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

We present new radial velocity measurements for both components of the close (ρ ≈ 0 1) visual binary Finsen 342 (ϕ342) in the Hyades. We combine speckle and visual observations with our velocities to derive an astrometric-spectroscopic orbital solution. This yields individual masses for the stars ( M A = 1.363 ± 0.073 M ☉ and M B = 1.253 ± 0.075 M ☉ ) as well as an orbital parallax for the system (π orb = 0 02144 ± 0 00067). We use relative proper motions to transfer this orbital parallax to 53 representative cluster members and derive a distance to the 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, & Latham from the orbital parallax for 51 Tauri, another astrometric-spectroscopic binary in the Hyades. We compare the masses and luminosities we derive for ϕ342, along with those of two other binaries in the Hyades (vB 22 and 51 Tau), with isochrones computed for the cluster and find that there is good agreement for an age of 630 Myr and metallicity Z = 0.027. Because the distances to these binaries have been determined directly, no arbitrary shifts have been applied, as is usually done when fitting main sequences. These results therefore provide a fundamental test of the mass-luminosity relation as predicted by recent stellar evolution models.