Radius Inflation at Low Rossby Number in the Hyades Cluster

Radius inflation continues to be explored as a peculiar occurrence among magnetically active, low-mass stars. Recently, Somers & Stassun showed that radius inflation among low-mass stars in the young open cluster M45 (Pleiades Cluster) is correlated to the rotation rate: faster rotators are more inflated. Here we extend that work to a sample of 68 stars of the older open Hyades Cluster. We derive the stars’ spectral energy distributions to measure their bolometric fluxes. With spectroscopically defined Teff and Gaia distances we calculate stellar radii using the Stefan–Boltzmann relation. We find numerous stars that exhibit significant (3–4σ) radius inflation relative to a nominal cluster isochrone. We compare these results to that of the younger Pleiades and consider radius inflation as a function of open cluster evolution. We find that unlike the Pleiades, there is not a statistically significant correlation between radius inflation and stellar rotation period. However, we do find that most inflated stars have (rapid) rotational Rossby numbers of 0.1–0.2, such that the correlation of radius inflation with Rossby number is statistically significant at 99.98% confidence. Because the canonical rotation-activity relation of low-mass stars is understood to result from the connection between magnetic activity and surface convection, our results imply that magnetic activity within the convective layers of low-mass stars is what preferentially drives radius inflation.