Formation and evolution of protostellar accretion disks: Towards a quantitative and predictive model

Abstract

The formation and subsequent evolution of accretion disks around young (Class 0/I) protostars are very important events in the formation of stars and planets. Such disks constitute an evolutionary bridge between the earliest stages of star formation, during which a pre-stellar core fragments out of its natal molecular cloud and commences dynamical contraction, and the later (Class II) stage, when an optically visible pre-main- sequence star is surrounded by a protoplanetary disk, with planet formation likely already underway.In this thesis, we attempt to develop a quantitative and predictive model of protostellar disks through a synergy of simulation and theory. We first develop and perform a 3D radiative non-ideal magnetohydrodynamic (MHD) simulation to investigate the formation and evolution of a young protostellar disk from a magnetized pre-stellar core. The simulation covers the first ∼10 kyr after protostar formation, and shows a hot, massive, and weakly magnetized disk that is gravitatinoally unstable. We then use our simulation results and a series of analytic arguments to construct a quantitative and predictive physical picture of Class 0/I protostellar disk evolution from several aspects, including (i) the angular-momentum redistribution in the disk, self-regulated by gravitational instability to make most of the disk marginally unstable; (ii) the thermal profile of the disk, well approximated by a balance between radiative cooling and accretion heating; and (iii) the magnetic-field strength and magnetic-braking rate inside the disk, regulated by non-ideal magnetic diffusion. Using these physical insights, we build a simple 1D semi-analytic model of disk evolution. This 1D model, when coupled to a computationally inexpensive simulation for the evolution of the surrounding pseudodisk, can be used reliably to predict disk evolution in the Class 0/I phase. This model can also be used for inferring the properties of observed disks by fitting multi-wavelength dust-continuum observations. Using data from a recent observational survey of protostellar disks, we find that the majority of the sample agree well with our model, and that these disks could be significantly more massive than previous observational estimates (which could be biased by high optical depth), with typical disk-to-star mass ratio of order unity.