A dynamical mass constraint for pre-main-sequence evolutionary tracks: Thebinary NTT 045251+3016

We present an astrometric-spectroscopic orbital solution for the pre-main-s equence binary NTT 045251 + 3016. Interferometric observations with the HST Fine Guidance Sensor No. 3 allowed stellar separations as small as 14 mas to be measured. Optical spectra provided 58 radial velocity measurements of the primary star, and near-infrared spectra provided two radial velocity m easurements of both the primary and secondary, giving a mass ratio for the binary system. The combination of these data allows the dynamical masses an d the distance of the stars to be derived. Our measurements for the primary and secondary masses are 1.45 +/- 0.19 and 0.81 +/- 0.09 respectively, and 145 +/- 8 pc for the distance of the system, consistent with prior estimat es for the Taurus-Auriga star-forming region. The evolutionary tracks of D' Antona & Mazzitelli, published in 1997, Baraffe et al., published in 1998, and Palla & Stahler, published in 1999, are tested against these dynamical mass measurements. Because of the intrinsic color-T-eff variation within th e K5 spectral class, each pre-main-sequence model provides a mass range for the primary. The theoretical mass range derived from the Baraffe et al. tr acks that use a mixing-length parameter of alpha = 1.0 is closest to our me asured primary mass, deviating between 1.3 and 1.6 sigma. The set of Baraff e et al. tracks that use alpha = 1.9 deviates between 1.6 and 2.1 sigma fro m our measured primary mass. The mass range given by the Palla & Stahler tr acks for the primary star deviate between 1.6 and 2.9 sigma. The D'Antona & Mazzitelli tracks give a mass range that deviates by at least 3.0 sigma fr om our derived primary mass, strongly suggesting that these tracks are inco nsistent with our observation. Observations of the secondary are less const raining than those of the primary, but the deviations between the dynamical mass of the secondary and the mass inferred for the secondary from the var ious pre-main-sequence tracks mirror the deviations of the primary star. Al l the pre-main-sequence tracks are consistent with coevality of the compone nts of NTT 045251 + 3016.