Dynamical Masses of Pre-Main Sequence Visual Binary Stars

The most fundamental parameter in determining the evo-lution of a star is its mass; unfortunately, it is also difficult to measure directly. Typically one instead uses theoretical mod-els to calculate the mass of a star. However, the most popu-lar models [1-6] give masses and ages are systematically dis-crepant by up to factors of two and ten, respectively, in the pre-main sequence (PMS). This obvious disparity stems from differences in the treatment of convection, opacity, equations of state, and atmospheres [7]. Thus, in spite of continual im-provement in the calculation of these quantities over the past 15 years, the ”correct ” model remains a debatable topic. In order to calibrate these models, it is essential to have well-determined dynamical masses for comparison. The cur-rent number of PMS stars with dynamical mass estimates is small, and these measurements are riddled with uncertaini-ties. Worse, there are no definitive mass estimates for stars 0.3 M (spectral types later than M2 at 2 Myrs), which encompasses the critical stellar/substellar boundary. Thus, it is imperative that the number of well-determined dynamical masses in the PMS be increased, particularly on the very low mass end. We present here the most recent results of our ongo-ing monitoring campaign of young, low-mass, visual binary stars using high angular resolution techniques. The goal of this project is to derive astrometric orbits for these stars and thereby enable further calibration of PMS evolutionary tracks. Our observations have been obtained using speckle interfer-ometry on the Keck I 10 m telescope and adaptive optics on the Keck II 10 m telescope. The requirements constraining the observed sample are that the binaries are resolvable with Keck and that they have periods that allow for the determi-nation of their orbital solutions within a reasonable period of time. These requirements lead to a sample that consists of bi-naries with