Benchmarking mesa isochrones against the Hyades single star sequence

Based on GAIA Early Data Release 3 (EDR3), we revisit and update our sample of bonafide single stars in the Hyades open cluster. The small observational uncertainties in parallax and photometry of EDR3 result in a tightly defined stellar sequence, which is ideal for the testing and calibration of theoretical stellar evolutionary tracks and isochrones. We benchmark the solar-scaled mesa evolutionary models against the single star sequence. We find that the non-rotating mesa models for [Fe/H] = +0.25 provide a good fit for stars with masses above 0.85, and very low mass stars below 0.25 M-circle dot. For stars with masses between 0.25 and 0.85 M-circle dot, the models systematically under predict the observed stellar luminosity. One potential limitation of the models for partially convective stars more massive than 0.35 M-circle dot is the prescription of (superadiabatic) convection with the mixing-length theory parameter alpha(ML) tuned to match the Solar model. Below 0.35 M-circle dot, the increased scatter in the stellar sequence might be a manifestation of the convective kissing instability, which is driven by variations in the He-3 nuclear energy production rate due to instabilities at the convective core to envelope boundary. For a Hyades-like stellar population, the application of solar-scaled models to subsolar mass stars could result in a significant underestimate of the age, or an overestimate of the metallicity. We suggest that future grids of solar-scaled evolutionary stellar models could be complemented by Hyades-scaled models in the mass range 0.25 to 0.85 M-circle dot.