Unresolved Rossby and gravity modes in 214 A and F stars showing rotational modulation

TL;DR

This study analyzes Kepler data of 214 A- and F-type stars, revealing the coexistence of Rossby and gravity modes, and their correlation with stellar spots and internal stellar processes, advancing understanding of stellar oscillations.

Contribution

It provides the first ensemble analysis linking Rossby and gravity modes with stellar activity and rotation in A- and F-type stars, highlighting their excitation mechanisms.

Findings

Strong correlation between spike amplitude and mode power.

Stars with higher r-mode power are more likely to show higher azimuthal order modes.

G-mode humps are observed only in hotter, more luminous stars.

Abstract

Here we report an ensemble study of 214 A- and F-type stars observed by \textit{Kepler}, exhibiting the so-called \textit{hump and spike} periodic signal, explained by Rossby modes (r~modes) -- the \textit{hump} -- and magnetic stellar spots or overstable convective (OsC) modes -- the \textit{spike} -- respectively. We determine the power confined in the non-resolved hump features and find additional gravity~modes (g~modes) humps always occurring at higher frequencies than the spike. Furthermore, we derive projected rotational velocities from FIES, SONG and HERMES spectra for 28 stars and the stellar inclination angle for 89 stars. We find a strong correlation between the spike amplitude and the power in the r and g~modes, which suggests that both types of oscillations are mechanically excited by either stellar spots or OsC modes. Our analysis suggests that stars with a higher power in $m=1$ r~modes humps are more likely to also exhibit humps at higher azimuthal orders ($m$ = 2, 3, or 4). Interestingly, all stars that show g~modes humps are hotter and more luminous than the observed red edge of the $\delta$ Scuti instability strip, suggesting that either magnetic fields or convection in the outer layers could play an important role.