Radiative-transfer modelling of the circumstellar environments of pre-main-sequence stars

Abstract

Circumstellar discs of pre-main sequence stars undergo different processes depending on the nature of the circumstellar environment, which is governed by stellar mass. I have performed numerical simulations of the circumstellar regions of classical T~Tauri stars (CTTs) and Herbig~Ae (HAe) stars using the radiative transfer code TORUS in order to test the paradigm of magnetospheric accretion in CTTs, and to ascertain the nature of the material in the inner regions of HAe discs.

The process of magnetospheric accretion (MA) involves disc material attaching to stellar magnetic field lines and impacting the photosphere, producing accretion shocks. When the magnetic field is inclined to the star, disc warps form which periodically occult the photosphere. With specific reference to the CTTs AA Tau I perform three-dimensional MA models to study this variability. By comparing synthetic photometry with observational data I show that the geometry of the system can be constrained. I go on to study Balmer line profiles in the context of MA and disc wind outflows. I present three-dimensional models of a system comprising the star, magnetosphere, disc, and disc wind, producing synthetic line profiles and images. Using these profiles I perform time-series fitting to observational data and demonstrate that the mass accretion rate, mass loss rate, and magnetosphere temperature can be constrained. I show that there is a degeneracy between wind temperature and wind acceleration which require alternative methods to constrain further. While an outflow model alternative to a sole disc wind may produce better fits to observations, MA models reproduce various observational features well.

Finally I test the hypothesis that refractory grains produce the innermost emission in HAe discs. Focussing on the HAe stars MWC 275 and AB Aur, I perform radiative equilibrium modelling to create synthetic images of these objects from which interferometric visibility profiles are produced. I show that the temperatures at which these refractory grains are required to survive are too high to be physically plausible. I also find that silicate dust is shielded when sufficiently high mass fractions of refractory grains are used, enabling the silicates to survive interior to the classical sublimation radius. While refractory dust may provide a significant contribution to the emission observed in these inner regions, this alone is not sufficient.