Spectra disentangling applied to the Hyades binary θ²   Tauri AB: new orbit, orbital parallax and component properties^⋆,⋆⋆

¹ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
e-mail: kbtorres2003@yahoo.com.br, patricia.lampens@oma.be
² GEPI & UMR CNRS 8111, Observatoire de Paris, 92195 Meudon Cedex, France
³ IPR Université de Rennes 1, Campus de Beaulieu 3, 35042 Rennes Cedex, France
⁴ Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic

Received: 7 June 2010
Accepted: 4 October 2010

Abstract

Aims.θ² Tau is a detached and single-lined interferometric-spectroscopic binary as well as the most massive binary system of the Hyades cluster. The system revolves in an eccentric orbit with a periodicity of 140.7 days. Its light curve furthermore shows a complex pattern of δ Scuti-type pulsations. The secondary has a similar temperature but is less evolved and fainter than the primary. In addition, it is rotating more rapidly. Since the composite spectra are heavily blended, the direct extraction of radial velocities over the orbit of component B was hitherto unsuccessful. Our aim is to reveal the spectrum of the fainter component and its corresponding Doppler shifts in order to improve the accuracy of the physical properties of this important “calibrator” system.

Methods. Using high-resolution spectroscopic data recently obtained with the Elodie (Observatoire de Haute-Provence, France) and Hermes (Roque de Los Muchachos, La Palma, Spain) spectrographs, and applying a spectra disentangling algorithm to three independent data sets including CfA spectra (Oak Ridge Observatory, USA), we derived an improved spectroscopic orbit. We next used a code based on simulated annealing and general least-squares minimization to refine the orbital solution by performing a combined astrometric-spectroscopic analysis based on the new spectroscopy and the long-baseline data from the Mark III optical interferometer.

Results. As a result of the performed disentangling, and notwithstanding the high degree of blending, the velocity amplitude of the fainter component is obtained in a direct and objective way. Major progress based on this new determination includes an improved computation of the orbital parallax (still consistent with previous values). Our mass ratio is in good agreement with the older estimates of Peterson et al. (1991, 1993), but the mass of the primary is 15–25% higher than the more recent estimates by Torres et al. (1997) and Armstrong et al. (2006).

Conclusions. The evolutionary status of both components is re-evaluated in the light of the revisited properties of θ² Tau AB. Due to the strategic position of the components in the turnoff region of the cluster, the new determinations imply stricter constraints for the age and the metallicity of the Hyades cluster. We conclude that the location of component B can be explained by current evolutionary models, but the location (and the status) of the more evolved component A is not trivially explained and requires a detailed abundance analysis of its disentangled spectrum. The improved accuracy (at the 2% level) on the stellar masses provides a useful basis for the comparison of the observed pulsation frequencies with suitable theoretical models.