Fundamental Parameters of Wolf-Rayet Stars VI
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Astron. Astrophys. 320, 500–524 (1997) ASTRONOMY AND ASTROPHYSICS Fundamental parameters of Wolf-Rayet stars VI. Large Magellanic Cloud WNL stars? P.A.Crowther and L.J. Smith Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK Received 5 February 1996 / Accepted 26 June 1996 Abstract. We present a detailed, quantitative study of late WN Key words: stars: Wolf-Rayet;mass-loss; evolution; fundamen- (WNL) stars in the LMC, based on new optical spectroscopy tal parameters – galaxies: Magellanic Clouds (AAT, MSO) and the Hillier (1990) atmospheric model. In a pre- vious paper (Crowther et al. 1995a), we showed that 4 out of the 10 known LMC Ofpe/WN9 stars should be re-classified WN9– 10. We now present observations of the remaining stars (except the LBV R127), and show that they are also WNL (WN9–11) 1. Introduction stars, with the exception of R99. Our total sample consists of 17 stars, and represents all but one of the single LMC WN6– Quantitative studies of hot luminous stars in galaxies are im- 11 population and allows a direct comparison with the stellar portant for a number of reasons. First, and probably foremost, parameters and chemical abundances of Galactic WNL stars is the information they provide on the effect of the environment (Crowther et al. 1995b; Hamann et al. 1995a). Previously un- on such fundamental properties as the mass-loss rate and stellar published ultraviolet (HST-FOS, IUE-HIRES) spectroscopy are evolution. In the standard picture (e.g. Maeder & Meynet 1987) presented for a subset of our programme stars. mass-loss significantly affects the evolution of massive O stars We find observational evidence for lower metallicities in as they proceed through an Of phase, then an unstable Luminous LMC WNL stars compared to the Galaxy, though this is not re- Blue Variable (LBV) phase, before becoming late WN (WNL) flected in their stellar properties. For Galactic and LMC stars we stars. The current theory of driving mass-loss through radiation find: (i) a similar range in temperature and luminosity, in contrast pressure predicts mass-loss to scale as the square-root of the metallicity (Kudritzki et al. 1989). The evolution of massive to evolutionary. predictions; (ii) comparable wind performance values (Mv∞/[L/c]) and hydrogen composition, with a broad stars should therefore be strongly dependent on the local envi- correlation between increasing helium content and wind perfor- ronment (Maeder 1991; Maeder & Meynet 1994). Thus stud- mance number; (iii) a general trend to lower wind velocities at ies of massive stars at different metallicities should enable a lower stellar temperature, with possibly slower winds for LMC direct comparison of observations with evolutionary and radia- WN9–11 stars. Some 30 Dor WNL stars show exceptional prop- tion driven wind theory. The question of exactly how mass-loss erties: Brey 89 (HD 38282, WN6h) has the highest luminos- scales with metallicity is of enormous importance. For exam- . −1 ity (log (L/L )∼6.25) and mass-loss rate (log (M/(M yr )) ple, it affects the early chemical enrichment of galaxies, and ∼−3.6) known for any WR star, while Brey 80 (R135, WN7h) the evolution of starbursts containing many thousands of O and has an enormous wind performance number of 50. The observed Wolf-Rayet (WR) stars (e.g. Leitherer & Heckman 1995). physical properties of our sample of LMC WNL stars supports In this series of papers we have studied the physical and the Crowther et al. (1995c) evolutionary scheme for Galactic chemical nature of predominantly Galactic Wolf-Rayet stars by stars, in that the most massive O stars, exclusive to 30 Dor, combining detailed model atmosphere calculations with high evolve directly to O3 If/WN6 and subsequently WN6–7 stars quality, multi-wavelength observations. In particular, we have (e.g. Brey 89), without passing through an intermediate LBV investigated WNL stars (Crowther et al. 1995a,b,c, hereafter phase. In contrast, lower initial mass stars evolve through a Papers I–III), weak-lined, early WN (WNE) stars (Crowther et LBV phase, encompassing a WN9–11 stage (e.g. BE294), with al. 1995d) and intermediate WN/C stars (Crowther et al. 1995e). WN8 stars being their immediate successors. Other related studies have been conducted on LBVs (L.J. Smith et al. 1994), Of stars (Crowther & Bohannan 1997), an M33 Send offprint requests to: P.A.Crowther ([email protected]) WR star (L.J. Smith et al. 1995), and strong-lined WNE stars ? Based on observations collected at the Anglo-Australian and Mount in the infrared (Crowther & Smith 1996). WC and WNE stars Stromlo Observatories, Australia