The Wolf-Rayet Content of M31

TL;DR

This study provides a comprehensive survey of Wolf-Rayet stars in M31, identifying 107 new stars mostly of WN-type, and analyzes their distribution and metallicity-related properties to test stellar evolution models.

Contribution

It presents the first complete optical survey of M31's Wolf-Rayet stars, detecting weaker-lined WN types and analyzing their distribution and metallicity implications.

Findings

154 confirmed WR stars in M31, 95% complete.

Most WRs are in OB associations around the Population I ring.

The WC-to-WN ratio is higher than models predict, indicating possible effects of Roche-lobe overflow or rotational velocities.

Abstract

Wolf-Rayet stars are evolved massive stars, and the relative number of WC-type and WN-type WRs should vary with the metallicity of the host galaxy, providing a sensitive test of stellar evolutionary theory. However, past studies of the WR content of M31 have been biased towards detecting WC stars, as their emission line signatures are much stronger than those of WNs. Here we present the results of a survey covering all of M31's optical disk (2.2 deg^2), with sufficient sensitivity to detect the weaker-lined WN-types. We identify 107 newly found WR stars, mostly of WN-type. This brings the total number of spectroscopically confirmed WRs in M31 to 154, a number we argue is complete to ~95%, except in regions of unusually high reddening. This number is consistent with what we expect from the integrated Halpha luminosity compared to that of M33. The majority of these WRs formed in OB associations around the Population I ring, although 5% are truly isolated. Both the relative number of WC to WN-type stars as well as the WC subtype distribution suggest that most WRs exist in environments with higher-than-solar metallicities, which is consistent with studies of M31's metallicity. Although the WC-to-WN ratio we find for M31 is much lower than that found by previous studies, it is still higher than what the Geneva evolutionary models predict. This may suggest that Roche-lobe overflow produces the excess of WC stars observed at high metallicity, or that the assumed rotational velocities in the models are too high.