Abstract
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.
Highlights
Until the late 1990s, most photometric studies of the variability in young stellar objects (YSOs) were limited to the optical part of the spectrum and to time-scales ranging from hours to weeks, focusing on studies of rotation periods due to surface spots and accretioninduced changes (e.g. Bouvier et al 1993; Fernandez & Eiroa 1996; Stassun et al 1999; Lamm et al 2004); see review by Herbst et al (2007)
Because we focus on well-known T Tauri stars, our sample contains many objects that have been studied over decades and have originally been discovered based on their variability, e.g. RW Aur, T Tau, and BP Tau (Joy 1945)
Most studies on the variability in T Tauri stars have focused on time-scales of days to a few months, but changes on time-scales significantly longer are well documented
Summary
This has been known for more than half a century, we have only started to gain a comprehensive understanding of the characteristics and the origins of these variations. Until the late 1990s, most photometric studies of the variability in young stellar objects (YSOs) were limited to the optical part of the spectrum and to time-scales ranging from hours to weeks, focusing on studies of rotation periods due to surface spots and accretioninduced changes With the advent of large-scale infrared surveys via satellites like Spitzer and WISE as well as large-scale variability surveys (e.g. PTF, CoRoT), our knowledge of YSO variability has improved rapidly over the past decade. A mixture of these processes seems to be present, leading to bewilderingly complex light curves
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