An Investigation into the Accretion Processes of Young Stellar Objects

The way in which young stars form is complex and fascinating, and is anything but a smooth, continuous process. Studying their dramatic, and sometimes violent, early years of accreting and ejecting material is vital in understanding how a star will evolve. Of these young objects, protostars are generally less well-studied than the more evolved pre-main sequence (PMS) stars due the necessity of infrared observations to probe through the dusty envelope surrounding protostars. Variability of these young stellar objects (YSOs) and whether or not they are capable of strong episodic accretion is also left relatively unstudied. Variability appears in the accretion and ejection processes of PMS stars as well. To establish a broader understanding of the evolution of young stars it is necessary to investigate different classes of protostar and PMS stars, as well as different timescales of the variability. This thesis aims to contribute to this understanding of the characterisation and sources of different kinds of accretion and ejection variability, by focusing on two young stars: IRS 54 and RU Lup. Firstly, the very low-mass star (VLMS) IRS 54, of mass M* ~ 0.1 - 0.2 Msun, which is still in its early stages of its evolution, was investigated. As episodic accretion is likely how young stars accrete most of their mass, it is important to understand on what kinds of timescales these events are occurring and how stellar properties influence them. Through multi-epoch, near-infrared (NIR) spectroscopic and photometric data obtained with the instruments Very Large Telescope/ISAAC and VLT/SINFONI, multiple accretion- (PaB and BrG) and outflow-tracing (H2 2.122um and [Fe II] 1.644um) emission lines present were investigated to examine physical properties and kinematics of the object. Taking advantage of VLT/SINFONI's integral field unit (IFU) capabilities, it was also possible to detect the red- and blue-shifted lobes of the protostellar jet launching material away from the object. The morphology of the jet and outflow cavities is discussed, providing more context about the system as a whole. The mass accretion rate (Macc) was calculated and compared across four epochs, roughly a few years apart. It was found to increase from ~1.7e-8 Msun/yr in 2005 to ~2.6e-7 Msun/yr in 2013, suggesting an accretion burst. This study explores the trends in visual extinction (Av), mass accretion rate and flux over these epochs and how they fit into the complex processes at work in the inner regions of IRS 54. Increases in all three of these quantities was found and is attributed to a potential EXor-type outburst, making IRS 54 one of the lowest mass YSOs to show this behaviour. Delving further into the kinds of variability found in these young stars, a slightly more evolved star (a PMS star) over the much shorter timescale of 15 days was investigated. The chosen target, RU Lup, has a mass of M* ~ 0.65 Msun and is a very strong accretor, which produces many strong emission lines in the optical spectrum. As RU Lup is a PMS star, its dusty envelope has almost completely dissipated, allowing it to be observed optically. This study analysed spectroscopic data from the instrument Canada-France-Hawaii-Telescope/ESPaDOnS and imaging data from the instrument Cerro Tololo Inter-American Observatory/ANDICAM to characterise the short-timescale variability of RU Lup over roughly 15 nights of observations. The accretion luminosity, mass accretion rate and veiling were calculated and their sources of variability were investigated. The photospheric lines of NaI 5688A, MnI 6021A and LiI 6707A were utilised to estimate the veiling present in RU Lup's spectrum. Additionally, wind-tracing emission lines ([OI] 6300A and [SII] 6730A) were observed to be very stable over the short time interval of these observations, indicating no dependence on the accretion variability detected. By comparing the variability in measurements of accretion luminosity found via accretion-tracing line profiles with the variability in measurements of veiling, a positive correlation between the two was found, which is independent of extinction. The types of variability explored in this thesis paint a dynamic picture of the process young stars undergo when evolving. These studies demonstrate the value of both optical and infrared observations in studying different ages of YSO. Observations at different wavelength ranges complement each other, highlighting different aspects of the star formation process, which demonstrates the advantage of simultaneous multi-wavelength observations when characterising the accretion and ejection of an object.

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