Rotation and convective core overshoot in θ Ophiuchi
LESIA, Observatoire de Paris-Meudon, UMR8109, Meudon, France
Accepted: 10 March 2010
Context. Recent work on several β Cephei stars has succeeded in constraining both their interior rotation profile and their convective core overshoot. In particular, a recent study focusing on θ Ophiuchi has shown that a convective core overshoot parameter of = 0.44 is required to model the observed pulsation frequencies, significantly higher than for other stars of this type.
Aims. We investigate the effects of rotation and overshoot in early type main sequence pulsators, such as β Cephei stars, and attempt to use the low order pulsation frequencies to constrain these parameters. This will be applied to a few test models and the β Cephei star θ Ophiuchi.
Methods. We use the 2D stellar evolution code ROTORC and the 2D linear adiabatic pulsation code NRO to calculate pulsation frequencies for 9.5 models evolved to an age of 15.6 Myr. We calculate low order p-modes ( ≤ 2) for models with a range of rotation rates and convective core overshoot parameters. These low order modes are the same range of modes observed in θ Ophiuchi.
Results. Using these models, we find that the convective core overshoot has a larger effect on the pulsation frequencies than the rotation, except in the most rapidly rotating models considered. When the differences in radii are accounted for by scaling the frequencies by √(GM/R(, the effects of rotation diminish, but are not entirely accounted for. Thus, this scaling emphasizes the differences produced by changing the convective core overshoot. We find that increasing the convective core overshoot decreases the large separation, while producing a slight increase in the small separations. We created a model frequency grid which spanned several rotation rates and convective core overshoot values. We used this grid to define a modified statistic in order to determine the best fitting parameters from a set of observed frequencies. Using this statistic, we are able to recover the rotation velocity and convective core overshoot for a few test models. We have also performed a “hare and hound” exercise to see how well 1D models can recover these parameters. Finally, we discuss the case of the β Cephei star θ Oph. Using the observed frequencies and a fixed mass and metallicity, we find a lower overshoot than previously determined, with = 0.28 ± 0.05. Our determination of the rotation rate agrees well with both previous work and observations, around 30 km s-1.
Key words: asteroseismology / stars: oscillations / stars: rotation
© ESO, 2010