| dc.description.abstract | T Tauri stars are young low-mass stellar objects. Studying how the progenitors of Sun-like stars form is vital to our understanding of stellar evolution and planetary formation. However, the mechanisms by which these stars accrete matter, power winds, and shed angular momentum remains elusive. Too distant and dim to probe with direct imaging or interferometry, the inner disks of T Tauri stars must be studied using spectroscopy. However, the atomic spectra of these stars are complex and highly variable. Radiative transfer modelling offers a powerful tool for understanding the physical processes that create T Tauri stars’ enigmatic hydrogen emission lines. In this thesis, I use the state-of-the-art radiative transfer code TORUS to explore synthetic T Tauri spectra. I present a detailed description of the radiative transfer code and the T Tauri model. Moreover, I develop and test a new polar stellar wind framework applicable to both axisymmetric and non-axisymmetric T Tauri simulations. In addition, I explore the effects of Stark and turbulent broadening. The optical and near-IR spectroscopy of 29 T Tauri stars is compared with a grid of synthetic line profiles created using TORUS. The archival T Tauri spectra, obtained with VLT/X-Shooter, provide simultaneous coverage of many optical and infrared hydrogen lines. The observations exhibit morphologies similar to those seen in other studies. The synthetic line profile grid was computed for the hydrogen transitions of Hydrogen alpha, Paschen gamma, Paschen beta, and Brackett gamma for a fiducial T Tauri model that included axisymmetric magnetospheric accretion and a polar stellar wind. Furthermore, I use the Reipurth classification scheme to study observed and synthetic profile morphology distributions. I show that the modelled infrared lines are narrower than the observations by ≈ 80 km/s. In other words, the mean width of the synthetic lines is only ≈ 50 per cent of the observed mean width. Additionally, the models predict a significantly higher proportion (≈ 90 per cent) of inverse P-Cygni profiles. The radiative transfer models suggest that the frequency of P-Cygni profiles depends on the ratio of mass loss to mass accretion rates, and blue-shifted sub-continuum absorption is predicted for mass-loss rates as low as 10 − 12 stellar masses per year. Several modifications to the T Tauri models are explored, including the effects of rotation and turbulence on the synthetic hydrogen line profiles. Finally, I present six 3D T Tauri models that include non-axisymmetric accretion flows and wind. For each model, line profiles are calculated at different azimuthal values, allowing me to study the rotational variability of the synthetic spectra. The models exhibit patterns of variability, correlation, and morphology, as seen in observations and previous theoretical studies. It was seen that non-axisymmetric accretion flows could reduce the frequency of inverse P-Cygni profiles and increase the HW10% of the infrared lines by ≈ 60 km/s. However, the width of both the infrared lines and Hydrogen alpha increased simultaneously. Additionally, the 3D models predict trends in the variation of equivalent width and profile half-width that could be tested against T Tauri time-series spectroscopy. The radiative transfer code developed in this work and the models presented in this thesis provide a modern foundation for future synthetic T Tauri line modelling. The models were show to successfully reproduced the hydrogen line profile characteristics similar to those of T Tauri stars. Moreover, the radiative transfer modelling predicted trends in variability and profile morphology that observations could verify. However, this thesis also highlights several problems with the current models and discusses how future work may solve these obstacles so that radiative transfer modelling can be used effectively as an effective diagnostic tool for T Tauri systems. | en_GB |
