The X-ray view of the Hyades cluster: updated

Abstract

We revisit the X-ray properties of the main-sequence Hyades members and the relation between X-ray emission and stellar rotation. As input catalog for Hyades members, we combined three recent membership lists derived from Gaia DR2 data including the Hyades core and its tidal tails. We searched for X-ray detections from the ROSAT all-sky survey (RASS) and pointings from ROSAT, Chandra, and XMM-Newton of the Hyades members and adopted rotation periods derived from Kepler's K2 mission and other resources. We find an X-ray detection for 281 of 1066 bona fide main-sequence Hyades members and provide statistical upper limits for the undetected sources. F- and G-type stars have the highest detection fraction (72 %), while K- and M-type dwarfs have lower detection rates (22 %). The X-ray luminosities of the detected members range from ~$2\times10^{27}$ for late M-type dwarfs to ~$2\times10^{30}$ erg s$^{-1}$ for active binaries. The X-ray luminosity distribution functions formally differ for the members in the core and tidal tails, which is likely caused by a larger fraction of field stars in our Hyades tails sample. Compared to previous studies, our sample is slightly fainter in X-rays due to differences in the used Hyades membership list, furthermore, we extend the X-ray luminosity distribution to fainter luminosities. The X-ray activity of F- and G-type stars is well defined at $F_X/F_bol=10^{-5}$ and increases to later spectral types reaching the saturation limit for members later than spectral type M3. The X-ray flux varies by less than a factor of three between epochs for the 104 Hyades members with multiple epoch data. Rotation periods are found for 204 Hyades members, with about half of them being detected in X-rays. The activity-rotation-relation derived for the coeval Hyades members has properties very similar to those obtained by other authors investigating stars of different ages.