Pulsation of the δ Scuti star θ2 Tau: new multisite photometry and modelling of instability

Abstract

The results of a multisite photometric campaign of θ2 Tau are reported. This binary system consists of an evolved and a main-sequence A star, both situated inside the instability strip. The 12th Delta Scuti Network campaign included 152 h of high-precision photometry obtained at four observatories. This leads to the derivation of 11 frequencies of pulsation in the 10.8 to 14.6 cycle d−1 range. These frequencies confirm the results from previous Earth-based (1982–1986) as well as satellite (2000) photometry, although amplitude variability on a time-scale of several years is present. We show that at least two high frequencies (26.18 and 26.73 cycle d−1) are also present in θ2 Tau. Arguments are given that these high frequencies originate in the main-sequence companion and are not combination frequencies, fi+fj, from the primary. Models for both the primary and the secondary components were checked for the instability against radial and non-radial oscillations. All hot models of the primary with Teff > 8000 K are stable in the observed frequency range. The best fit between the theoretical and observed frequency ranges is achieved for models with Teff≈ 7800 K (or slightly higher), in agreement with photometric calibrations. The instability range spans two or three radial orders in the range p4 to p6 for radial modes. Post-main-sequence models (with or without overshooting) are preferable for the primary, but main-sequence models with overshooting cannot be excluded. For the less-luminous secondary component the instability range is wider and spans 5 to 7 radial orders from p2 to p8. The observed frequencies lie around radial modes p5 to p6. The main uncertainties of these results are caused by a simple treatment of the convective flux in the hydrogen ionization zone.