TESS photometry of helium-rich hot subdwarfs: r modes in BD+37 442 and BD+37 1977

TL;DR

This study uses TESS photometry to analyze helium-rich hot subdwarfs BD+37 442 and BD+37 1977, revealing multi-periodic variability likely caused by Rossby waves (r modes), indicating rapid stellar rotation.

Contribution

First detection of r modes in helium-rich hot subdwarfs using TESS data, suggesting a new understanding of their pulsation mechanisms and rotation.

Findings

Stars exhibit multi-periodic low-amplitude variability.

Principal periods suggest rapid rotation at 0.7 and 0.3 times breakup speed.

Rossby waves are the most probable cause of observed variability.

Abstract

TESS photometry of the extremely helium-rich hot subdwarfs BD+37 442 and BD+37 1977 demonstrates multi-periodic low-amplitude variability with principal periods of 0.56 and 1.14 d, respectively, and with both first and second harmonics present. The lightcurves are not perfectly regular, implying additional periodic and/or non-periodic content. Possible causes are examined, including the binary hypothesis originally introduced to explain X-ray observations, differentially rotating surface inhomogeneities, and pulsations. If the principal photometric periods correspond to the rotation periods, the stars are rotating at approximately 0.7 and 0.3 x breakup, respectively. Surface Rossby waves (r modes) therefore provide the most likely solution.