Stellar Population Diagnostics of the Massive Star Binary Fraction

TL;DR

This study uses synthetic stellar populations to analyze how binary star systems influence the observed ratios of massive star types, revealing degeneracies and potential correlations with metallicity, which impact age and metallicity diagnostics.

Contribution

It introduces a method to account for binary stars in population diagnostics, improving the interpretation of massive star observations and their evolutionary implications.

Findings

Binary stars significantly affect stellar count ratios.

Degeneracies exist between metallicity, age, and binary fraction.

Including binaries alters age estimates by over 0.1 dex.

Abstract

Populations of massive stars are directly reflective of the physics of stellar evolution. Counting subtypes of massive stars and ratios of massive stars in different evolutionary states have been used ubiquitously as diagnostics of age and metallicity effects. While the binary fraction of massive stars is significant, inferences are often based upon models incorporating only single-star evolution. In this work, we utilize custom synthetic stellar populations from the Binary Population and Stellar Synthesis (BPASS) code to determine the effect of stellar binaries on number count ratios of different evolutionary stages in both young massive clusters and galaxies with massive stellar populations. We find that many ratios are degenerate in metallicity, age, and/or binary fraction. We develop diagnostic plots using these stellar count ratios to help break this degeneracy, and use these plots to compare our predictions to observed data in the Milky Way and the Local Group. These data suggest a possible correlation between the massive star binary fraction and metallicity. We also examine the robustness of our predictions in samples with varying levels of completeness. We find including binaries and imposing a completeness limit can both introduce $\gtrsim0.1$ dex changes in inferred ages. Our results highlight the impact that binary evolution channels can have on the massive star population.