Formation and Evolution of Protoplanetary Disks

Sammanfattning: A star forms with a surrounding protoplanetary disk after the collapse of a molecular cloud core. Subsequently, over a period of several Myr this protoplanetary disk of dust and gas is accreted onto the host star. We model the formation and evolution of such a protoplanetary disk using a one-dimensional numerical model. We find that disks form on a time scale of less than 1 Myr and that the size and mass of the disk at the end of formation depend on the angular momentum of the molecular cloud core, which may explain the large diversity in observed disk masses and radii. The initial disk size subsequently sets its viscous lifetime. In order for the star to accrete the disk within 5 Myr we find that disks need to form very compact (within about 3 au). % We also find that there is no single relation between the stellar accretion rate and the disk mass. Instead, at a given accretion rate the corresponding disk mass will depend on the initial conditions of the cloud core. Dust disk lifetimes are investigated assuming that particles sizes are held constant by the combined effects of coagulation, bouncing and fragmentation. For dust sizes of 0.1 and 0.01 cm we find that the dust disk drains significantly faster than the gas disk, having lifetimes that are at least 2-3 Myr shorter than the 5 Myr simulation. For 0.001 cm sized dust, the dust disk only begins to rapidly drain inwards towards the end of the 5 Myr simulation, but maintaining such small particles would require very low coagulation efficiencies in the outer disk. For these particles with fixed sizes, we find that pebbles can pile up and could form planetesimals though the streaming instability early in the disk evolution at a wide range of orbital radii. With this in mind, we also briefly look into the potential of planet formation via pebble accretion, starting with embryos placed in these streaming instability active regions. We find that giant planets cores of about 10 M$_\Earth$ can emerge after the disk formation phase has ended in a timespan of about 0.25 Myr yr for 0.1 cm sized pebble at an orbit of about 10 au. For 0.01 cm sized pebbles we find that planets are able to grow to masses from a few Mars masses to a few Earth masses, both in the inner disk ( < 5 au) and the outer disk ( < 10 au).