shown. The numbers given here for the RSG phase differ from
those taken from Garcia-Segura et al. (1996b) because the latter are
integrated over several time-steps of the RSG phase whereas the
former represent single time-steps in the evolutionary model
tabulations.
The finding that the observed nebular N enrichment for AG Car is
too low for CNO-equilibrium abundances depends on the predictions of theoretical stellar evolutionary tracks for LBV surface
abundances. A He mass fraction of Ystar = 0.63 has been determined for AG Car from a model atmosphere analysis (Smith,
Crowther & Prinja 1994). This value is very similar to the mean
Ystar = 0.66 ± 0.08 derived for nine LMC Ofpe/WN9 stars (these
stars are believed to represent quiescent LBVs and many have
nebulae) by Crowther & Smith (1997) and Pasquali et al. (1997). In
the models of Meynet et al. (1994), Langer et al. (1994) and
Pasquali et al. (1997) this value of Ystar occurs very close to the
beginning of the LBV phase and does correspond to CNO-equilibrium abundances. Although we do not have an He abundance for
the AG Car nebula, the similarity of the N abundance to WR
nebulae, which have measured He abundances, suggests that
Yneb = 0.5 for AG Car. Overall, the small nebular N enhancement
and the inferred lower Y abundance compared with the current
surface abundances suggest that the AG Car nebula may have been
formed before the LBV phase. The observation that essentially all
known LBVs have nebulae (Nota et al. 1995) also supports this
possibility.
Another possibility to consider is that the AG Car nebula was
originally composed of material with CNO-equilibrium abundances that has been progressively diluted through mixing with
material from the 0 star wind as the nebula expands into the
wind-blown bubble created by the 0 star. The fact that the AG
Car nebula and the four WR ejecta nebulae have similar N
enhancements but different dynamical ages (Table 5) argues against
this possibility. It can also easily be shown that the mass of 0 star
material required is far in excess of that likely to be swept up.
Following Esteban et al. (1991), if we assume that the total mass of
the nebula Mneb is given by the sum of the mass of ejected material
with CNO-equilibrium abundances MCNO and the mass of the
swept-up material Msw' then in terms of the mass fraction of an
element X, the ratio of Msw/MCNO is given by
Msw
MCNo
X CNO
-
= Xneb -
Xneb
Xsw .
Taking the observed N mass fraction of the AG Car nebula to
represent Xneb , the CNO-equilibrium N abundance of 0.013 for
XCNO ' and the initial N mass fraction of 0.001 to represent Xsw
(Garcia-Segura et al. 1996a), we find that the mass of material
from the 0 star wind has to be six times the amount ejected with
CNO-equilibrium abundances or 3.6 M0 if we take the total AG Car
nebular mass as 4.2M0 (Nota et al. 1992). For comparison, the
model of Garcia-Segura et al. (1996a) predicts that during the 0 star
phase, 32 M0 of material will be trapped inside a wind-blown
bubble of radius 50 pc. Thus, assuming that the density of the
bubble is uniform, the mass swept-up out to a radius of 0.5 pc (the
observed radius of the AG Car nebula) is 3.2 x 10-5 M0 or a factor
of 105 lower than that required to dilute the nebular N abundance
from its CNO-equilibrium value to the currently observed value.
We now consider the possibility that the AG Car nebula was
formed during a BSG phase as suggested by the surface abundances
of the 60-85 M0 models of Meynet et al. (1994) given in Table 5.
These evolutionary tracks use mass-loss rates enhanced by a factor
of 2 over standard mass-loss rates to obtain better agreement with
observations, e.g., that 0 stars are He-enriched (Herrero et al.
1992). It does not appear feasible for AG Car to have formed a
nebula at this evolutionary point because the observed size of the
nebula is far too small when either the evolutionary time-scales or
likely ejection velocities are considered. For the two models, a BSG
takes 3-7x105 yr to reach the observed surface He abundance for
AG Car of Ystar = 0.63. Since the radius of the nebula is given
approximately by the product of the age and the expansion velocity,
the observed radius of the AG Car nebula of 0.5 pc implies an
expansion velocity of at most 2 km s-1 ! Since the BSG is a normal
H-burning star and is not close to the Eddington limit or the
Humphreys-Davidson limit (Humphreys & Davidson 1979), any
material will probably be ejected at the escape velocity for a BSG of
500-800km s-1 (Lamers, Snow & Lindholm 1995) for the mass
range considered here. The material then has to be braked to the
observed expansion velocity of 70km S-1 and the evolutionary
time-scale reduced by a factor of 100 to produce a nebula of the
correct size. We therefore conclude that the Meynet et al. (1994)
models are not able to produce the AG Car nebula with the correct
chemical composition and size because the enhanced surface
abundances occur far too early in the models.
