The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations

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

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In a previous paper on the Magellanic Clouds, we demonstrated that coeval clusters provide a powerful tool for probing the progenitor masses of Wolf-Rayet (W-R) stars and luminous blue variables (LBVs). Here we extend this work to the higher metallicity regions of the Milky Way, studying 12 Galactic clusters. We present new spectral types for the unevolved stars and use these, plus data from the literature, to construct H-R diagrams. We find that all but two of the clusters are highly coeval, with the highest mass stars having formed over a period of less than 1 Myr. The turnoff masses show that at Milky Way metallicities some W-R stars (of early WN type) come from stars with masses as low as 20–25 M⊙. Other early-type WN stars appears to have evolved from high masses, suggesting that a large range of masses evolve through an early WN stage. On the other hand, WN7 stars are found only in clusters with very high turnoff masses, over 120 M⊙. Similarly, the LBVs are only found in clusters with the highest turnoff masses, as we found in the Magellanic Clouds, providing very strong evidence that LBVs are a normal stage in the evolution of the most massive stars. Although clusters containing WN7 stars and LBVs can be as young as 1 Myr, we argue that these objects are evolved, and that the young age simply reflects the very high masses that characterize the progenitors of such stars. In particular, we show that the LBV η Car appears to be coeval with the rest of the Trumpler 14/16 complex. Although the WC stars in the Magellanic Clouds were found in clusters with turnoff masses as low as 45 M⊙, the three Galactic WC stars in our sample are all found in clusters with high turnoff masses (>70 M⊙); whether this difference is significant or due to small number statistics remains to be seen. The bolometric corrections of Galactic W-R stars are hard to establish using the cluster turnoff method but are consistent with the "standard model" of Hillier.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/563804327c82a65c89eee782e331e4ed63801648

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Citations in the corpus

A Study of Cygnus OB2: Pointing the Way toward Finding Our Galaxy’s Super-Star Clusters (2003, background): “Citing proximity arguments, Massey et al. (2001) dismissed the van der Hucht et al. (1981) claim that all three W-R stars were members of Cyg OB2.”
A New Spectral Classification System for the Earliest O Stars: Definition of Type O2 (2002, background): “Pismis 24-17 was first classified as O3 by Massey et al. (2001).”
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).”
Progenitors of Core-Collapse Supernovae (2009, background): “Massey, DeGioia-Eastwood, and Waterhouse (2001) studied the WR population in twelve Galactic clusters and show that at solar metallicity the minimum initial mass to produce a WR through single star evolution is above 25M⊙.”
On the massive stellar population of the super star cluster Westerlund 1 (2005, background): “D-0039). within clusters, since this enables us to study a coeval population at a uniform metallicity, where the MS turnoff mass and hence post-MS progenitor mass may be accurately determined (e.g., Massey et al. 2001).”
THE GALACTIC O-STAR SPECTROSCOPIC SURVEY (GOSSS). II. BRIGHT SOUTHERN STARS (2013, background): “This star was not included in Máız Apellániz et al. (2004) but was classified as an O star by Massey et al. (2001).”
A census of the Wolf–Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy (2006, methodology): “From membership of WR stars in open clusters, Schild & Maeder (1984) and Massey, DeGioia-Eastwood & Waterhouse (2001) investigated the initial masses of WR stars empirically, from the membership of WR stars in Milky Way clusters, to which we can now add Westerlund 1, which hosts more WR stars than a”
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).”