A search for radius inflation among active M-dwarfs in Praesepe

Abstract

Rotation periods from Kepler K2 are combined with projected rotation\nvelocities from the WIYN 3.5-m telescope, to determine projected radii for\nfast-rotating, low-mass ($0.15 \\leq M/M_{\\odot} \\leq 0.6$) members of the\nPraesepe cluster. A maximum likelihood analysis that accounts for observational\nuncertainties, binarity and censored data, yields marginal evidence for radius\ninflation -- the average radius of these stars is $6\\pm4$ per cent larger at a\ngiven luminosity than predicted by commonly-used evolutionary models. This\nover-radius is smaller (at 2-sigma confidence) than was found for similar stars\nin the younger Pleiades using a similar analysis; any decline appears due to\nchanges occurring in higher mass ($>0.25 M_{\\odot}$) stars. Models\nincorporating magnetic inhibition of convection predict an over-radius, but do\nnot reproduce this mass dependence unless super-equipartition surface magnetic\nfields are present at lower masses. Models incorporating flux-blocking by\nstarspots can explain the mass dependence but there is no evidence that spot\ncoverage diminishes between the Pleiades and Praesepe samples to accompany the\ndecline in over-radius. The fastest rotating stars in both Praesepe and the\nPleiades are significantly smaller than the slowest rotators for which a\nprojected radius can be measured. This may be a selection effect caused by more\nefficient angular momentum loss in larger stars leading to their progressive\nexclusion from the analysed samples. Our analyses assume random spin-axis\norientations; any alignment in Praesepe, as suggested by Kovacs (2018), is\nstrongly disfavoured by the broad distribution of projected radii.\n