Investigation of the rotation-activity relation for a sample of SuperWASP and ASAS-SN field stars

TL;DR

This study investigates the relationship between stellar rotation and X-ray activity in a large sample of field stars, revealing a power-law relation consistent with canonical models and evidence of supersaturation in the fastest rotators.

Contribution

It provides a detailed analysis of the rotation-activity relation using combined photometric and X-ray data, highlighting the reliability issues of automatic light curve classification.

Findings

Power-law index in unsaturated regime: -1.84±0.18

Consistent with canonical value of -2

Evidence of supersaturation in fastest K-type rotators

Abstract

It is well established that late-type main-sequence stars display a relationship between X-ray activity and the Rossby number, $Ro$, the ratio of rotation period to the convective turnover time. This manifests itself as a saturated regime (where X-ray activity is constant) and an unsaturated regime (where X-ray activity anti-correlates with the Rossby number). However, this relationship breaks down for the fastest rotators. We cross-correlated SuperWASP visually classified photometric light curves and ASAS-SN automatically classified photometric light curves with XMM-Newton X-ray observations to identify 3,178 stars displaying a photometrically defined rotational modulation in their light curve and corresponding X-ray observations. We fitted a power-law to characterise the rotation-activity relation of 900 main-sequence stars. We identified that automatically classified rotationally modulated light curves are not as reliable as visually classified light curves for this work. We found a power-law index in the unsaturated regime of G- to M-type stars of $\beta=-1.84\pm0.18$ for the SuperWASP catalogue, in line with the canonical value of $\beta=-2$. We find evidence of supersaturation in the fastest rotating K-type stars, with a power-law index of $\beta_{s}=1.42\pm0.26$.