850 SRG/eROSITA Sources in the Pleiades

Abstract

Using SRG/eROSITA X-ray sky survey data and the optical catalog of 2209 Pleiades cluster members constructed from Gaia data, we have found 850 X-ray sources associated with cluster stars. More than 650 of them have been detected in X-rays for the first time. At the distance of the Pleiades the nominal sensitivity of eROSITA in this sky region corresponds to a \(0.3{-}2.3\) keV luminosity \(L_{X}\sim 1.6\times 10^{28}\) erg s\({}^{-1}\). The eROSITA sources associated with the Pleiades radiate with a total luminosity \(L_{X,\textrm{total}}\sim 1.3\times 10^{32}\) erg s\({}^{-1}\), which exceeds the X-ray luminosity of the quiet Sun by a million times. A strong, more than a factor of 10, X-ray variability has been recorded for 27 sources. Most of them are known as eruptive optical variable dM stars. \(R_{X}=\log(L_{X}/L_{\textrm{bol}})\) for Pleiades stars increases with decreasing effective stellar temperature from \(R_{X}\approx{-}5\) to \(R_{X}\approx{-}2\). The \(R_{X}\) distribution of stars is bimodal. The left peak at \(R_{X}\sim{-}4.3\) is formed by FGK stars, while the right peak at \(R_{X}\sim{-}3.1\) is mostly populated by M stars. The dependence of \(R_{X}\) on the Rossby number \(Ro\) is different for different spectral types. At small Rossby numbers for K and M stars \(R_{X}\sim{-}3\) and depends weakly on \(Ro\). At \(Ro\gtrsim 0.25\) a rapid decrease in \(R_{X}\) is observed for K stars, while there are no M stars with large Rossby numbers in our sample. Most of the F and G stars lie in the range of lower values, \(R_{X}\sim{-}4.5\), but the size of our sample is not enough to characterize in more detail the \(R_{X}{-}Ro\) relation for them.