Ice condensation as a planet formation mechanism

Sammanfattning: In models of dust growth in protoplanetary discs focus is typically on coagulation, a mechanism which unfortunately seems incapable of forming particles larger than centimeters. The main reasons for this are that collisions between larger particles lead to bouncing or fragmentation rather than sticking, and that particles approaching meter-sizes drift radially inwards in the disc due to interaction with the surrounding gas. The concept of ice lines is often overlooked, but is of great importance as particles can grow also by condensation. As a volatile species drifts in towards the central star, it sublimates as it passes the ice line of that particular species. The vapour diffuses back across the ice line and condenses onto existing dust grains, leading to significant growth. We model the dynamical behaviour of ice particles close to the water ice line, around 3 AU from the central star. Ice particles and water vapour move in a damped random walk, due to turbulence in the gas, gravity towards the midplane and radial drift towards the central star. The ice line is curved, and we therefore model it as being composed of the radial ice line, separating the hot region close to the star from the outer cold region of the disc, and the atmospheric ice line, separating the hot midplane and the colder outer layers. Main focus is on diffusion over the atmospheric ice line, but we also look at the effect of including the radial ice line. Condensation and sublimation are modelled with a Monte Carlo approach. The effect of varying the turbulent alpha value, as well as the distance from the atmospheric ice line to the midplane, is investigated. Our results indicate that, with a turbulent alpha-value of 0.01, growth from millimeter-sized to at least decimeter-sized particles is possible in the vicinity of the ice line, on a time scale of 1 000 years. The resulting particle layer may be dense enough to be sensitive to dynamical instabilities, such as the streaming instability, which causes further growth into planetesimals. From planetesimals to planets growth is possible via gravitational interactions.