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Optically thin core accretion: how planets get their gas in nearly gas-free discs

Lee, Eve J. and Chiang, Eugene and Ferguson, Jason W. (2018) Optically thin core accretion: how planets get their gas in nearly gas-free discs. Monthly Notices of the Royal Astronomical Society, 476 (2). pp. 2199-2208. ISSN 0035-8711. doi:10.1093/mnras/sty389.

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Models of core accretion assume that in the radiative zones of accreting gas envelopes, radiation diffuses. But super-Earths/sub-Neptunes (1–4 R_⊕, 2–20 M_⊕) point to formation conditions that are optically thin: their modest gas masses are accreted from short-lived and gas-poor nebulae reminiscent of the transparent cavities of transitional discs. Planetary atmospheres born in such environments can be optically thin to both incident starlight and internally generated thermal radiation. We construct time-dependent models of such atmospheres, showing that super-Earths/sub-Neptunes can accrete their ∼1 per cent-by-mass gas envelopes, and super-puffs/sub-Saturns their ∼20 per cent-by-mass envelopes, over a wide range of nebular depletion histories requiring no fine tuning. Although nascent atmospheres can exhibit stratospheric temperature inversions affected by atomic Fe and various oxides that absorb strongly at visible wavelengths, the rate of gas accretion remains controlled by the radiative–convective boundary (rcb) at much greater pressures. For dusty envelopes, the temperature at the rcb T_(rcb) ≃ 2500 K is still set by H₂ dissociation; for dust-depleted envelopes, T_(rcb) tracks the temperature of the visible or thermal photosphere, whichever is deeper, out to at least ∼5 au. The rate of envelope growth remains largely unchanged between the old radiative diffusion models and the new optically thin models, reinforcing how robustly super-Earths form as part of the endgame chapter in disc evolution.

Item Type:Article
Related URLs:
URLURL TypeDescription
Lee, Eve J.0000-0002-1228-9820
Chiang, Eugene0000-0002-6246-2310
Ferguson, Jason W.0000-0002-8216-6084
Additional Information:We are grateful to Richard Freedman and Mark Marley for in-depth conversations that helped us understand opacities better. We also thank Jeff Cuzzi, Kevin Heng, Jack Lissauer, Nikku Madhusudhan, Eliot Quataert, and Leslie Rogers for helpful and motivating discussions. Andrew Youdin provided positive and constructive feedback on our submitted manuscript, and Michiel Lambrechts delivered a thoughtful and encouraging referee's report that led to substantive improvements. EJL was supported in part by NSERC of Canada under PGS D3, the Berkeley Fellowship, and the Sherman Fairchild Fellowship at Caltech. EC acknowledges support from the NSF. This research used the Savio computational cluster resource provided by the Berkeley Research Computing programme at the University of California, Berkeley, supported by the UC Berkeley Chancellor, Vice Chancellor for Research, and Chief Information Officer.
Group:TAPIR, Walter Burke Institute for Theoretical Physics
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)PGS D3
University of California, BerkeleyUNSPECIFIED
Sherman Fairchild FoundationUNSPECIFIED
Issue or Number:2
Record Number:CaltechAUTHORS:20230119-806240100.1
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118856
Deposited By: Tony Diaz
Deposited On:19 Jan 2023 20:34
Last Modified:19 Jan 2023 20:34

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