The activity of weak-lined T tauri stars. I. V410 tauri

From two intensive series of spectra we have studied the phenomenology of magnetic activity and its distribution on and around V410 Tau. Large-amplitude, quasisinusoidal radial velocity variations at the stellar rotation period appear to be due to large-scale photospheric temperature inhomogeneity that has been present at least since 1988, and not to orbital motion. Other radial velocity variations imply more localized spot groups. The radial velocity variations highlight the point that radial velocity searches for brown dwarfs and planets orbiting cool stars must take stellar activity into account. The activity-related radial velocity variations seen in V410 Tau are thousands of times larger than expected radial velocity reflex motion of low-mass stars due to planets. Variations of absorption linewidths are consistent with surface temperature inhomogeneities implied by the radial velocity variations. The implied temperature distribution is in qualitative agreement with Doppler images made from one of our series of spectra. We found that the cooler regions of the photosphere were associated with greater Balmer emission during the first observing run, but about 90° out of phase 13 months later. The level of Balmer emission increased from one season to the next, and flares were observed only during the later season, suggesting an activity cycle. The net Balmer emission apparently has two strong contributors: a global chromospheric component and a variable asymmetric "halo." Prominence-like structures appear not to be a large contributor to the nonphotospheric Balmer features. Transient absorption features probably are due to an infall of previously ejected material.