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Close-in super-Earths: The first and the last stages of planet formation in an MRI-accreting disc

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Title: Close-in super-Earths: The first and the last stages of planet formation in an MRI-accreting disc
Authors: Jankovic, MR
Owen, JE
Mohanty, S
Item Type: Journal Article
Abstract: We explore in situ formation and subsequent evolution of close-in super-Earths and mini-Neptunes. We adopt a steady-state inner protoplanetary gas disc structure that arises from viscous accretion due to the magneto-rotational instability (MRI). We consider the evolution of dust in the inner disc, including growth, radial drift, and fragmentation, and find that dust particles that radially drift into the inner disc fragment severely due to the MRI-induced turbulence. This result has two consequences: (1) radial drift of grains within the inner disc is quenched, leading to an enhancement of dust in the inner regions that scales as dust-to-gas-mass-flux-ratio at ∼1 au; (2) however, despite this enhancement, planetesimal formation is impeded by the small grain size. Nevertheless, assuming that planetary cores are present in the inner disc, we then investigate the accretion of atmospheres on to cores and their subsequent photoevaporation. We then compare our results to the observed exoplanet mass–radius relationship. We find that (1) the low gas surface densities and high temperatures in the inner disc reduce gas accretion on to cores compared to the minimum mass solar nebula, preventing the cores from growing into hot Jupiters, in agreement with the data; (2) however, our predicted envelope masses are still typically larger than observed ones. Finally, we sketch a qualitative picture of how grains may grow and planetesimals form in the inner disc if grain effects on the ionization levels and the MRI and the back reaction of the dust on the gas (both neglected in our calculations) are accounted for.
Issue Date: 1-Apr-2019
Date of Acceptance: 12-Dec-2018
URI: http://hdl.handle.net/10044/1/69799
DOI: https://dx.doi.org/10.1093/mnras/stz004
ISSN: 0035-8711
Publisher: Oxford University Press (OUP)
Start Page: 2296
End Page: 2308
Journal / Book Title: Monthly Notices of the Royal Astronomical Society
Volume: 484
Issue: 2
Copyright Statement: © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). This is a pre-copy-editing, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The definitive publisher-authenticated version Marija R Jankovic, James E Owen, Subhanjoy Mohanty, Close-in Super-Earths: The first and the last stages of planet formation in an MRI-accreting disc, Monthly Notices of the Royal Astronomical Society, Volume 484, Issue 2, April 2019, Pages 2296–2308, https://doi.org/10.1093/mnras/stz004 is available online at: https://dx.doi.org/10.1093/mnras/stz004
Sponsor/Funder: The Royal Society
Funder's Grant Number: UF150412
Keywords: Science & Technology
Physical Sciences
Astronomy & Astrophysics
planets and satellites: formation
protoplanetary discs
MAGNETOROTATIONAL INSTABILITY
PROTOPLANETARY DISKS
SURFACE-DENSITY
DUST PARTICLES
MASS
GAS
FRAGMENTATION
ATMOSPHERES
VELOCITIES
IMPACTS
0201 Astronomical and Space Sciences
Publication Status: Published
Open Access location: https://arxiv.org/abs/1901.01800
Online Publication Date: 2019-01-05
Appears in Collections:Physics
Astrophysics
Faculty of Natural Sciences