Pulsation of the Delta Scuti star Theta2 Tau: New multisite photometry and modelling of instability

The results of a multisite photometric campaign of Theta2 Tau are reported. This binary system consists of evolved and main-sequence A stars inside the instability strip. The 12th Delta Scuti Network campaign included 152 hours 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 c/d 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 c/d are also present in Theta 2 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 nonradial oscillations. All hot models of the primary with Teff > 8000K are stable in the observed frequency range. The best fit between the theoretical and observed frequency ranges is achieved by models with Teff approximately equal to 7800K (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.