The evolutionary models of Schaller et al. (1992), incorporating
standard mass-loss rates, are more successful but have other
problems. For the 60-M0 case, the star reaches the surface N
abundance corresponding to the AG Car nebula after the end of
core hydrogen burning. To prevent redward evolution, the mass-loss
rate is increased to _10- 3 M0 and the surface becomes enriched at
the point where the redward evolution is halted at a surface
temperature of -6000 K. The star then evolves back to the blue
as an LBV and reaches CNO-equilibrium abundances at the start of
the late-WN stage, much later than the Meynet et al. (1994) tracks.
Comparison of the observed AG Car nebular abundances with the
40-M0 track of Meynet et al. (1994), which has an intermediate
RSG rather than an LBV phase (Table 5), shows that the best
agreement occurs during the RSG phase. The surface temperature at
this point is 5700 K, which is very similar to the Schaller et al.
(1992) 60-M0 track and thus, strictly, the star is a yellow rather than
a red supergiant. The exact temperature reached, however, during
this evolutionary phase depends critically on the adopted mass-loss
rate.
Overall the comparisons presented in Table 5 reveal that, within
the errors, the AG Car nebula has a similar N enhancement to four
WR ejecta nebulae, to the inner ring of SN 1987A, believed to
consist ofRSG material, and to the predicted N surface abundances
during the RSG phase for the Meynet et al. (1994) 4O-M0 model.
The N enrichment appears to be too low for material that has
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
AG Car: evidence for a red supergiant progenitor?
reached CNO-equilibrium abundances. In their study of WR
nebulae, Esteban et al. (1992) show (their fig. 5) that their N/O
ratios versus Y values are only reproduced by evolutionary models
with initial masses between 25 and 40 Mo at the end of the RSG
stage. They therefore proposed that a short period of very high mass
loss or shell ejection at, or very near to, the end of the RSG phase
could plausibly explain the origin of the enriched material in the
WR ejecta nebulae. The luminosities of the WR central stars
(logliLo = 5.2-5.5: Hamann, Koesterke & Wessolowski 1993;
Crowther et al. 1995) place them below the Humphreys-Davidson
limit (logliLo = 5.7-5.8), in agreement with a post-RSG status.
The similarity of the N enrichment in the AG Car nebula to the
four WR nebulae suggests that it also is probably composed of RSG
material. The nebula does not appear to be composed of material
lost from the surface of AG Car during its current LBV phase since
it is mildly processed in comparison to the surface composition.
This suggests that the so-called giant eruption event that formed the
nebula probably occurred before AG Car became an LBV. We find
that a BSG origin, as suggested by the Meynet et al. (1994) 60- and
85-Mo models, for the AG Car nebula is not consistent with the
small size of the nebula because enhanced surface abundances
occur too early in these models.
4.2 An RSG progenitor for AG Car?
The luminosity of AG Car is well determined (given that its revised
distance of 6 kpc is correct: Humphreys et al. 1989), with
10gliLo = 6.0 (Leitherer et al. 1994; Smith et ·al. 1994). This
places AG Car well above the Humphreys-Davidson limit which
represents the empirical upper luminosity boundary for cool supergiants (Humphreys & Davidson 1979). This creates the considerable problem of whether or not it is feasible to have an RSG
progenitor for AG Car when no RSGs are observed to exist of
comparable luminosity. LBVs are, however, known to exist both
above and below the Humphreys-Davidson limit. It is therefore
necessary to invoke two different evolutionary schemes to explain
this; one with no intermediate RSG phase for those above, and the
other with an RSG phase for those below (e.g. Humphreys &
Davidson 1994).
To explain the nebular abundances, we require the LBV progenitor to move rapidly across the HR diagram to the RSG region,
dump its H-rich envelope, and then quickly move back to become a
classical LBV. The period spent in this intermediate phase needs to
be short compared to the LBV lifetime otherwise cool supergiants
would be observed above the Humphreys-Davidson limit. In most
stellar evolutionary models, evolution to the red is largely prevented
by very high mass-loss rates of ~ 10-3 Mo yr- I in the LBV phase.
The main problem with this is that the observed continuous massloss rates of LBVs (Humphreys & Davidson 1994) are at least a
factor of 10 below the rate required to halt evolution to the red. The
60-Mo track of Meynet et al. (1994) reaches a minimum temperature of 6500 K for log liLo = 6.0 whereas the 60-Mo track of
Langer et al. (1994) evolves much farther to the red during the LBV
phase with a minimum temperature of 2500 K for log liLo = 5.9.
