Temperaments of young stars: rapid mass accretion rate changes in T Tauri and Herbig Ae stars

Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here, we present a study of H alpha variability in 15 T Tauri and Herbig Ae stars (K7 - B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in H alpha with typical time-scales of 10 min, which occurs in 10 per cent of the total covered observing time. The mean accretion rate changes, inferred from the line fluxes, are 0.01-0.07 dex for time-scales of <1 h, 0.04-0.4 dex for time-scales of days, and 0.13-0.52 dex for time-scales of years. In Costigan et al., we derived an upper limit finding that the intermediate (days) variability dominated over longer (years) variability. Here, our new results, based on much higher cadence observations, also provide a lower limit to accretion rate variability on similar time-scales (days), thereby constraining the accretion rate variability physics in a much more definitive way. A plausible explanation for the gradual variations over days is an asymmetric accretion flow resulting in a rotational modulation of the accretion-related emission, although other interpretations are possible as well. In conjunction with our previous work, we find that the time-scales and the extent of the variability is similar for objects ranging in mass from similar to 0.1 to similar to 5 M-circle dot. This confirms that a single mode of accretion is at work from T Tauri to Herbig Ae stars - across a wide range of stellar masses.