The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga

Context. The Taurus-Auriga star-forming complex hosts the only population of T Tauri stars in which an anticorrelation of X-ray activity and rotation period has been observed. Aims. We aim to explain the origin of the X-ray activity‐rotation relation in Taurus-Auriga. We also aim to put the X-ray activity of these stars into the context of the activity of late-type mai n-sequence stars and T Tauri stars in the Orion Nebula Cluster. Methods. We have used XMM-Newton’s European Photon Imaging Cameras to perform the most sensi tive survey to date of Xray emission (0.3-10 keV) from young stars in Taurus-Auriga. We investigated the dependences of X-ray activity measures ‐ X-ray luminosity, LX, its ratio with the stellar luminosity, LX/L⋆, and the surface-averaged X-ray flux, FXS ‐ on rotation period and compared them with predictions based solely on the observed dependence of LX on a star’s L⋆ and whether it is accreting or not. We tested for differences in the distributions of LX/L⋆ of fast and slow rotators, accretors and non-accretors, and compared the dependence of LX/L⋆ on the ratio of the rotation period and the convective turnover timescale, the Rossby number, with that of late-type main-sequence stars. Results. We found significant anticorrelations of LX and FXS with rotation period, but these could be explained by the typically higher stellar luminosity and effective temperature of fast-rotators in Taurus-Auriga and a near-linear dependence of LX on L⋆. We found no evidence for a dependence of LX/L⋆ on rotation period, but for accretors to have lower LX/L⋆ than non-accretors at all rotation periods. The Rossby numbers of accretors and non-accretors were found to be the same as those of late-type main-sequence stars showing saturated X-ray emission. Conclusions. Non-accreting T Tauri stars show X-ray activity entirely consistent with the saturated activity of fast-rotating late -type main-sequence stars. Accreting T Tauri stars show lower X-ray activity, but this cannot be attributed to their slower ro tation.