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

The results of a multisite photometric campaign of theta(2) 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 theta(2) Tau. Arguments are given that these high frequencies originate in the main-sequence companion and are not combination frequencies, f(i) + f(j), 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 T-eff > 8000 K are stable in the observed frequency range. The best fit between the theoretical and observed frequency ranges is achieved for models with T-eff approximate to 7800 K (or slightly higher), in agreement with photometric calibrations. The instability range spans two or three radial orders in the range p(4) to p(6) 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 p(2) to p(8). The observed frequencies lie around radial modes p(5) to p(6). The main uncertainties of these results are caused by a simple treatment of the convective flux in the hydrogen ionization zone.