Peculiar rotation in evolved binary systems: stellar and tidal evolution of TZ Fornacis
Instituto de Astrofísica de Andalucía,
CSIC, Apartado 3004,
Accepted: 8 September 2010
Aims. TZ Fornacis is an evolved binary system with an orbital period of 75.7 days and a circular orbit. The two stars have similar masses (2.045 ± 0.055 and 1.945 ± 0.027 M⊙) but very different radii: 8.33 ± 0.12 and 3.966 ± 0.088 R⊙. One of its most interesting characteristics is that while the primary rotates synchronously with the mean orbital angular velocity, the secondary is rotating 16 times faster than this reference value. The stellar and tidal evolution of such a system was investigated in the past using some simplifications: integration of the time scales for circularization and synchronization that are valid only for low eccentricities and small departures from the synchronism, which are not fully valid for the case of TZ Fornacis. Another detected problem is the inconsistency between the observed levels of circularization/synchronization and the theoretical critical times and the inferred age (the time of synchronization of the secondary was found to be shorter than its age). The main goal of the present paper is to advance a little more in our understanding of the stellar and tidal evolution of TZ For.
Methods. In order to improve our understanding of the tidal evolution of TZ For, we adopt new absolute dimensions and compute specific stellar evolutionary models for the precise observed masses. We explicitly integrate the differential equations that govern the tidal evolution (eccentricity, angular velocities and orbital period) by using these stellar models.
Results. The stellar models indicate that there are two possibilities for the position of the primary on the HR diagram: case A (primary on the clump) and case B (primary on the first ascendent branch). The integration of the complete differential equations of tidal evolution was crucial to shed some light on the tidal evolution of TZ For. The observed levels of circularization/synchronization are now compatible with the inferred age and with the times of circularization/synchronization in both cases. The mentioned inconsistency is no longer detected. In particular, we have found that the primary is synchronized with the orbital period, while the secondary is rotating 17.6 (or 16.2) times faster depending on the selected model, which agrees well with the observations.
Key words: stars: evolution / stars: interiors / binaries: eclipsing / stars: rotation / binaries: general
© ESO, 2011