Welcome to my home page! I'm a second year astronomy student in Tsinghua University , Beijing, China, under the advice of Chris Ormel. My research interest covers planet formation and planet atmosphere .
Accretion of aerodynamically large pebbles, MNRAS, 522, 2241
Pebbles -- the aerodynamically active particles in protoplanetary disks -- are essential in driving planet growth. This is mainly because they drift efficiently in the disk, such that the planet growth is not limited by the local solid budget. When a pebble drifts across the planet orbit, gas drag dissipates its energy, resulting in their capture by the planet. The Stokes number (St) describes a pebble's aerodynamical size. Larger pebbles or a lower gas density both increase the Stokes number. This work focuses on the accretion of those pebbles more loosely coupled to gas: St > 1 but where drift is still significant. In contrast to the St < 1 pebbles, the regime of large pebble accretion has not been investigated thoroughly before.
We performed numerical simulations to integrate the large pebble's orbit in a 2D, global reference frame. The planet moves in fixed Keplerian orbits and the pebble undergoes both gas drag and gravity from the star and the planet. We varied the pebble's Stokes number in different simulations. It is found that for St > 1 pebbles, they are more likely to directly hit the finite surface of the planet, rather than settling down the planet's gravitational well, due to the combination of gravity and drag. We found that pebbles of Stokes number 70 < St < 400 are most favorable to be accreted, with the accretion efficiency approaching 100%. That is, almost every pebble in this size range that drifts past the planet will be swallowed by the planet. For higher Stokes numbers, the drift of pebble is so slow that it will be captured outside of the planet's orbit in a mean motion resonance. However, we found that the collision velocity among these pebbles are so high that they are likely to fragment to smaller sizes, which are highly likely to be accreted.
The St > 1 pebbles may be produced when planetesimals collide with each other, or when the gas density becomes low. The latter scenario could be achieved in the debris disk phase. We proposed a debris disk model where the primordial H/He gas is blown away by fast photoevaporation and the diluted CO gas is replenished by the outgassing from solids. We followed the drift and accretion of ~10μm-sized dusts particles, which could be produced during collision of larger particles. In such low density debris disk, the 30μm dust will become St > 1 pebbles and be accreted at high efficiency. We find up to ~0.3 Earth mass of these pebbles will be accreted by an Earth-mass planet, mostly in the debris disk phase as St > 1 pebbles. In conclusion, planets could still accrete solid material in the late phase of disk evolution, mainly by small-sized but aerodynamically big pebbles. This late accretion could contribute several percentage of the planet mass and shape the chemical composition of these planets atmospheres.
