Observations of the Hyades Open Cluster during the SRG/eROSITA All-Sky Survey

Abstract

Using data from the all-sky X-ray survey with the eROSITA telescope onboard the SRG orbital observatory and the optical catalog of Hyades cluster members constructed from Gaia data, we have investigated the X-ray emission from cluster stars. Of the 395 cluster members located in the eastern Galactic part of the sky, eROSITA detects X-ray emission from 290 stars, 171 of which have been found for the first time in X-rays. At the Hyades distance (\({\sim}47\) pc) the median eROSITA sensitivity in this sky region corresponds to a 0.3–2.3 keV luminosity \(L_{X}\sim 2.6\times 10^{27}\) erg s\({}^{-1}\), allowing 100\(\%\) of the stars with spectral types from F0 to K5 to be detected within the Hyades tidal radius. We have also investigated objects in the Hyades tidal tail, where X-ray emission was recorded from 114 of the 281 known tidal tail members, 59 of which have been found in X-rays for the first time. For six sources a strong (more than a factor of 10) X-ray variability on a time scale of six months has been recorded. All of them are late-M stars with binarity. The \(R_{X}=\log(L_{X}/L_{\textrm{bol}})\) distribution of Hyades stars is distinctly bimodal. The left peak at \(R_{X}\sim{-}4.7\) is formed predominantly by FGK stars, while the right peak at \(R_{X}\sim{-}3.2\) consists mostly of M stars. The dependence of \(R_{X}\) on the Rossby number \(Ro\) is different for M and FGK stars. For M stars at small Rossby numbers \(R_{X}\sim{-}3\) and depends weakly on \(Ro\), while at \(Ro\gtrsim 0.2\) a rapid decrease in \(R_{X}\) is observed. At the same time, most of the F, G, and K stars lie in a fairly compact region with \(Ro\sim 0.3{-}0.8\) and \(R_{X}\sim{-}4.7\).