Jesper Tjoa   (The Netherlands)

tjoa @ mps.mpg.de

The Formation of schmall, icy planetesimals

The formation pathway of small planetesimals is debated. Gravitational instability preferentially forms larger objects, whereas hierarchical coagulation is prohibitively slow. The specific mechanism that leads to km-scale planetesimals is unknown. Contemporary representatives of this class of objects include comets, such as 67P/Churyumov-Gerasimenko, and Kuiper Belt Objects (KBOs), such as 486958 Arrokoth, recently visited by New Horizons. Common among these objects is a bilobate structure, which may shed light on what mechanisms controlled their formation. We investigate the structural, morphological and material consequences of collisional growth of small planetesimals, as a function of local protoplanetary disk parameters. We use the hydrodynamics code GIZMO (Hopkins, 2015) to model pebble- to boulder-sized objects composed of ices and silicates colliding, embedded in a gas that provides drag and turbulence.

The material's state is given by the Tillotson equation of state, which was developed to model high velocity solid impacts and vaporization (Tillotson 1962). We simulate a cloud of tracer particles representing pebbles piles that are gravitationally coalescing, registering two particles approaching within some threshold distance as a "collision". We subsequently simulate these "collisions" on a smaller scale, to determine whether the logged close approach was indeed a collision, as well as investigate the material and structural consequences of the event. We thus attempt to recreate the bilobate morphology observed in many comets/KBOs, as well as ascertain whether small planetesimal formation according to the present paradigm is at all possible. If not, we will then proceed to destroy our formed multiple-km planetesimals to thus obtain the 100 m - km sized objects which cover the lower mass end of the cometary mass spectrum

Supervisor:    Hubert Klahr   (MPIA)