On the initial masses and evolutionary origins of Wolf-Rayet stars

Based on a study of the HR diagrams of the stellar associations and clusters with definite and probable Wolf-Rayet star members, the authors find that the lower limit to the initial masses of the Wolf-Rayet stars is greater than 30 M_sun;, and that 80% of them have initial masses greater than 50 M_sun;. No significant difference is found between the WN and WC stars for the lower limits to their initial masses. It has been suggested that some Wolf-Rayet stars have previously passed through an M supergiant stage. However, most red supergiants have evolved from stars with initial masses between 15 and 30 M_sun;; and therefore have too low an initial mass to evolve to the Wolf-Rayet stage. The authors also discuss the evidence for a gradient in the M supergiant to Wolf-Rayet star ratio in our galaxy and M33.