The Stellar Activity - Rotation Relationship

TL;DR

This study investigates the relationship between stellar rotation and activity, finding a steeper power law slope than previously thought, and explores super-saturation effects and a new X-ray population model for the Galaxy.

Contribution

It provides a precise measurement of the rotation-activity relationship slope and introduces a new X-ray population synthesis model for the Galaxy.

Findings

The power law slope of the unsaturated regime is -2.70±0.13.

Super-saturation occurs for the fastest rotators and depends on specific parameters.

The XStar model can reproduce Galactic X-ray observations and aid in stellar evolution studies.

Abstract

Using a new catalog of 824 solar and late-type stars with X-ray luminosities and rotation periods we have studied the relationship between rotation and stellar activity. From an unbiased subset of this sample the power law slope of the unsaturated regime, $L_X/L_{bol}\propto Ro^\beta$, is fit as $\beta=-2.70\pm0.13$. This is inconsistent with the canonical $\beta=-2$ slope to a confidence of 5$\sigma$ and argues for an interface-type dynamo. Super-saturation is observed for the fastest rotators in our sample and its parametric dependencies are explored. Significant correlations are found with both the corotation radius and the excess polar updraft, the latter theory being supported by other observations. We also present a new X-ray population synthesis model of the mature stellar component of our Galaxy and use it to reproduce deep observations of a high Galactic latitude field. The model, XStar, can be used to test models of stellar spin-down and dynamo decay, as well as for estimating stellar X-ray contamination rates for non-stellar studies.