Probing accretion variability of young stars with TESS and high-resolution spectroscopy in the Chamaeleon I star-forming region

The accretion of material from the protoplanetary disk onto the star is essential in the formation of Sun-like stars and in setting the properties of the disk at the time when planet formation happens. Although often described by simple and static models, the accretion process is inherently variable. For this reason, variability of young stars is perhaps the most important source of information on the accretion process and on the behavior of the innermost region of the systems. We designed a program to study six young stars in the Chamaeleon I star-forming region with the aim of characterizing the accretion process in time, to study the accretion diagnostic parameters and to examine the innermost part of the circumstellar disk. We combined the high-precision optical photometric observations of the TESS space telescope with contemporaneous ground-based IJHK-band photometry, and multi-epoch high-resolution optical spectroscopic observations obtained by the VLT/ESPRESSO and the 2.2m /FEROS spectrographs. Besides uncovering the periodic light variations that can be attributed to the stellar rotation, the uninterrupted TESS observations allowed us to examine the shorter timescale fluctuations due to accretion variability and also revealed flare-like events in the light curves. Our high-resolution spectroscopy allowed us to examine the amplitude, timescale and pattern of variability in accretion tracers which carry information on the distribution and the kinematics of the accreting material, the density structure of the inner disk, stellar activity, and the presence of outflows or jets. Where we have contemporaneous photometry and spectroscopy we linked the photometric variability to spectroscopic variations.