have not been widely investigated to date since we anticipate P.A.Crowther & L.J. Smith: Fundamental parameters of Wolf-Rayet stars. VI 501 that their parameters will be the most susceptible to the effects Table 1. A complete list of LMC WN6–8 and Ofpe/WN9 stars (exclud- of line blanketing, given their dense winds and high excitation. ing O3 If/WN6 stars). We preferentially use Breysacher (1981, Brey), In this, the final paper of the current series, we present a study Henize (1956, S) and Bohannan & Epps (1974, BE) catalogue numbers, of WNL stars in the Large Magellanic Cloud (LMC). These stars as indicated with an underline. Other catalogue references are Feast et have the distinct advantage over their Galactic counterparts in al. (1960, R), Westerlund & Smith (1964, WS), Sanduleak (1969, Sk), that they lie at a known distance and suffer only moderate in- Fehrenbach et al. (1976, FD), Azzopardi & Breysacher (1985, AB) terstellar extinction. It is thus much easier to determine reliable and Melnick (1985, Mk). WR spectral classifications are taken from absolute fluxes and mass-loss rates. A direct comparison with L.F. Smith et al. (1996), Melnick (1985) and herein Galactic WNL stars (Papers I–III) will allow us to investigate any differences in fundamental properties and evolution, as a result of the lower metallicity of the LMC, by a factor of 2– Brey HD(E) WS FD BE S R Sk/other Sp Type Note 3 (Spite & Spite 1991). The first quantitative comparison of 10b 9 −66◦40 WN10h ◦ the properties of LMC and Galactic WR stars was performed 13 33133 8 12 14 −66◦51 WN8h • by L.J. Smith & Willis (1983) who found no differences ex- 18 269227 12 17 543 91 84 −69◦79 WN9h ◦ cept that the LMC WN stars had stronger He ii emission lines. 24 18 23 28 −65◦55 WN6h • More recently, Koesterke et al. (1991) suggested that, while the 269445 261 30 99 −68◦73 Of/WN∗ • LMC WN stars showed no clear differences from their Galactic 26 36063 19 24 569 161 −71◦21 WN6(h)+? •† counterparts in mass-loss rates or wind velocities, they do have 36 27 32 280 108 −69◦141 WN8h • ◦ ∗ lower stellar luminosities. Nevertheless, it is well known that 269582 294 −69 142a Of/WN • 44a AB18 WN8–9∗ • our Galaxy and the LMC exhibit quite different WN:WC ratios; ◦ ∗ in the solar neighbourhood this ratio is 1:1 and in the LMC it is 269687 335 119 −69 175 Of/WN • 47 42 −68◦115 WN6h • 5:1. It has been suggested that this difference may be directly 57 53 WN7h+? (4) † attributed to metallicity effects (e.g. L.F. Smith 1991). It is also 58 AB4 WN5–6 (1) interesting to note that the frequency distribution of WN stars 64 381 WN9h ◦ among the subtypes is different between the two galaxies; in 65 269828 38 55 383 −69◦209a WN7+.. (2) † the Galaxy there is a broad peak between WN5–6 whereas the 269858f 397 128 127 −69◦220 Of/WN LMC shows a sharp peak at WN4 (L.F. Smith et al. 1996). 71 269883 60 −69◦233 WN7h • Our previous Galactic studies (Papers I–III; Crowther & Bo- 73 AB10 WN7+OB? (2) † hannan 1997) have led to the discovery of different evolutionary 75 63 134 WN6(h) • routes for the formation of WNL stars. In particular, we find 76a Mk37Wa WN7? (3) † 79 Mk49 WN6(h)+? (4) † that very massive early Of stars (≥ 60 M ) evolve to WN6–7 stars with an Ofpe or WNL+abs (hereafter WNLha following 80 64 135 WN7h • 81 Mk53 WN8(h)+? •† L.F. Smith et al. 1996) intermediate stage. For less massive pro- ◦ 82c 38268c 66 609 136c −69 241c WN7o?+OB (3) † genitors, evolution instead proceeds through an intermediate 89 38282 46 70 420 133 144 −69◦246 WN6h • LBV stage to the WN8 spectral type, with WN9–11 stars rep- 90 269928 47 71 421 145 −69◦248 WN6(h)+? •† resenting either dormant or post-LBVs. This evolutionary sce- 91 269927c −69◦249c WN9h ◦ nario naturally explains the observed dichotomy among WNL 470 142 −69◦297 Of/WN∗ • stars (Moffat 1989). However, this scenario is based principally 153 61 −67◦266 Of/WN∗ • on studies of Galactic objects, and it is therefore of interest to examine if it is viable in lower metallicity environments such •: programme star as the LMC. ◦: star previously studied in Paper I 00 In Sects. 2–3 we present and discuss new ultraviolet and op- †: object considered to be multiple at a spatial resolution ≥1 tical observations of a substantial sample of LMC WNL stars. ∗: programme Of/WN stars and AB18 are re-classified in Sect. 3 The fundamental parameters of our sample are derived and dis- References: (1) Breysacher (1981); (2) Walborn et al. (1995b); (3) Parker (1993); (4) see text cussed in Sect. 4. Finally in Sect. 5, we compare the results of this analysis with our Galactic sample and discuss the implica- tions of our results for radiatively-driven winds and evolutionary theories of massive stars.