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arXiv:1708.05366 [astro-ph.EP]AbstractReferencesReviewsResources

Effects of Planetesimal Accretion on the Thermal and Structural Evolution of Sub-Neptunes

Sourav Chatterjee, Howard Chen

Published 2017-08-17Version 1

A remarkable discovery from NASA's Kepler mission is the wide diversity in the average densities of planets that defy traditional planet formation theories. After the dissipation of gas disk, planets could interact with nearby planetesimals from a remnant planetesimal disk. Typical low planetary densities, their proximity to the host stars, and the high star-planet mass ratios often lead to planetesimal accretion by the planets as a result of these interactions. Here we present calculations using the open-source stellar evolution toolkit MESA (Modules for Experiments in Stellar Astrophysics) modified to include the deposition of planetesimals into the hydrogen/helium envelopes of sub-Neptunes (~sim 1-20 MEarth). We show that such accretion can alter the mass-radius isochrones for these planets. Due to the inherent stochasticity, the same initial planet as a result of the same total accreted mass of planetesimals can have average densities different by up to ~5% several ~Gyr after the last accretion. Of course, during the active accretion period these differences are more dramatic. Furthermore, the additional energy deposition from the accreted planetesimals increases the ratio between the planet's radius to that of the core, which in turn leads to enhanced loss of atmospheric mass. Planet mean densities, long after the phase of high accretion rate are also altered by planetesimal accretion, especially for planets initially less massive than ~10 MEarth and with envelope mass fraction less than ~10%. These results indicate that planetesimal accretion may be an important contributor in shaping the properties and diverse densities of sub-Neptune sized planets in addition to the more well-studied effects of photo-evaporation.

Comments: 17 Pages, 9 Figures, 1 Table; Submitted to the Astrophysical Journal
Categories: astro-ph.EP
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