Mode excitation by turbulent convection in rotating stars. I. Effect of uniform rotation

TL;DR

This paper investigates how uniform rotation and the Coriolis acceleration influence the stochastic excitation of oscillation modes in stars, revealing that rotation causes measurable asymmetries in mode excitation rates, especially in faster rotators like HD 49933.

Contribution

The study extends existing formalism to include the Coriolis effect and analyzes its impact on mode excitation asymmetries in rotating stars, focusing on uniform rotation cases.

Findings

Rotation causes up to 10% asymmetry in mode excitation rates.

The Coriolis acceleration's direct effect on mode driving is negligible.

Faster rotation increases excitation rate asymmetries in stars like HD 49933.

Abstract

We focus on the influence of the Coriolis acceleration on the stochastic excitation of oscillation modes in convective regions of rotating stars. Our aim is to estimate the asymmetry between excitation rates of prograde and retrograde modes. We extend the formalism derived for obtaining stellar $p$- and $g$-mode amplitudes (Samadi & Goupil 2001, Belkacem et al. 2008) to include the effect of the Coriolis acceleration. We then study the special case of uniform rotation for slowly rotating stars by performing a perturbative analysis. This allows us to consider the cases of the Sun and the CoRoT target HD 49933. We find that, in the subsonic regime, the influence of rotation as a direct contribution to mode driving is negligible in front of the Reynolds stress contribution. In slow rotators, the indirect effect of the modification of the eigenfunctions on mode excitation is investigated by performing a perturbative analysis of the excitation rates. It turns out that the excitation of solar $p$ modes is affected by rotation with excitation rates asymmetries between prograde and retrograde modes of the order of several percents. Solar low-order $g$ modes are also affected by uniform rotation and their excitation rates asymmetries are found to reach up to 10 %. The CoRoT target HD 49933 is rotating faster than the Sun ($\Omega / \Omega_\odot \approx 8$) and we show that the resulting excitation rates asymmetry is about 10 % for the excitation rates of $p$ modes. We have then demonstrated that $p$ and $g$ mode excitation rates are modified by uniform rotation through the Coriolis acceleration. Study of the effect of differential rotation is dedicated to a forthcoming paper.