Long-term variability of T Tauri stars using WASP

TL;DR

This study analyzes long-term optical variability of young T Tauri stars using WASP data, revealing that most exhibit low-level variability dominated by rotational modulation, with some showing long-term changes linked to disk and accretion processes.

Contribution

It provides a comprehensive analysis of T Tauri star variability over years, highlighting the connection between long-term variability and inner disk and accretion activity, using extensive WASP data.

Findings

Most CTTS have low-level variability (<0.3mag) over weeks.

21% of stars show strong variability (>0.3mag).

Long-term variability correlates with disk geometry and accretion indicators.

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.