###
Chris Ormel

Research Group LeaderAnton Pannekoek Institute for Astronomy

University of Amsterdam (UvA)

The Netherlands

c.w.ormel AT uva DOT nl

Anton Pannekoek Institute for Astronomy

University of Amsterdam (UvA)

The Netherlands

c.w.ormel AT uva DOT nl

December 2015

[top]

Broadly, there are two distinct philosophies for solving the dust coagulation problem in disks: (i) make the model as precise as you can; (ii) include the key physics, but optimize it for speed. Because of its toy model nature, the latter approach is often preferred as it better lends itself to explore a vast parameter space. Here, we have developed such a panoptic model for planet formation, where we model the first stages of dust coagulation in the disk. The figure shows an example of trajectories of coagulating dust particles. As is well known, dust first grows (vertical lines) and then drifts (horizontal lines). For each of these trajectories we also follow the particle size, their porosity (not shown) and the density (background shading). Lines terminate when particles have grown large enough to form planetesimals. Here we see that the largest radius where this occurs is ~5 AU, which we predict is the radius where the first planets can form (because they will sweepup the remainder pebbles that drift in). |

November 2015

[top]

It has been shown that pebble accretion can be very efficient for optimal conditions of the pebble (aerodynamical) and protoplanet properties. But when does pebble accretion start? In particular, when gravity is weak small particles are carried with the flow streamlines around planetesimals, hence avoiding accretion. In this work, we have conducted a systematic study on how efficient gas drag-mediated interactions are and determined the collisional cross section, see figure. Values >1 imply gravitational focusing (fast growth), values <1 aerodynamic deflection (grow stalls). The two dotted lines denote the transition from the geometric (collisional=geometric cross section) regime to aerodynamic deflection and Safronov regimes, respectively. Also, the curved dashed line separates ballistic from settling encounters. Pebble accretion start to the right of this line. For typical disk, we find that planetesimals of around 100 km grow the slowest, indicating a bottleneck for sweepup growth at this size. |

June 2015

[top]

June 2015

[top]