The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds

By Philip Massey, Elizabeth Waterhouse, K. DeGioia-Eastwood. 2000

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We combine new CCD UBV photometry and spectroscopy with those from the literature to investigate 19 Magellanic Cloud OB associations that contain Wolf-Rayet (W-R) and other types of evolved, massive stars. Our spectroscopy reveals a wealth of newly identified interesting objects, including early O-type supergiants, a high-mass, double-lined binary in the SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV; R85), a newly discovered W-R star (Sk -69°194), and a newly found luminous B[e] star (LH 85-10). We use these data to provide precise reddening determinations and construct physical H-R diagrams for the associations. We find that about half of the associations may be highly coeval, with the massive stars having formed over a short period (Δτ < 1 Myr). The (initial) masses of the highest mass unevolved stars in the coeval clusters may be used to estimate the masses of the progenitors of W-R and other evolved stars found in these clusters. Similarly, the bolometric luminosities of the highest mass unevolved stars can be used to determine the bolometric corrections (BCs) for the evolved stars, providing a valuable observational basis for evaluating recent models of these complicated atmospheres. What we find is the following: (1) Although their numbers is small, it appears that the W-R stars in the SMC come from only the highest mass (greater than 70 M⊙) stars. This is in accord with our expectations that at low metallicities only the most massive and luminous stars will have sufficient mass loss to become W-R stars. (2) In the LMC, the early-type WN (WNE) stars occur in clusters whose turnoff masses range from 30 to 100 M⊙ or more. This suggests that possibly all stars with mass greater than 30 M⊙ pass through a WNE stage at LMC metallicities. (3) The one WC star in the SMC is found in a cluster with a turnoff mass of 70 M⊙, the same as that for the SMC WN stars. In the LMC, the WC stars are found in clusters with turnoff masses of 45 M⊙ or higher, similar to what is found for the LMC WN stars. Thus we conclude that WC stars come from essentially the same mass range as do WN stars and indeed are often found in the same clusters. This has important implications for interpreting the relationship between metallicity and the WC/WN ratio found in Local Group galaxies, which we discuss. (4) The LBVs in our sample come from very high mass stars (greater than 85 M⊙), similar to what is known for the Galactic LBV η Car, suggesting that only the most massive stars go through an LBV phase. Recently, Ofpe/WN9 stars have been implicated as LBVs after one such star underwent an LBV-like outburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br 18, which we find in clusters with much lower turnoff masses (25–35 M⊙). We suggest that Ofpe/WN9 stars are unrelated to "true" LBVs: not all "LBV-like outbursts" may have the same cause. Similarly, the B[e] stars have sometimes been described as LBV-like. Yet, the two stars in our sample appear to come from a large mass range (30–60 M⊙). This is consistent with other studies, suggesting that B[e] stars cover a large range in bolometric luminosities. (5) The bolometric corrections of early WN and WC stars are found to be extreme, with an average BC(WNE) of -6.0 mag and an average BC(WC4) of -5.5 mag. These values are considerably more negative than those of even the hottest O-type stars. However, similar values have been found for WNE stars by applying Hillier's "standard model" for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars (BC = -2 to -4 mag), also consistent with recent analysis done with the standard model. Extension of these studies to the Galactic clusters will provide insight into how massive stars evolve at different metallicities.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/da5642732cf3cd408703451cb10f3d86e7b9d8d5

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Citations in the corpus

Massive Stars in the Local Group: Implications for Stellar Evolution and Star Formation (2003, methodology): “F or p er so na l u se o nl y. MASSIVE STARS IN THE LOCAL GROUP 35 Figure 6 The progenitor masses are shown for various evolved massive stars for the Milky Way (filled circled), LMC (open circles), and SMC (asterisks).”
The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample (2005, methodology): “This star was classified as O3 If by Testor & Niemela (1998) and again as O3 If* by Massey et al. (2000); the data for the latter are the same as what are used here.”
A New Spectral Classification System for the Earliest O Stars: Definition of Type O2 (2002, background): “The high-quality data for ST 2-22 and W28-23 illustrated by Massey et al. (2000) provide an excellent definition of type O4 III(f+) in the present system.”
The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class (2014, background): “However, according to Massey et al. (2000), this is an alias of TSWR 1 which was resolved into multiple components by Walborn et al. (1999) and incorporated in the BAT99 catalog with the number BAT99 78.”
A UBVR CCD Survey of the Magellanic Clouds (2001, background): “We used the ‘‘ Harris set ’’ UBVR filters (Massey et al. 2000a).”
SPITZER SAGE INFRARED PHOTOMETRY OF MASSIVE STARS IN THE LARGE MAGELLANIC CLOUD (2009, background): “Notes. a Star designations: Breysacher et al. (BAT99; 1999); Sanduleak (Sk; 1970); Westerlund (W; 1961); Brunet et al. (BI; 1975); Lucke (LH; 1972); Henize (S; 1956). b Reference key: (B01) Bartzakos et al. 2001; (B09) Bonanos 2009; (C86) Conti et al. 1986; (E06) Evans et al. 2006; (F88) Fitzpatrick”
A Study of Cygnus OB2: Pointing the Way toward Finding Our Galaxy’s Super-Star Clusters (2003, background): “As discussed by Massey & Hunter (1998) and Massey et al. (2000), the Vacca et al. values represent a significant upward trend from previous effective temperature scales (Conti 1973; Humphreys & McElroy 1984; Chlebowski & Garmany 1991).”
THE COSMIC CORE-COLLAPSE SUPERNOVA RATE DOES NOT MATCH THE MASSIVE-STAR FORMATION RATE (2011, background): “From Galactic stellar clusters and stellar clusters in the LMC, such WR stars are estimated to have evolved from main-sequence masses 25 M⊙ and above (Massey et al. 2000, 2001).”
Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars (2014, background): “…of relatively rapid rotation (Georgy et al. 2012).6 At first glance, models and observations would seem to be reasonably well aligned (Massey 2003): MWR is inferred to be about 25 M⊙ at Z⊙ (Crowther 2007), increasing to about 30 and 70 M⊙ at ZLMC and ZSMC , respectively (Massey et al. 2000).”
Luminous blue variables are antisocial: their isolation implies that they are kicked mass gainers in binary evolution (2014, methodology): “As such, this method produces a very different result from cluster turnoffs discussed by Massey et al. (2000).”
A Survey of Local Group Galaxies Currently Forming Stars. III. A Search for Luminous Blue Variables and Other Hα Emission-Line Stars (2007, background): “…surveys of UV-bright objects (Massey et al. 1996), He-emission objects (Corral &Herrero 2003), orH -emission stars (Neese et al. 1991; Corral1996; King et al. 1998), or by just blundering across them spectroscopically (Massey et al. 2000; Massey 2006) in spectroscopic surveys of bright blue stars.”