Maximum mass of planetary embryos that formed in core-accretion models

Alibert, Yann (2017). Maximum mass of planetary embryos that formed in core-accretion models. Astronomy and astrophysics, 606(A69), A69. EDP Sciences 10.1051/0004-6361/201630051

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Context. In the core-accretion model, the typical size of solids that are accreted to form planetary embryos and planetary cores is debated. First, models assumed that the main part of planetary cores came from large-sized planetesimals, but other more recent models
are based on the accretion of small-sized pebbles.
Aims. The goal of this paper is to compute the maximum mass a growing planetary embryo can reach depending on the size of accreted planetesimals or pebbles, and to infer the possibility of growing the cores of giant planets and giant planets themselves.
Methods. We computed the internal structure of the gas envelope of planetary embryos to determine the core mass that is necessary to bind an envelope large enough to destroy planetesimals or pebbles while they are gravitationally captured. We also considered the
effect of the advection wind originating from the protoplanetary disk, following the results of Ormel et al. (2015).
Results.We show that for low-mass pebbles the envelope is large enough to destroy and vaporize pebbles completely before they can reach the core once the planetary embryo is larger than a fraction of the Earth mass. The material constituting pebbles is therefore
released in the planetary envelope and is later on dispersed in the protoplanetary disk if the advection wind is strong enough. As a consequence, the growth of the planetary embryo is stopped at a mass that is so low that Kelvin-Helmholtz accretion cannot lead to
the accretion of significant amounts of gas. For larger planetesimals, a similar process occurs but at much higher mass, on the order of ten Earth masses, and it is followed by rapid accretion of gas.
Conclusions. If the effect of the advection wind is as effcient as described in Ormel et al. (2015), the combined effect of the vaporization of accreted solids in the envelope of forming planetary embryos and of this advection wind prevents the growth of the planets
at masses lower than or similar to the Earth mass in the case of formation by pebble accretion, up to a distance on the order of 10 AU. In the case of formation by accretion of high-mass planetesimals, the growth of the planetary core is limited at masses on the order of ten Earth masses. However, in contrast to the case of pebble accretion, further growth is still possible and proceeds either through the accretion of gas or through the accretion of solids that are destroyed in the planetary envelope when the effect of the advection wind has ceased and the planetary Hill radius becomes comparable to the disk scale height.

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