http://arxiv.org/abs/1708.05392
Context: Planet formation by pebble accretion is an alternative to classical core accretion. One of the main differences with classical core accretion is the increased thermal ablation rate experienced by pebbles. This changes the process of core growth. Aims: To describe and compute core growth in the pebble accretion model. We aim to predict core masses and compositions that can form by pebble accretion and compare them to the case of planetesimals. Methods: We have written a code containing both an impact and a planet evolution model to simulate the early growth of a proto-planet self-consistently. The region where high-Z material can exist in vapor form is determined by the temperature-dependent vapor pressure. We include enrichment effects by locally modifying the mean molecular weight of the envelope and determine when direct core growth of the planet terminates. Results: We have identified three phases of core growth in pebble accretion. In the first phase, pebbles impact the core without significant ablation. During the second phase, ablation becomes increasingly severe. A layer of high-Z vapor starts to form around the core that absorbs a small fraction of the ablated mass. The rest of the material either rains out to the core or mixes outwards instead, slowing core growth. In the third phase, the high-Z inner region expands outwards, absorbing an increasing fraction of the ablated material as vapor. Rainout ends before the core mass reaches 0.6 M_Earth, terminating direct core growth. Conclusions: Our results indicate that pebble accretion can directly form rocky cores up to only 0.6 M_Earth, and is unable to form icy cores. This result contrasts classical core accretion models, which can directly form massive cores of both rocky and icy compositions. Subsequent core growth can proceed indirectly when the planet cools, provided it is able to retain its high-Z material.
M. Brouwers, A. Vazan and C. Ormel
Mon, 21 Aug 17
10/44
Comments: 10 pages, 10 figures. Submitted to A&A. Comments welcome