The accretion shock in planet formation

Gabriel-Dominique Marleau, University of Bern - 19.12.16

Abstract:

In the core-accretion (CA) formation scenario of gas giants, most of the gas accreting onto a planet is processed through an accretion shock. This shock is key in setting a planet's post-formation luminosity, with differences of several orders of magnitude at a given mass. In turn, processes at and ahead of the shock determine the radiative loss efficiency, i.e. the fraction of the accreting gas's kinetic energy which is radiated away from the system (roughly the Hill sphere). We use one-dimensional radiation-hydrodynamical simulations to compute this efficiency and to obtain post-shock temperatures and pressures and thus entropies. With as a first step an ideal-gas equation of state but constant or tabulated opacities, the shock is found to be isothermal and supercritical for a range of conditions relevant to the CA formation scenario, with the entropy of the gas decreasing by a significant amount across the shock. Also, the efficiencies are as low as roughly 40 percent, implying that a significant fraction of the total accretion energy is brought into the planet. We compare these results to semi-analytical estimates, highlight the relevant (micro)physics at the shock, and finally discuss possible observational implications.