THE MASS-RADIUS(-ROTATION?) RELATION FOR LOW-MASS STARS

The fundamental properties of low-mass stars are not as well understood as those of their more massive counterparts. The best method for constraining these properties, especially masses and radii, is to study eclipsing binary systems, but only a small number of late-type (>= M0) systems have been identified and well characterized to date. We present the discovery and characterization of six new M dwarf eclipsing binary systems. The 12 stars in these eclipsing systems have masses spanning 0.38-0.59 M-circle dot and orbital periods of 0.6-1.7 days, with typical uncertainties of similar to 0.3% in mass and similar to 0.5%-2.0% in radius. Combined with six known systems with high-precision measurements, our results reveal an intriguing trend in the low-mass regime. For stars with M = 0.35-0.80 M-circle dot, components in short-period binary systems (P less than or similar to 1 day; 12 stars) have radii which are inflated by up to 10% (mu = 4.8% +/- 1.0%) with respect to evolutionary models for low-mass main-sequence stars, whereas components in longer-period systems (>1.5 days; 12 stars) tend to have smaller radii (mu = 1.7% +/- 0.7%). This trend supports the hypothesis that short-period systems are inflated by the influence of the close companion, most likely because they are tidally locked into very high rotation speeds that enhance activity and inhibit convection. In summary, very close binary systems are not representative of typical M dwarfs, but our results for longer-period systems indicate that the evolutionary models are broadly valid in the M similar to 0.35-0.80 M-circle dot regime.