Enhanced stellar activity for slow antisolar differential rotation?

TL;DR

This study reports unprecedented enhanced stellar activity in slowly rotating stars with large Rossby numbers, suggesting the possible presence of antisolar differential rotation, supported by observational data from M67 and theoretical models.

Contribution

It provides observational evidence and theoretical support for enhanced activity and antisolar differential rotation in slow rotators, a phenomenon not previously documented.

Findings

Enhanced activity correlates with large Rossby numbers in M67 stars.

Data supports the existence of antisolar differential rotation in slow rotators.

Rotation--activity relation aligns with theoretical predictions for antisolar DR.

Abstract

High precision photometry of solar-like members of the open cluster M67 with Kepler/K2 data has recently revealed enhanced activity for stars with a large Rossby number, which is the ratio of rotation period to the convective turnover time. Contrary to the well established behavior for shorter rotation periods and smaller Rossby numbers, the chromospheric activity of the more slowly rotating stars of M67 was found to increase with increasing Rossby number. Such behavior has never been reported before, although it was theoretically predicted to emerge as a consequence of antisolar differential rotation (DR) for stars with Rossby numbers larger than that of the Sun, because in those models the absolute value of the DR was found to exceed that for solar-like DR. Using gyrochronological relations and an approximate age of 4 Gyr for the members of M67, we compare with computed rotation rates using just the B-V color. The resulting rotation--activity relation is found to be compatible with that obtained by employing the measured rotation rate. This provides additional support for the unconventional enhancement of activity at comparatively low rotation rates and the possible presence of antisolar differential rotation.