The Radius Discrepancy in Low-mass Stars: Single versus Binaries

A long-standing issue in the theory of low-mass stars is the discrepancy between predicted and observed radii and effective temperatures. In spite of the increasing availability of very precise radius determinations from eclipsing binaries and interferometric measurements of radii of single stars, there is no unanimous consensus on the extent (or even the existence) of the discrepancy and on its connection with other stellar properties (e. g., metallicity, magnetic activity). We investigate the radius discrepancy phenomenon using the best data currently available (accuracy less than or similar to 5%). We have constructed a grid of stellar models covering the entire range of low-mass stars (0.1-1.25 M-circle dot) and various choices of the metallicity and mixing length parameter, alpha. We used an improved version of the Yale Rotational stellar Evolution Code, implementing surface boundary conditions based on the most up-to-date PHOENIX atmosphere models. Our models are in good agreement with others in the literature and improve and extend the low mass end of the Yale-Yonsei isochrones. Our calculations include rotation-related quantities, such as moments of inertia and convective turnover timescales, useful in studies of magnetic activity and rotational evolution of solar-like stars. Consistent with previous works, we find that both binaries and single stars have radii inflated by about 3% with respect to the theoretical models; among binaries, the components of short orbital period systems are found to be the most deviant. We conclude that both binaries and single stars are comparably affected by the radius discrepancy phenomenon.