The stellar rotation–activity relation for a sample of SuperWASP and ASAS-SN field stars

Abstract

AbstractIt is well established that late-type main-sequence (MS) 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 Rossby number). However, this relationship breaks down for the fastest rotators. We cross-correlated SuperWASP visually classified photometric light curves and All-Sky Automated Survey for Supernovae 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 MS 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$\beta=-1.84\,\pm\,0.18$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$\beta_{s}=1.42\pm0.26$.