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

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 successi ve nights. Our spectral range extends from 3800 to 7300 Angstrom, including the Balmer series lines from Hot to H delta, Ca II H+K and various Fe II l ines. We construct time series of equivalent widths for each species to stu dy the variations of the strengths of the different lines, and the correlat ions between them. We can discern a range of physical processes at work, su ch as the slow rotation of the stars (time-scales of days), magnetic flarin g activity (time-scales of about an hour), variable accretion (timescales o f several hours) and obscuration by circumstellar material (again, time-sca les 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 act ivity) to those with only slowly varying line activity consistent with rota tion (AA Tau). For objects between these extremes, we differentiate the act ivity according to time-scale, short time-scale events often appearing supe rimposed on longer time-scale variations. We examine the correlation in tim e between the rapid activity of the different emission features, which are formed under a wide range of excitation conditions. The lower Balmer lines (H alpha and H beta) and Ca II K are usually strongly correlated with one a nother, as are the higher Balmer lines (H gamma and H delta), but the behav iour of these two groups sometimes becomes decoupled. In one particular cas e, for the active object DR Tau, we see well-correlated but time-lagged act ivity, occurring sequentially with the high-energy lines varying first, fol lowed 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.