Alternatively, Stothers & Chin (1993, 1994, 1995, 1996) have
developed an evolutionary scheme for LBV s which has the potential of explaining the nebular abundance data since their tracks
briefly enter the RSG region.
Stothers & Chin (1993) proposed that an ionization-induced
dynamic instability occurs in the outer envelope of the star to
explain the LBV phenomenon. They find that when H is exhausted
in the core, the star rapidly crosses the HR diagram. The dynamical
273
instability is then triggered as a result of the envelope tr)'ing to
expand and the H-rich envelope is quickly lost. The star then moves
back to the blue to become a true LBV. Since this part of the
evolution is rapid, it seems possible to have an RSG phase that
would not be observable given the small number of known LBVs
above the Humphreys-Davidson limit. Stothers & Chin (1994,
1995) find that a second period of dynamical instability occurs
during the longer blue LB V phase characterized by moderate massloss cycles, similar to those observed in AG Car, which has a wellknown 1O-yr cycle. Their predicted mass-loss rates during the
dynamically unstable portion of the cycle of 4-8x1O- 5 Mo yr-I
agree well with observation.
Stothers & Chin (1996) discuss the formation of nebulae around
LBVs. They find that during the yellow or red phase, the star will
lose a large fraction of its envelope in a single large ejection episode
consisting of a series of closely spaced multiple outbursts. The
amount of material lost varies between 4 and 20 Mo for initial
masses of 45 and 90 Mo, and depends on the stellar luminosity with
a slope agreeing with the observational relationship of Hutsemekers (1994) who found that the masses of LBV nebulae are
correlated with the luminosity of the central star. This relationship
argues against the nebulae being formed through continuous mass
loss (as the mass would be a function of age), but rather by a single
violent ejection event.
There is other independent observational evidence that supports
the idea that AG Car may have experienced a cool supergiant phase.
Robberto et al. (1993) use a dynamical model for the interaction of
the current AG Car wind with a slower wind from the pre-LBV
phase to deduce that a progenitor wind terminal velocity of 2040km s-I and amass-loss rate ofl-4 x 10-4 Mo yr-I is required to
reproduce the observed mass, expansion velocity and radius of the
nebula. Nota et al. (1995) find that the easiest way to explain the
axisymmetric shape of LBV nebulae is through interacting winds
where the LBV wind interacts with a pre-existing density contrast
created by a slower wind. They discuss various mechanisms for
producing the required density contrast including an asymmetric
RSG wind. Nota et al. (1996) have investigated the large- and smallscale structure of the AG Car nebula, as revealed by HST images,
using hydrodynamic models. They find that to reproduce the
observed parameters of the AG Car nebula, a progenitor with a
dense, slow and cool wind is required. Another piece of observational evidence that points to LBV nebulae being composed ofRSG
material is the puzzle of why dust (Hutsemekers 1994) has formed
in such a harsh environment. This problem disappears if the dust is
formed in a cool RSG phase.
The unique object IRC + 10420 is a possible observational example
of a massive post-RSG star that is rapidly evolving back to the blue to
become either an LBVor WR star. In a detailed study, Jones et al.
(1993) classify it as an F8 supergiant and find that it has
10gliLo = 5.8, which places it at the Humphreys-Davidson limit.
IRC + 10420 has a dusty circumstellar envelope which is believed to
have been ejected during the RSG phase. The envelope extends to
~0.25 pc and contains ~ 5 Mo of material with a dynamical age
of ~5OOOyr (Kastner & Weintraub 1995). In a recent study,
Oudmaijer et al. (1996) find that IRC + 10420 is evolving rapidly as
the surface temperature has increased by ~ 1000 K over the last 20 yr
and it now has an A supergiant spectrum. From CO observations, they
find an envelope outflow velocity of 40 km s-I and a gas mass-loss rate
of ~5 x 1O-4 Mo yr- I. The envelope parameters ofIRC +10420 are
broadly similar to but somewhat smaller than those observed for the
AG Car nebula, suggesting that it is feasible for the dusty envelope
around IRC + 10420 to evolve into an LBV-type nebula.
