Limit to Protoplanet Growth by Accretion of Small Solids

Presenter: Mohamad ALI-DIB
We identify limits on the growth of a rock-ice planetary core by the accretion of small solids from a gaseous disk. Pebbles falling into a gaseous envelope disintegrate into small grains long before reaching the core, due to sublimation, mutual collisions, and sandblasting. The sublimation of water ice creates a temperature plateau in the
envelope, and a strong inward peak in the density of hydrogen and water vapor. But the high accretion rate needed to build the core implies a high envelope metallicity and grain opacity, effectively shutting off radiative cooling and pushing convective motions out beyond the Bondi radius. Then the convective expulsion of dust-enriched gas can compensate ongoing accretion of pebbles and cooler metal-poor gas, effectively quenching protoplanet growth. We construct a time-dependent model of the envelope that includes ongoing accretion, pebble destruction, convection in the mixing-length approximation, dust transport and size evolution, radiative diffusion with a realistic dust opacity, feedback of sublimation on the thermodynamics of the envelope, multiple luminosity sources, and tabulated equations of state for hydrogen and water.

Ali-Dib & Thompson (2019), ApJ, arxiv: 1902.09449