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

TL;DR

This paper investigates how angular momentum transfer between stellar oscillations and rotation can explain observed variations in rotational splittings in a hybrid subdwarf B pulsator, using nonadiabatic models and observed g-modes.

Contribution

It introduces a model for angular momentum exchange between oscillations and rotation to explain observed splitting variations in a hybrid pulsator.

Findings

The mechanism can produce splitting changes matching observations.

G-modes amplitude influences the magnitude of splitting variations.

The model supports internal angular momentum transfer as a plausible explanation.

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 large number of p- and g-modes including a triplet and a quintuplet-like structures which are interpreted as p-mode frequency splittings due to stellar rotation. Photometric observations undertaken in two subsequent years revealed a change in these rotational splittings of 0.24 - 0.58 {\mu}Hz/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 nonadiabatic linear approximation for a stellar structure model that is representative of the target star. The fact that g-modes are also observed in Balloon09 makes it 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.