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

Abstract

We present a seismic study of the beta Cephei star theta Ophiuchi. Our analysis is based on the observation of one radial mode, one rotationally split l = 1 triplet and three components of a rotationally split l = 2 quintuplet for which the m values were well identified by spectroscopy. We identify the radial mode as fundamental, the triplet as p<SUB>1</SUB> and the quintuplet as g<SUB>1</SUB>. Our non-local thermodynamic equilibrium 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. mixture but with a Ne abundance about 0.3 dex larger, the matching of the three independent modes enables us to deduce constrained ranges for the mass (M = 8.2 +/- 0.3M<SUB>solar</SUB>) and central hydrogen abundance (X<SUB>c</SUB> = 0.38 +/- 0.02) of theta Oph and to prove the occurrence of core overshooting (alpha<SUB>ov</SUB> = 0.44 +/- 0.07). We also derive an equatorial rotation velocity of 29 +/- 7 km s<SUP>-1</SUP>. Moreover, we show that the observed non-equidistance of the l = 1 triplet can be reproduced by the second-order effects of rotation. Finally, we show that the observed rotational splitting of two modes cannot rule out a rigid rotation model.