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The Dependence of Theoretical Synthetic Spectra on $α$-enhancement in Young, Binary Stellar Populations

The enhancement of $α$ 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 $α$-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 [$α$/Fe]. A number of features such as ultraviolet line indices (e.g. at 1719 and 1853Å) 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, $α$-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 [$α$/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.