Long-term variability of T Tauri stars using WASP
  We present a reference study of the long-term optical variability of young stars using data from the WASP project. Our primary sample is a group of well-studied classical T Tauri stars (CTTS), mostly in Taurus-Auriga. WASP lightcurves cover timescales up to 7 years and typically contain 10000-30000 datapoints. We quantify the variability as function of timescale using the time-dependent standard deviation 'pooled sigma'. We find that the overwhelming majority of CTTS has low-level variability with sigma<0.3mag dominated by timescales of a few weeks, consistent with rotational modulation. Thus, for most young stars monitoring over a month is sufficient to constrain the total amount of variability over timescales up to a decade. The fraction of stars with strong optical variability (sigma>0.3mag) is 21% in our sample and 21% in an unbiased control sample. An even smaller fraction (13% in our sample, 6% in the control) show evidence for an increase in variability amplitude as a function of timescale from weeks to months or years. The presence of long-term variability correlates with the spectral slope at 3-5mu, which is an indicator of inner disk geometry, and with the U-B band slope, which is an accretion diagnostics. This shows that the long-term variations in CTTS are predominantly driven by processes in the inner disk and in the accretion zone. Four of the stars with long-term variations show periods of 20-60d, significantly longer than the rotation periods and stable over months to years. One possible explanation are cyclic changes in the interaction between the disk and the stellar magnetic field. 
Laura Rigon (St Andrews), Aleks Scholz (St Andrews), David Anderson   (Keele), Richard West (Leicester)
astro-ph.SR