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 main-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 sensitive survey to date of X-ray emission (0.3-10 keV) from young stars in Taurus-Auriga. We investigated the dependences of X-ray activity measures - X-ray luminosity, L-X, its ratio with the stellar luminosity, L-X/L-star, and the surface-averaged X-ray flux, F-XS - on rotation period and compared them with predictions based solely on the observed dependence of L-X on a star's L-star and whether it is accreting or not. We tested for differences in the distributions of L-X/L-star, of fast and slow rotators, accretors and non-accretors, and compared the dependence of L-X/L-star 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 L-X and F-XS 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 L-X on L-star. We found no evidence for a dependence of L-X/L-star on rotation period, but for accretors to have lower L-X/L-star 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 rotation.