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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. REFERENCES Brocklehurst M., 1972, MNRAS, 157,211 Clegg R E. S., 1987, MNRAS, 221, 31p Conti P. S., 1984, in Maeder A., Renzini A., eds, Proc. lAU Symp. 105, Observational Tests of the Stellar Evolution Theory. Kiuwer, Dordrecht, p.233 Crowther P. A., Smith L. J., 1996, A&A, 305, 541 Crowther P. A., Smith L. J., 1997, A&A, 320, 500 Crowther P. A., Smith L. 1., Hillier D. J., Schmutz W., 1995, A&A, 293, 427 Davidson K., Dufour R J., Walborn N. R, Gull T. R, 1986, ApJ, 305, 867 de Freitas Pacheco J. A., Darnineli Neto A., Costa R D. D., Viotti R, 1992, A&A, 266, 360 de Koter A., Lamers H. J. G. L. M., Schmutz W, 1996, A&A, 306, 501 Esteban C., Vilchez J. M., 1992, ApJ, 390, 536 Esteban C., Vilchez 1. M., Smith L. 1., Manchado A., 1991, A&A, 244, 205 Esteban C., Vilchez J. M., Smith L. J., Clegg R E. S., 1992, A&A, 259, 629 Fitzsimmons A., Dufton P. L., Rolleston W R J., 1992, MNRAS, 259, 489 Garcia-Segura G., Mac Low M.-M., Langer N., 1996a, A&A, 305, 229 Garcia-Segura G., Langer N., Mac Low M.-M., 1996b, A&A, 316,133 Hamann W-R, Koesterke L., Wessolowski U., 1993, A&A, 274, 397 Herrero A., Kudritzki R. P., Vilchez J. M., Kunze D., Butler K., Haser S., 1992, A&A, 261,209 Howarth I. D., 1983, MNRAS, 203, 301 Hummer D. G., Storey P. J., 1987, MNRAS, 224, 801 Humphreys R M., Davidson K., 1979, ApJ, 232, 409 Humphreys R M., Davidson K., 1994, PASP, 106, 1025 Humphreys R M., Lamers H. J. G. L. M., Hoekzema N., Cassatella A., 1989, A&A218,Ll7 © 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? Hutsemekers D., 1994, A&A, 281, L81 Johnson D. R. H., Barlow M. J., Drew J. E., Brinks E., 1992, MNRAS, 255, 261 Jones T. J. et al., 1993, ApJ, 411, 323 Kastner J. H., Weintraub D. A., 1995, ApJ, 452,833 Kilian J., 1992, A&A, 262,171 Kingsburgh R. L., Barlow M. J., 1994, MNRAS, 271, 257 Lamers H. J. G. L. M., Snow T. P., Lindholm D. M., 1995, ApJ, 455, 269 Lamers H. J. G. L. M. et al., 1996, A&A, 315, L225 Langer N., Hamann W.-R., Lennon M., Najarro F., Pauldrach A. W. A., PuIs J., 1994, A&A, 290, 819 Leitherer C. et al., 1994, ApJ, 428,292 Meynet G., Maeder A., Schaller G., Schaerer D., Charbonnel C., 1994, A&AS, 103, 97 Mitra P. M., Dufour R. J., 1990, MNRAS, 242, 98 Nota A., Leitherer C., Clampin M., Greenfield P., Golimowski D. A., 1992, ApJ, 398, 621 Nota A., Livio M., Clampin M., Schulte-Ladbeck R., 1995, ApJ, 448, 788 Nota A., Clampin M., Garcia-Segura G., Leitherer C., Langer N., 1996, in Benvenuti P., Macchetto F. D., Schreier E. J., eds, Science with the Hubble Space Telescope - II. Space Telescope Science Institute, Baltimore, MD, p. 398 Nota A., Smith L. J., Pasquali A., Clampin M., Stroud M. P., 1997, ApJ, in press Oudmaijer R. D., Groenewegen M. A. T., Matthews H. E., Blomrnaert J. A. D. L., Sahu K. C., 1996, MNRAS, 280,1062 Panagia N., Scuderi S., Gilmozzi R., Kirshner R. P., 1994, BAAS, 26, 1445 Paresce F., Nota A., 1989, ApJ, 341, L83 Pasquali A., Langer N., Schmutz W., Leitherer C., Nota A., Hubeny I., Moffat A. F. J., 1997, ApJ, 470, 340 275 Robberto M., Ferrari A., Nota A., Paresce F., 1993, A&A, 269, 330 Rolleston W. R. J., Brown P. J. F., Dufton P. L., Howarth I. D., 1996: A&A, 315,95 Rosa M. R., 1987, in Appenzeller I., Jordan C., eds, Proc. lAU Symp. 122, Circurnstellar Matter. Kluwer, Dordrecht, p. 457 Rosa M. R., Mathis J. S., 1990, in Garmany C. D., ed., ASP Conf. Ser. 7, Properties of Hot Luminous Stars. Astron. Soc. Pac., San Francisco, p.135 Schaller G., Schaerer D., Meynet G., Maeder A., 1992, A&AS, 96, 269 Schulte-Ladbeck R. E., Clayton G. C., Meade M. R., 1993, in Cassinelli J. P., Churchwell E. B., eds, ASP Conf. Ser. 35, Massive Stars: Their Lives in the Interstellar Medium. Astron. Soc. Pac., San Francisco, p. 237 Shaver P. A., McGee R. X., Newton L. M., Danks A. C., Pottasch S. R., 1983, MNRAS,204,53 SmithL. J., 1991, in van derHucht K. A., Hidayat B., eds, Proc. lAU Symp. 143, Wolf-Rayet Stars and Interrelations with other Massive Stars in Galaxies. Kluwer, Dordrecht, p. 385 Smith L. J., Crowther P. A., Prinja R. K., 1994, A&A, 281,833 Stone R. P. S., Baldwin J. A., 1983, MNRAS 204, 347 Stothers R. B., Chin C.-w., 1993, ApJ, 408, L85 Stothers R. B., Chin C.-w., 1994, ApJ, 426, L43 Stothers R. B., Chin C.-w., 1995, ApJ, 451, L61 Stothers R. B., Chin C.-w., 1996, ApJ, 468, 842 Thackeray A. D., 1950, MNRAS, 110, 524 Thackeray A. D., 1977, MNRAS, 180, 95 Vl1chez J. M., Esteban c., 1996, MNRAS, 280, 720 This paper has been typeset from a TEXILA TEX file prepared by the author. © 1997 RAS, MNRAS 290, 265-275 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System