Hertzsprung-Russell diagrams and mass distributions of Kepler high-α stars at three different metallicities. The [Fe/H] range and median in each panel is given at the top. Top panels: HR diagrams of the selected stars. The colour coding spans the metallicities in each bin with darker colours being more metal poor. Solid lines represent MESA models of the median [Fe/H], median [α/Fe], and median mass of each bin, shifted by −126 K, consistent with our analysis in Sect. 4.1. Bottom panels: The Kepler mass histograms based on asteroseismology from Yu et al. (2018) and Gaia DR3 parallax zero-points from Lindegren et al. (2021). The mass distributions of RGB and HeCB stars are calculated using Eq. (1), and for the RGB stars also using PARAM with Δν and ν_max as input. The number of stars N and the median mass of each evolutionary phase are shown along with the mass loss, ΔM, the difference in median mass between the HeCB and RGB distributions.