DYNAMICAL MASS CONSTRAINTS ON LOW-MASS PRE–MAIN-SEQUENCE STELLAR EVOLUTIONARY TRACKS: AN ECLIPSING BINARY IN ORION WITH A 1.0 M PRIMARY AND A 0.7 M SECONDARY

We report the discovery of a double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We analyze the system spectroscopically and photometrically to empirically determine precise, distance-independent masses, radii, effective temperatures, and luminosities for both components. The measured masses for the primary and secondary, accurate to 1%, are 1.01 and 0.73 M, respectively; thus, the primary is a definitive pre–main-sequence solar analog, and the secondary is the lowest-mass star yet discovered among pre–main-sequence eclipsing binary systems. We use these fundamental measurements to test the predictions of pre–main-sequence stellar evolutionary tracks. None of the models we examined correctly predict the masses of the two components simultaneously, and we implicate differences between the theoretical and empirical effective tem-perature scales for this failing. All of the models predict the observed slope of the mass-radius relationship reasonably well, though the observations tend to favor models with low convection efficiencies. Indeed, con-sidering our newly determined mass measurements together with other dynamical mass measurements of pre– main-sequence stars in the literature, as well as measurements of Li abundances in these stars, we show that the data strongly favor evolutionary models with inefficient convection in the stellar interior, even though such models cannot reproduce the properties of the present-day Sun