The Factory and the Beehive. IV. A Comprehensive Study of the Rotation X-ray Activity Relation in Praesepe and the Hyades

TL;DR

This study updates the rotation-X-ray activity relation in Praesepe and Hyades clusters, revealing that only M dwarfs remain saturated in X-ray emission at around 700 million years, with implications for stellar magnetic activity evolution.

Contribution

It provides the most comprehensive catalogs of cluster members with rotation and X-ray data, and analyzes the magnetic activity dependence on rotation for low-mass stars in these clusters.

Findings

Only M dwarfs remain X-ray saturated at ~700 Myr.

A tight power-law relation between Rossby number and X-ray luminosity is established.

No difference in coronal parameters between binary and single stars.

Abstract

X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses lower than $\approx 1\ M_{\odot}$. We present an updated study of the rotation X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods, an increase of 6$\times$ relative to what was previously available. We find that at $\approx$700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosity $L_\mathrm{X}/L_\mathrm{bol}$ in unsaturated single members, suggesting a power-law index between $-3.2$ and $-3.9$. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars' atmospheres, thereby informing the development of robust age-rotation-activity relations.