THE RADIUS DISCREPANCY IN LOW-MASS STARS: SINGLE VERSUS BINARIES

Abstract

A long-standing issue in the theory of low mass stars is the discrepancy\nbetween predicted and observed radii and effective temperatures. In spite of\nthe increasing availability of very precise radius determinations from\neclipsing binaries and interferometric measurements of radii of single stars,\nthere is no unanimous consensus on the extent (or even the existence) of the\ndiscrepancy and on its connection with other stellar properties (e.g.\nmetallicity, magnetic activity). We investigate the radius discrepancy\nphenomenon using the best data currently available (accuracy about 5%). We have\nconstructed a grid of stellar models covering the entire range of low mass\nstars (0.1-1.25 M_sun) and various choices of the metallicity and of the mixing\nlength parameter \\alpha. We used an improved version of the Yale Rotational\nstellar Evolution Code (YREC), implementing surface boundary conditions based\non the most up-to-date PHOENIX atmosphere models. Our models are in good\nagreement with others in the literature and improve and extend the low mass end\nof the Yale-Yonsei isochrones. Our calculations include rotation-related\nquantities, such as moments of inertia and convective turnover time scales,\nuseful in studies of magnetic activity and rotational evolution of solar-like\nstars. Consistently with previous works, we find that both binaries and single\nstars have radii inflated by about 3% with respect to the theoretical models;\namong binaries, the components of short orbital period systems are found to be\nthe most deviant. We conclude that both binaries and single stars are\ncomparably affected by the radius discrepancy phenomenon.\n