© 1997 RAS, MNRAS 290, 265-275
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1997MNRAS.290..265S
274
L. J. Smith et ai.
In summary, a post-RSG scenario for AG Car is probably
plausible given the current uncertainties surrounding theoretical
evolutionary tracks. There are, however, some problems associated
with the models of Stothers & Chin (1996). While they predict that
the major mass-loss episode occurs in a brief yellow or red supergiant phase, they find that LBVs are in an advanced He-burning
state with Ystar =0.78-0.88. This is much higher than the value of
Ystar = 0.66 ± 0.08 derived from the model atmosphere analyses
discussed above. Another problem is the dynamical time-scale of
8500 yr for the AG Car nebula. Stothers & Chin (1996) find that the
dynamical ages of LBV nebulae are too short for them to be
composed of RSG ejecta and propose instead that the nebulae are
formed by continuous mass-loss during the blue LBV phase. The
dynamical age of the AG Car nebula is not such a problem, however,
if the ejection occurs at the end of the RSG phase because evolution
back to the blue is expected to be rapid, as observed for IRC + 10420
by Oudmaijer et al. (1996). In addition, dynamical studies of the AG
Car nebula (Smith 1991; Nota et al. 1992) show that it is complex
with components moving at slower velocities than the overall
expansion velocity of 70 km s -1, suggesting that the dynamical
time-scale could be increased.
ejection event caused by an ionization-induced dynamic instability
while in a brief yellow or red supergiant phase.
Regarding future work: it is important to obtain abundances for
more LBV nebulae to establish that mildly CNO-processed material
is characteristic of the nebulae. It would also be interesting to search
for material with CNO-equilibrium abundances in LBV nebulae
occurring either as a result of continuous mass loss from the star or
through small outbursts during the LBV phase itself. The young
nebula associated with P Cyg would be a good starting point. Since
many LBV s are cool and can only ionize their immediate surroundings, a search should be made for neutral shells from a prior RSG
phase. Infrared Space Observatory observations of the HR Car
nebula by Lamers et al. (1996) indicate that there is cool dust
outside of the ionized nebula. Concerning the dynamics of the
nebulae, it would be worthwhile to search for evidence of slowmoving material in the AG Car nebula. The question of whether or
not a short RSG phase is possible both observationally and
theoretically should be addressed. An investigation of the hydrodynamic evolution and interaction of the LBV wind with the RSG
envelope to assess the dynamical time-scales and likely morphology would also be very valuable.
ACKNOWLEDGMENTS
5
CONCLUSIONS
A detailed abundance study of the nebula surrounding the LBV AG
Car shows that it consists of mildly processed stellar material, and
not material composed of CNO-equilibrium products, as predicted
for LBV nebulae by Garcia-Segura et al. (1996a). It is found that the
observed N enrichment of the AG Car nebula is similar to those
derived for four WR nebulae by Esteban et al. (1992). This result
can only be understood if the nebulae were ejected at the same point
in the evolution of the central stars. For AG Car, this point probably
occurred before the beginning of the LBV phase because of the nonCNO-equilibrium N abundance observed in the nebula. The fact
that the nebulae have the same abundance pattern also indicates that
they cannot be composed of CNO-equilibrium material that has
been gradually diluted by sweeping up 0 star wind material because
the nebulae have very different dynamical ages, and the mass
needed to be swept up is far too high. We find that the evolutionary
tracks of Meynet et al. (1994) for stars of similar luminosity to AG
Car are incompatible with the observed nebular N abundance and
size, because the stellar surface reaches the required N abundance
too early while the star is still in the H-burning BSG phase.
Comparison of the observed N abundance with that observed for
the inner ring of SN 1987A (Panagia et al., in preparation) and the
predicted N surface abundance for a 40-M8 evolutionary model
(Meynet et al. 1994) shows that the best agreement occurs during
the RSG phase. This, therefore, suggests that the AG Car nebula was
formed through a bulk ejection of material in an RSG stage. This
finding conflicts with the observed absence of RSGs above the
Humphreys-Davidson limit. To avoid this conflict, the lifetime of
the intervening RSG phase has to be shorter than the LBV lifetime.
Another possibility is that objects in this evolutionary phase are
highly obscured and thus might be difficult to detect. We note that
the unusual object IRC +10420 is an example of a massive postRSG star with a dusty envelope at the Humphreys-Davidson limit.
It is evolving rapidly back to the blue and may become an LBV with
a ring nebula. The requirement that the nebula is formed by a bulk
ejection of RSG material is met by the evolutionary models of
Stothers & Chin (1993, 1996). Their model predicts that the major
episode of mass loss occurs before the LBV phase during a single
Fig. 1 has been reproduced, with permission, from Astrophysical
Journal, published by The University of Chicago Press (© 1992 by
the American Astronomical Society. All rights reserved).
We thank Nino Panagia for sending us the abundances for the
inner ring of SN 1987Ain advance of publication, and Antonella
Nota, Norbert Langer, Mike Barlow and Paul Crowther for many
useful discussions. MPS acknowledges the financial support of
PPARC. CE and JMV were partially funded through grant No.
PB91-0531 from the DGYCIT of the Spanish Ministerio de Educacion y Ciencia.
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