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Dynamics and evolution of planetesimals in gaseous protopanetary disks - the role of gas dynamical friction

TYPEAstrophysics Seminar
Speaker:Evgeni Grishin
Location:Lidow 620
Remark:Master talk
Abstract: The growth of small planetesimals into large planetary embryos occurs much before the dispersal of the gas from the protoplanetary disk. The planetesimal - gaseous-disk interactions give rise to migration and orbital evolution of the planetesimals/planets. Small planetesimals are dominated by aerodynamic gas drag. Large protoplanets,  >1000km in size, are dominated by type I migration torque. There is an additional size range, 200-1000 km, of intermediate mass planetesimals (IMPs), where gravitational interactions with the disk dominate over aerodynamic gas drag, but for which such interactions were typically neglected. Here we model these interactions using the gas dynamical friction (GDF) approach, previously used to study the disk-planet interactions at the planetary mass range. We find the critical size where GDF dominates over gas drag, and then study the implications of GDF on single IMPs. We find that planetesimals with small inclinations and eccentricities circularize rapidly. Planetesimals of higher masses inspiral on a time-scale of a few Myrs, leading to an embryonic migration to the inner disk. This can lead to an over-abundance of rocky material (in the form of IMPs) in the inner protoplanetary disk ( <1 AU) and induce rapid planetary growth. This can explain the origin of super-Earth planets. In addition, GDF damps the velocities of IMPs, thereby cooling the planetesimal disk and affecting its collisional evolution through quenching the effects of viscous stirring by the large bodies.

In addition to single planetesimals, many binary planetesimals have been observed in the Solar System, indicating that binary planetesimals are very common.  We find that the GDF can induce binary mergers on timescales shorter than the disk lifetime for sizes above >500km.  Such mergers can affect the structure of merger-formed planetesimals, and the GDF-induced binary hardening can play a role in the evolution of the planetesimal disk. In addition, binaries on eccentric orbits around the central object may evolve in the supersonic regime, in which case they can exhibit considerable softening, that in turn enhance the cross section for planetesimal encounters and affect planet formation.