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

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 Σ of 4.03 ± 0.20 ⊙. We also combined our visual orbit with three recent spectroscopic orbits to find three spectroscopically based estimates of Σ and compared these to the Σ from the visual orbit and parallax. We chose the spectroscopic orbit that agreed best and used its mass ratio to estimate individual masses A,B of 2.15 ± 0.12 and 1.87 ± 0.11 ⊙. From the interferometry, we determine Δm = 1.13 ± 0.05 mag across the 450-850 nm band. The parallax then implies absolute V magnitudes MA,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.