Planetary Accretion Shocks with a Realistic Equation of State

TL;DR

This study uses advanced 1D radiation hydrodynamic simulations to analyze accretion shocks during gas giant formation, revealing how shock luminosity affects hydrogen dissociation and the entropy of accreted gas, with implications for planetary imaging.

Contribution

First self-consistent simulation of hydrogen dissociation, ionization, and radiation transport in planetary accretion shocks across a range of giant planet masses and accretion rates.

Findings

Shock luminosity can fully dissociate molecular hydrogen.

Post-shock entropy falls into 'cold' and 'hot' groups.

Distinction in entropy states influences interpretation of young exoplanets.

Abstract

The final stage of gas giant formation involves accreting gas from the parent protoplanetary disk. In general, the infalling gas likely approaches a free-fall velocity, creating an accretion shock, leading to strong shock heating and radiation. We investigate the kinematics and energetics of such accretion shocks using 1D radiation hydrodynamic simulations. Our simulations feature the first self-consistent treatment of hydrogen dissociation and ionization, radiation transport, and realistic grey opacity. By exploring a broad range of giant planet masses (0.1-3 M$_{J}$) and accretion rates ($10^{-3}$-$10^{-2}$M$_{\oplus}\cdot\rm{yr}^{-1}$), we focus on global shock efficiency and the final entropy of the accreted gas. We find that radiation from the accretion shock can fully disassociate the molecular hydrogen of the incoming gas when the shock luminosity is above a critical luminosity. Meanwhile, the post-shock entropy generally fall into "cold" ($<12k_{\rm{B}}/m_{\rm H}$) and "hot" ($>16k_{\rm{B}}/m_{\rm H}$) groups which depends on the extent of the endothermic process of $\rm{H}_2$ dissociation. While 2D or 3D simulations are needed for more realistic understandings of the accretion process, this distinction likely carries over and sheds light on the interpretation of young direct imaging planets.