Stellar Population Diagnostics of the Massive Star Binary Fraction
  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.
