Stellar X-ray activity and habitability revealed by ROSAT sky survey

TL;DR

This study analyzes stellar X-ray activity using ROSAT data, revealing detailed activity-rotation relations, phenomena like super-saturation, and implications for planetary habitability, especially atmospheric loss for Earth-like planets.

Contribution

It provides a comprehensive analysis of X-ray activity-rotation relations for single and binary stars, uncovering new fine structures and their potential causes, and assesses planetary atmospheric loss due to stellar X-ray emissions.

Findings

Identified two main regions in activity-rotation relation: weak and rapid decay.

Discovered super-saturation in extremely fast rotators.

Estimated atmospheric loss for Earth-like planets in habitable zones.

Abstract

Using the homogeneous X-ray catalog from ROSAT observations, we conducted a comprehensive investigation into stellar X-ray activity-rotation relations for both single and binary stars. Generally, the relation for single stars consists of two distinct regions: a weak decay region, indicating a continued dependence of the magnetic dynamo on stellar rotation rather than a saturation regime with constant activity, and a rapid decay region, where X-ray activity is strongly correlated with the Rossby number. Detailed analysis reveals more fine structures within the relation: in the extremely fast rotating regime, a decrease in X-ray activity was observed with increasing rotation rate, referred to as super-saturation, while in the extremely slow rotating region, the relation flattens, mainly due to the scattering of F stars. This scattering may result from intrinsic variability in stellar activities over one stellar cycle or the presence of different dynamo mechanisms. Binaries exhibit a similar relation to that of single stars while the limited sample size prevented the identification of fine structures in the relation for binaries. We calculated the mass loss rates of planetary atmosphere triggered by X-ray emissions from host stars. Our findings indicate that for an Earth-like planet within the stellar habitable zone, it would easily lose its entire primordial H/He envelope (equating to about 1% of the planetary mass).