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

Abstract

We combine new CCD UBV photometry and spectroscopy with that from the literature to investigate 19 Magellanic Cloud OB associations that contain Wolf-Rayet (WR) 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 LBV (R 85), a newly discovered WR star (Sk-69 194), and a newly found luminous B[e] star (LH85-10). We use the data to evaluate the extent that each association is coeval, and for the ones with little age spread, determine the turn-off masses of the unevolved stars, as a means of finding the progenitor masses of LBVs and WRs. Our data is also used to find the (minimum) bolometric corrections that apply to these evolved objects. We find that WRs in the SMC come from only the highest mass stars (>70Mo), while in the LMC the WNEs come from a wide range of masses---possibly all stars with masses >30Mo pass though a WNE stage at LMC metallicities. In both the SMC and LMC the WCs appear to come from the same mass range as do the WNs, a finding that has important implications for interpreting the WC/WN ratio among Local Group galaxies. LBVs appear to come from the very highest mass stars. The Ofpe/WN9 stars, sometimes implicated in the "LBV phenonomon" come from very modest mass stars, 25-35Mo, suggesting a different origin for their variability. The BCs of WRs are considerably more negative than those of O-type stars, with BC(WNE)=-6.0 mag and BC(WC4)=-5.5mag; more modest values are found for the Ofpe/WN9 stars (-2 to -4 mag). These findings are consistent with recent analysis done with Hillier's "standard model".