Hydrodynamics of Embedded Planets' First Atmospheres. II. A Rapid Recycling of Atmospheric Gas

Abstract

Following Paper I we investigate the properties of atmospheres that form around small protoplanets embedded in a protoplanetary disc by conducting hydrodynamical simulations. These are now extended to three dimensions, employing a spherical grid centred on the planet. Compression of gas is shown to reduce rotational motions. Contrasting the 2D case, no clear boundary demarcates bound atmospheric gas from disc material; instead, we find an open system where gas enters the Bondi sphere at high latitudes and leaves through the midplane regions, or, vice versa, when the disc gas rotates sub-Keplerian. The simulations do not converge to a time-independent solution; instead, the atmosphere is characterized by a time-varying velocity field. Of particular interest is the timescale to replenish the atmosphere by nebular gas, $t_{\mathrm{replenish}}$. It is shown that the replenishment rate, $M_{\mathrm{atm}}/t_{\mathrm{replenish}}$, can be understood in terms of a modified Bondi accretion rate, $\sim$$R_{\mathrm{Bondi}}^2{\rho }_{\mathrm{gas}}{v}_{\mathrm{Bondi}}$, where $v_{\mathrm{Bondi}}$ is set by the Keplerian shear or the magnitude of the sub-Keplerian motion of the gas, whichever is larger. In the inner disk, the atmosphere of embedded protoplanets replenishes on a timescale that is shorter than the Kelvin-Helmholtz contraction (or cooling) timescale. As a result, atmospheric gas can no longer contract and the growth of these atmospheres terminates. Future work must confirm whether these findings continue to apply when the (thermodynamical) idealizations employed in this study are relaxed. But if shown to be broadly applicable, replenishment of atmospheric gas provides a natural explanation for the preponderance of gas-rich but rock-dominant planets like super-Earths and mini-Neptunes.

15 pages, 13 figures, submitted to MNRAS. Comments welcome

References in corpus (9)

• PLUTO: a Numerical Code for Computational Astrophysics
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• An Atmospheric Structure Equation for Grain Growth
• Gas accretion onto planetary cores: three-dimensional self-gravitating radiation hydrodynamical calculations
• Grain opacity and the bulk composition of extrasolar planets. II. An analytical model for the grain opacity in protoplanetary atmospheres
• Growing and moving low-mass planets in non-isothermal disks
• Hydrodynamics of Embedded Planets' First Atmospheres. I. A Centrifugal Growth Barrier for 2D Flows
• Accretion and Evolution of ~2.5 Earth-mass Planets with Voluminous H/He Envelopes
• Formation of Isothermal Disks around Protoplanets. I. Introductory Three-Dimensional Global Simulations for Sub-Neptune-Mass Protoplanets

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