The Dependence of Theoretical Synthetic Spectra on $\alpha$-enhancement
  in Young, Binary Stellar Populations
  The enhancement of $\alpha$ elements such as oxygen is an important phase in
the chemical evolution of the early Universe, with nebular material becoming
enriched in these elements sooner than iron. Here we present models which
incorporate stellar spectra with $\alpha$-enhanced compositions, focusing on
the impact on the integrated light of young stellar populations, including
those with large binary star fractions using the Binary Populations and
Spectral Synthesis (BPASS) framework, while using Solar-scaled stellar
evolution models. We find that broad spectrum outputs such as production of
ionising flux, the ultraviolet spectral slope and optical colours are only
weakly affected by a change in [$\alpha$/Fe]. A number of features such as
ultraviolet line indices (e.g. at 1719 and 1853\r{A}) and optical line indices
(such as MgB) are sensitive to such changes in composition for a continuously
star-forming population and a single starburst population respectively. We find
that at ages of more than 1Gyr, $\alpha$-enhanced stellar populations appear
bluer than their Solar-scaled counterparts, and show expected sensitivity of
optical line indices to composition, in agreement with previous work. The
ultraviolet stellar absorption lines are relatively insensitive to subtleties
in the abundances ratios, although with sufficient measurement precision, a
combination of UV line indices may enable a simultaneous measurement of total
metallicity mass fraction and [$\alpha$/Fe] in young stellar populations. The
output models are designated as BPASS v2.3 and made available to the community
with the aim of assisting interpretation of observations of high-redshift
galaxies with the James Webb Space Telescope.
