The Hyades Binary Finsen 342 (70 Tauri): A Double‐lined Spectroscopic Orbit, the Distance to the Cluster, and the Mass‐Luminosity Relation

Abstract

We present new radial velocity measurements for both components of theclose ({ρ} ~ 0.1'') visual binary Finsen 342 ({\phis}342) in the Hyades.We combine speckle and visual observations with our velocities to derivean astrometric-spectroscopic orbital solution. This yields individualmasses for the stars (M_{A} = 1.363 +/- 0.073 M_{⊙}and M_{B} = 1.253 +/- 0.075 M_{⊙}) as well as anorbital parallax for the system ({π}_{orb} = 0.02144'' +/-0.00067''). We use relative proper motions to transfer this orbitalparallax to 53 representative cluster members and derive a distance tothe cluster of 47.4 +/- 2.3 pc (m - M = 3.38 +/- 0.11), which is only 1%closer than the cluster distance derived recently by Torres, Stefanik,{\amp} Latham from the orbital parallax for 51 Tauri, anotherastrometric-spectroscopic binary in the Hyades.We compare the masses and luminosities we derive for {\phis}342, alongwith those of two other binaries in the Hyades (vB 22 and 51 Tau), withisochrones computed for the cluster and find that there is goodagreement for an age of 630 Myr and metallicity Z = 0.027. Because thedistances to these binaries have been determined directly, no arbitraryshifts have been applied, as is usually done when fitting mainsequences. These results therefore provide a fundamental test of themass-luminosity relation as predicted by recent stellar evolutionmodels.Some of the observations reported here were obtained with the MultipleMirror Telescope, a joint facility of the Smithsonian Institution andthe University of Arizona.