Angular momentum transfer between oscillations and rotation in subdwarf B hybrid pulsators

Abstract

Context. Subdwarf B pulsators exhibit pressure (p) and/or gravity (g) modes. Their frequency spectra range from very simple, with few frequencies, to very rich, with more than fifty peaks in some cases. Balloon09 is a hybrid pulsating subdwarf B, showing a great number of p-and g-modes including triplet and quintuplet-like structures, which are interpreted as p-mode frequency splittings caused by stellar rotation. Photometric observations undertaken in two subsequent years revealed a change in these rotational splittings of 0.24-0.58 μHz/yr. Aims. We analyse the possibility of angular momentum interchange between stellar rotation and internal gravity waves as a mechanism for the observed rotational splitting variations. Methods. The expected change in the rotational splitting of eigenmodes resulting from this mechanism are computed in the non-adiabatic linear approximation for a stellar structure model that is representative of the target star. Balloon09 is a particularly suitable candidate for our study because the change in the rotational splittings are proportional to the amplitude squared of the g-modes which, in this case, can be estimated from the observations. Results. We find that this mechanism is able to predict changes in the splittings that can be of the same order of magnitude as the observed variations. © 2011 ESO.