The Red Supergiant Binary Fraction as a Function of Metallicity in M31 and M33

TL;DR

This study investigates how the binary fraction of red supergiants varies with metallicity in M31 and M33, revealing a strong metallicity dependence in M33 but not in M31, likely due to differences in OB binary distributions.

Contribution

It extends the analysis of RSG binary fractions to M31 and M33, demonstrating a metallicity-dependent trend in M33 and providing insights into binary population variations.

Findings

RSG binary fraction in M33 varies with galactocentric distance.

Higher metallicity environments have higher RSG binary fractions.

No significant binary fraction variation observed in M31 regions.

Abstract

Recent work measuring the binary fraction of evolved red supergiants (RSGs) in the Magellanic Clouds points to a value between 15-30%, with the majority of the companions being un-evolved B-type stars as dictated by stellar evolution. Here I extend this research to the Local Group galaxies M31 and M33, and investigate the RSG binary fraction as a function of metallicity. Recent near-IR photometric surveys of M31 and M33 have lead to the identification of a complete sample of RSGs down to a limiting $\log L/L_{\odot} \geq 4.2$. To determine the binary fraction of these M31 and M33 RSGs, I used a combination of newly obtained spectroscopy to identify single RSGs and RSG+OB binaries as well as archival UV, visible and near-IR photometry to probabilistically classify RSGs as either single or binary based on their colors. I then adjusted the observed RSG+OB binary fraction to account for observational biases. The resulting RSG binary fraction in M33 shows a strong dependence on galactocentric distance with the inner regions having a much higher binary fraction ($41.2^{+12.0}_{-7.3}$%) than the outer regions ($15.9^{+12.4}_{-1.9}$%). Such a trend is not seen in M31; instead, the binary fraction in lightly reddened regions remains constant at $33.5^{+8.6}_{-5.0}$%. I conclude the changing RSG binary fraction in M33 is due to a metallicity dependence with higher metallicity environments having higher RSG binary fractions. This dependence most likely stems not from changes in the physical properties of RSGs due to metallicity, but changes in the parent distribution of OB binaries.