On the Correlation between the Magnetic Activity Levels, Metallicities, and Radii of Low‐Mass Stars

The recent burst in the number of radii measurements of very low-mass starsfrom eclipsing binaries and interferometry of single stars has opened morequestions about what can be causing the discrepancy between the observed radiiand the ones predicted by the models. The two main explanations being proposedare a correlation between the radius of the stars and their activity levels ortheir metallicities. This paper presents a study of such correlations using allthe data published to date. The study also investigates correlations betweenthe radii deviation from the models and the masses of the stars. There is noclear correlation between activity level and radii for the single stars in thesample. Those single stars are slow rotators with typical velocities v_rot sini< 3.0 km s^-1. A clear correlation however exists in the case of the fasterrotating members of binaries. This result is based on the of X-ray emissionlevels of the stars. There also appears to be an increase in the deviation ofthe radii of single stars from the models as a function of metallicity, aspreviously indicated by Berger et al. (2006). The stars in binaries do not seemto follow the same trend. Finally, the Baraffe et al. (1998) models reproducewell the radius observations below 0.30-0.35Msun, where the stars become fullyconvective, although this result is preliminary since almost all the samplestars in that mass range are slow rotators and metallicities have not beenmeasured for most of them. The results in this paper indicate that stellaractivity and metallicity play an important role on the determination of theradius of very low-mass stars, at least above 0.35Msun.Comment: 22 pages, 4 figures. Accepted for publication on Ap