Protostars are formed when giant molecular clouds of gas and dust gravitationally collapse, thereby converting gravitational potential energy into thermal energy. However, random motion of gas and dust particles, along with conservation of angular momentum, prevents all of the material from directly accreting onto the star. Instead, the leftover material settles into a geometrically thin, flared disk around the midplane of the newly formed star. While these disks are nearly ubiquitous around stars of a few million years (Myrs) in age, there is a dramatic decline in the number of observed disks for stars after about 7 Myrs, suggesting that disk dispersion occurs rapidly and almost simultaneously over a large range of radii. The relatively few transition disks that have been discovered further supports the idea of a quick dispersal of both gas and dust.
Photoevaporation is largely accepted as one of the mechanisms behind this rapid dispersal phase of the disk. Stellar UV and X-ray photons irradiate and heat a thin surface layer of the disk causing the gas to expand. In regions far enough from the central star, this expanding gas can escape the pull of gravity altogether and, thus, provides an effective means whereby gas and dust (via aerodynamic drag) can be removed simultaneously over large areas of the disk. Although simple photoevaporation models have been effective at reproducing certain key features observed in protoplanetary disks, there are still substantial uncertainties in the relative abundances and roles of different wavelengths throughout the entire process. Furthermore, photoevaporation has not yet been implemented alongside fully coupled gas and dust evolution models where both large and small dust-to-gas ratios coexist in the same disk.
The main focus of my Ph.D. will be using smoothed particle hydrodynamics to further explore the effect of dust dynamics on photoevaporative winds with the hope that our results will better constrain the uncertainties currently associated with photoevaporation models.
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