The Close Binary Frequency of Wolf-Rayet Stars as a Function of Metallicity in M31 and M33

TL;DR

This study investigates whether the frequency of close Wolf-Rayet binaries varies with metallicity in M31 and M33, finding no significant dependence and suggesting other factors influence WC star abundance at high metallicity.

Contribution

It provides the first empirical assessment of close Wolf-Rayet binary frequency across different metallicities in nearby galaxies.

Findings

Close binary frequency of Wolf-Rayet stars is not metallicity dependent.

Approximately 100 Wolf-Rayet binaries identified through radial velocity variations.

Results challenge the hypothesis that binary interactions explain WC star overabundance at high metallicity.

Abstract

Massive star evolutionary models generally predict the correct ratio of WC-type and WN-type Wolf-Rayet stars at low metallicities, but underestimate the ratio at higher (solar and above) metallicities. One possible explanation for this failure is perhaps single-star models are not sufficient and Roche-lobe overflow in close binaries is necessary to produce the "extra" WC stars at higher metallicities. However, this would require the frequency of close massive binaries to be metallicity dependent. Here we test this hypothesis by searching for close Wolf-Rayet binaries in the high metallicity environments of M31 and the center of M33 as well as in the lower metallicity environments of the middle and outer regions of M33. After identifying ~100 Wolf-Rayet binaries based on radial velocity variations, we conclude that the close binary frequency of Wolf-Rayets is not metallicity dependent and thus other factors must be responsible for the overabundance of WC stars at high metallicities. However, our initial identifications and observations of these close binaries have already been put to good use as we are currently observing additional epochs for eventual orbit and mass determinations.