2022ApJ...925L..14C

The final stage of gas giant formation involves accreting gas from the parent protoplanetary disk. In general, the infalling gas likely approaches a freefall 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 gray opacity. By exploring a broad range of giant-planet masses (0.1-3M _J) and accretion rates (10^–3-10^–2 M _⊕ 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 falls into "cold" (<=12k _B/m _H) and "hot" (>=16k _B/m _H) groups which depend on the extent of the endothermic process of H₂ 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.