Theory and evidence of global Rossby waves in upper main-sequence stars: r-mode oscillations in many Kepler stars

TL;DR

This paper identifies and analyzes the presence of global Rossby waves (r modes) in many Kepler stars, revealing their frequency ranges, visibility, and potential for measuring stellar rotation across various types of upper main-sequence stars.

Contribution

It provides the first comprehensive analysis of r modes in Kepler data, deriving their expected frequencies and demonstrating their presence in diverse stellar types.

Findings

r modes appear as frequency groups slightly below m times the rotation frequency

r modes are detected in many gamma Dor, spotted, and Heartbeat stars

r modes can be used to estimate stellar rotation rates

Abstract

Asteroseismic inference from pressure modes (p modes) and buoyancy, or gravity, modes (g modes) is ubiquitous for stars across the Hertzsprung--Russell diagram. Until now, however, discussion of r modes (global Rossby waves) has been rare. Here we derive the expected frequency ranges of r modes in the observational frame by considering the visibility of these modes. We find that the frequencies of r modes of azimuthal order $m$ appear as groups at slightly lower frequency than $m$ times the rotation frequency. Comparing the visibility curves for r modes with Fourier amplitude spectra of Kepler light curves of upper main-sequence B, A and F stars, we find that r modes are present in many $\gamma$ Dor stars (as first discovered by Van Reeth et al. 2016), spotted stars, and so-called Heartbeat stars, which are highly eccentric binary stars. We also find a signature of r modes in a frequently bursting Be star observed by Kepler. In the amplitude spectra of moderately to rapidly rotating $\gamma$ Dor stars, r-mode frequency groups appear at lower frequency than prograde g-mode frequency groups, while in the amplitude spectra of spotted early A to B stars, groups of symmetric (with respect to the equator) r-mode frequencies appear just below the frequency of a structured peak that we suggest represents an approximate stellar rotation rate. In many Heartbeat stars, a group of frequencies can be fitted with symmetric $m=1$ r modes, which can be used to obtain rotation frequencies of these stars.