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SDSS-IV MaStar: [α/Fe] for the MaNGA Stellar Library from Synthetic Model Spectra


 We calculate the α-enhancement ratio [α/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 - Teff, 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 [α/Fe]. Using the full spectral fitting code pPXF, we are able to fit multiple lines sensitive to [α/Fe] for a more robust measurement. Quality flags based on the convergence of the sampler, errors in [α/Fe] and a cut in the χ2 of the model fit are used to clean the final catalogue, returning 17,214 spectra and values in the range of −0.25 < [α/Fe] <0.48. Comparing our calculated [α/Fe] with literature values reveals a degeneracy in cool stars with log g ≥∼4; this comparison is then used to create an alternative and calibrated parameter set. We also plot the final catalogue in an [Fe/H] vs. [α/Fe] diagram and recover the expected result of increasing [α/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 which 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 [α/Fe], high [Fe/H] and enhanced [α/Fe], low [Fe/H].