Steamworlds: atmospheric structure and critical mass of planets accreting icy pebbles

Abstract

In the core accretion model, gas-giant planets first form a solid core, which then accretes gas from a protoplanetary disk when the core exceeds a critical mass. Here, we model the atmosphere of a core that grows by accreting ice-rich pebbles. The ice fraction of pebbles evaporates in warm regions of the atmosphere, saturating it with water vapor. Excess water precipitates to lower altitudes. Beneath an outer radiative region, the atmosphere is convective, following a moist adiabat in saturated regions due to water condensation and precipitation. Atmospheric mass, density and temperature increase with core mass. For nominal model parameters, planets with core masses (ice + rock) between 0.08 and 0.16 Earth masses have surface temperatures between 273 K and 647 K and form an ocean. In more massive planets, water exists as a super-critical convecting fluid mixed with gas from the disk. Typically, the core mass reaches a maximum (the critical mass) as a function of the total mass when the core is 2-5 Earth masses. The critical mass depends in a complicated way on pebble size, mass flux, and dust opacity due to the occasional appearance of multiple core-mass maxima. The core mass for an atmosphere of 50 percent hydrogen and helium may be a more robust indicator of the onset of gas accretion. This mass is typically 1-3 Earth masses for pebbles that are 50 percent ice by mass, increasing with opacity and pebble flux, and decreasing with pebble ice/rock ratio.

Accepted for publication in The Astrophysical Journal

References in corpus (7)

• Particle Stirring in Turbulent Gas Disks: Including Orbital Oscillations
• Separating gas-giant and ice-giant planets by halting pebble accretion
• Ab initio Equation of State data for hydrogen, helium, and water and the internal structure of Jupiter
• Formation of Jupiter using opacities based on detailed grain physics
• Massive planet migration: Theoretical predictions and comparison with observations
• Grain opacity and the bulk composition of extrasolar planets. II. An analytical model for the grain opacity in protoplanetary atmospheres
• Critical core mass for enriched envelopes: the role of H2O condensation

Cited by in corpus (2)

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