The Hyades Binary 51 Tauri: Spectroscopic Detection of the Primary, the Distance to the Cluster, and the Mass‐Luminosity Relation

Abstract

51 Tauri has long been known as a double-lined spectroscopic binary (orbital period = 11 yr) with the peculiarity that only the velocities of the secondary component could be measured. We report here new spectroscopic observations of this system that have enabled us to measure velocities for the rapidly rotating primary using TODCOR, a two-dimensional cross-correlation technique, and we have derived a double-lined orbit. In addition, 51 Tau is a visual binary resolved by speckle interferometry. By combining the astrometric and spectroscopic data, we have obtained the first complete visual-spectroscopic solution for the system, from which we derive the orbital parallax (?orb = 00179 ? 00006) and the individual masses (MA = 1.80 ? 0.13 M? and MB = 1.46 ? 0.18 M?). Based on this orbital parallax, we use relative proper motions for a representative sample of cluster members to obtain a distance modulus for the cluster of m - M = 3.40 ? 0.07 (47.8 ? 1.6 pc). We compare the empirical mass-luminosity relation from all available direct mass determinations with recent theoretical models, and we take advantage of the unusual opportunity that we have absolute luminosities and colors to derive an age estimate of 600 Myr for the Hyades, by comparing with stellar evolution models without any arbitrary adjustments.