  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%
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 hour, 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. 2012 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 ~ 0.1
to ~ 5 Msol. 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.

Solar and Stellar Astrophysics