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Title: Photoevaporation of circumstellar discs
Author: Alexander, R. D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2005
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In this thesis I investigate how discs around young stars, such as T Tauri stars (TTs), evolve, I concentrate on models which combine photoevaporation by ionizing radiation with viscous evolution of the disc, as previous work suggests that such models can reproduce the rapid disc dispersal seen in observations of TTs. However these models require that TTs produce a rather large ionizing flux (of order 1041 ionizing photons per second), and it is clear if TTs can produce such a large flux. It first consider the ionizing flux resulting from the accretion of disc material on to the stellar surface; and if does not have a significant effect on the disc evolution. My next chapter considers whether or not TT chromospheres can provide sufficiently large ionizing fluxes and I conclude that TT chromospheres can provide ionizing fluxes sufficient to drive disc photoevaporation. I then consider the effect of X-rays on disc evolution. I use radiative transfer modelling, combined with a simple hydrostatic disc model, to study the effects of X-rays on the disc. The X-rays heat the disc, producing a warm (~5000K) layer above the cold disc midplane. However, this attenuates the X-rays significantly, and I find that most of the X-rays are absorbed close to the star. Consequently the disc wind that can be driven by X-ray absorption at large radii has a rather low mass-loss rate, and will not influence disc evolution significantly. Having thus shown that TTs produce ionizing fluxes which have a significant effect on disc evolution, I apply this result to models of disc evolution. It first set up a simple disc model and show that a basic photoevaporation model breaks down at late times in the evolution (as the geometry of the radiative transfer problem changes). I construct hydrodynamic models of the wind produced by direct ionization of the outer part of TT discs, and show that this wind dominates the evolution at late times. I then incorporate this wind into the disc evolution model, and show that my model is consistent with current observational data. To date this is the only model which has successfully reproduced the rapid disc dispersal seen in observations of TTs, and this has important consequences for theories of disc evolution and planet formation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available