The Hyades Binary θ² Tauri: Confronting Evolutionary Models with Optical Interferometry

Abstract

We determine the masses and magnitude difference of the components of the Hyades spectroscopic binary θ² Tauri. We find that both components appear to be less massive and/or brighter than predicted from some recent evolutionary models. The rapid rotation and unknown rotational inclination of both components introduce uncertainty in their luminosities and colors, but not enough to reconcile both of them with the evolutionary models. We measured the visual orbit with the Mark III optical interferometer and the Navy Prototype Optical Interferometer and combined it with the Hipparcos proper-motion-based parallax to find a total system mass ΣM of 4.03+/-0.20 M_solar. We also combined our visual orbit with three recent spectroscopic orbits to find three spectroscopically based estimates of ΣM and compared these to the ΣM from the visual orbit and parallax. We chose the spectroscopic orbit that agreed best and used its mass ratio to estimate individual masses M_A,B of 2.15+/-0.12 and 1.87+/-0.11 M_solar. From the interferometry, we determine Δm=1.13+/-0.05 mag across the 450-850 nm band. The parallax then implies absolute V magnitudes M_A,B of 0.48+/-0.05 and 1.61+/-0.06 mag. If the components are rotating near breakup velocity and seen nearly pole-on, the true luminosities may be as faint as 1.03 and 2.13 mag; even in that case, however, the secondary is too blue by ~0.07 mag in B-V.