PhD Thesis

Summary

Young stellar objects are still in that early stage of stellar evolution, when they are surrounded by a rich circumstellar environment made of gas and dust, from which they accrete material. The accretion mechanism is essential in the star formation process, and the star-disk interaction plays an important role in establishing the physical properties of the circumstellar disk where planets might form. Although often described with simple and static models, the accretion process is inherently time variable. The aim of this thesis is examining the variable accretion process of young stars, understanding the relationship between the inhomogeneous disk structure and the time-variable accretion, and to identify the different physical mechanisms behind the variations by studying a few young stellar objects with diverse accretion and circumstellar properties.

V582 Aur is an FU Orionis-type object (FUor), which is currently in an outbursting state but shows photometric variations due to an orbiting dust clump. I studied the optical–infrared photometric variability of the system in order to examine its evolution. The new data presented in this thesis imply that changing extinction is the dominant physical mechanism behind the light variations, and suggest the viscous spreading of the dust particles along the orbit. The long-term measurements, as well as an accretion disk modeling hint at a general fading of V582 Aur, suggesting that the source will reach the quiescent level in ∼80 yr. 

For a sample of objects in the Chamaeleon I star forming region, we arranged ground-based optical and near-infrared photometric, and high-resolution spectroscopic (VLT/ESPRESSO, 2.2/FEROS) measurements contemporaneously with the TESS space telescope observations in order to examine the accretion process, analyze the effects of hot/cold spots, and identify the signs of line-of-sight obscuration by circumstellar matter. For one particular object, the CR Cha single T Tauri star, I was able to study the accretion process on timescales from hours to a decade by including earlier data from the AAT/UCLES and the HARPS instruments. These show that the accretion variations increase on timescales from hours to days/weeks, after which it saturates, and the overall accretion variability is within the factor of ∼3 on timescales of a decade. The near-infrared JHK-band light curves reveal an interesting pattern in the color-magnitude diagrams: they show that CR Cha becomes redder as it brightens. This unusual pattern may be explained by a disk model, which states that this behaviour may be caused by the changing accretion rate, or by changes in the inner disk structure. 

For an other object, the VW Cha multiple system, four high-resolution spectra were taken besides the TESS and the ground-based photometry. Spectra were obtained in both fainter and brighter photometric states of the system, allowing me to examine the origin of a photometric brightening event. The results show that this brightening event can be explained by increased accretion. In addition, the new spectroscopic data also suggest that the primary component of VW Cha is a spectroscopic binary with a speculated orbital period around 10 days.

While the studies presented in this thesis are examining individual stars, they well represent the accretion phenomenon in general, including both moderately accreting T Tauri stars and an outbursting FUor. This work also showed the importance of multifilter and multi-instrument campaigns when identifying and distinguishing the mechanisms causing the variability. I also demonstrated that time domain astronomy is becoming a very efficient tool to study these phenomena.