An asteroseismic study of the β Cephei star θ Ophiuchi: constraints on global stellar parameters and core overshooting

We present a seismic study of the β Cephei star θ Ophiuchi. Our analysis is based on the observation of one radial mode, one rotationally split ` = 1 triplet and three components of a rotationally split ` = 2 quintuplet for which the m-values were well identified by spectroscopy. We identify the radial mode as fundamental, the triplet as p1 and the quintuplet as g1. Our NLTE abundance analysis results in a metallicity and CNO abundances in full agreement with the most recent updated solar values. With X ∈ [0.71, 0.7211] and Z ∈ [0.009, 0.015], and using the Asplund et al. (2005) mixture but with a Ne abundance about 0.3 dex larger (Cunha et al. 2006), the matching of the three independent modes, enables us to deduce constrained ranges for the mass (M = 8.2±0.3 M ) and central hydrogen abundance (Xc = 0.38±0.02) of θ Oph and to prove the occurrence of core overshooting (αov = 0.44±0.07). We also derive an equatorial rotation velocity of 29±7 km s−1. Moreover, we show that the observed non-equidistance of the ` = 1 triplet can be reproduced by second order effects of rotation. Finally, we show that the observed rotational splitting of two modes cannot rule out a rigid rotation model.