Rapid variations of T Tauri spectral features: clues to the morphology of the inner regions

Abstract

We have monitored the optical spectra of six T Tauri stars, AA Tau, BP Tau, CI Tau, DR Tau, RY Tau and SU Aur, approximately hourly over five successive nights. Our spectral range extends from 3800 to 7300 A, including the Balmer series lines from H α to H δ, CaII H+K and various Fe II lines. We construct time series of equivalent widths for each species to study the variations of the strengths of the different lines, and the correlations between them. We can discern a range of physical processes at work, such as the slow rotation of the stars (time-scales of days), magnetic flaring activity (time-scales of about an hour), variable accretion (time-scales of several hours) and obscuration by circumstellar material (again, time-scales of several hours). The sample objects show a range of activity in each of these categories, from DR Tau (dominated entirely by hour-time-scale activity) to those with only slowly varying line activity consistent with rotation (AA Tau). For objects between these extremes, we differentiate the activity according to time-scale, short time-scale events often appearing superimposed on longer time-scale variations. We examine the correlation in time between the rapid activity of the different emission features, which are formed under a wide range of excitation conditions. The lower Balmer lines (H α and H β) and Ca II K are usually strongly correlated with one another, as are the higher Balmer lines (H γ and H δ), but the behaviour of these two groups sometimes becomes decoupled. In one particular case, for the active object DR Tau, we see well-correlated but time-lagged activity, occurring sequentially with the high-energy lines varying first, followed by lines of successively lower excitation temperature. We tentatively conclude that this may be the signature of an accretion shock moving over the stellar limb.