Dynamical Mass Measurements of Pre-Main-Sequence Stars: Fundamental Tests of the Physics of Young Stars

There are now 23 dynamical mass measurements for PMS stars of less than 2 Mo, with most of the measured stars having masses greater than 0.5 Mo. The masses of two PMS brown dwarfs have also been precisely measured. The most important application of these dynamical mass measurements has been to provide tests of theoretical masses derived from PMS stellar evolution models. On average most models in use today predict stellar masses to within 20%; however, the predictions for individual stars can be in error by 50% or more. Now that dynamical mass measurements are relatively abundant, and will become more so with the application of ground-based optical/infrared interferometers, the primary limitations to such tests have become systematic errors in the determination of the stellar properties necessary for the comparison with evolutionary models, such as effective temperature, luminosity, and radii. Additional dynamical mass determinations between 0.5 Mo and 2 Mo will not likely improve the constraints on evolutionary models until these systematic uncertainties in measurements of stellar properties are reduced. The nature and origin of these uncertainties, as well as the dominant physical issues in theoretical PMS stellar evolution models, are discussed. There are immediately realizable possibilities for improving the characterizations of those stars with dynamical mass measurements. Additional dynamical mass measurements for stars below 0.5 Mo are also very much needed.