SDSS-IV MaStar: [alpha/Fe] for the MaNGA Stellar Library from synthetic model spectra

We calculate the alpha-enhancement ratio [alpha/Fe] for the Mapping Nearby Galaxies at APO (MaNGA) Stellar Library (MaStar) while also fitting for the fundamental atmospheric parameters effective temperature, surface gravity, and metallicity - T-eff, log g, [Fe/H]. This approach builds upon a previous catalogue of stellar parameters, whereby only the fundamental atmospheric parameters are fit with solar-scaled models. Here, we use the same Markov Chain Monte Carlo method with the additional free parameter [alpha/Fe]. Using the full spectral fitting code pPXF, we are able to fit multiple lines sensitive to [alpha/Fe] for a more robust measurement. Quality flags based on the convergence of the sampler, errors in [alpha/Fe] and a cut in the chi(2) of the model fit are used to clean the final catalogue, returning 17 214 spectra and values in the range of -0.25 < [alpha/Fe] < 0.48. Comparing our calculated [alpha/Fe] with literature values reveals a degeneracy in cool stars with log g >= similar to 4; this comparison is then used to create an alternative and calibrated parameter set. We also plot the final catalogue in an [Fe/H] versus [alpha/Fe] diagram and recover the expected result of increasing [alpha/Fe] with decreasing [Fe/H] for Milky Way disc-halo stars. We apply our method to a subsample of spectra of uniform resolution and higher signal to noise that finds that our results are independent of this higher signal to noise. In the context of stellar population models, we are able to cover a parameter space for the creation of intermediate to old age models at solar-scaled [alpha/Fe], high [Fe/H] and enhanced [alpha/Fe], low [Fe/H].