Warm-start planets from core accretion, and H α from accreting planets: Thermal and radiative properties of the accretion shock

In the core-accretion formation scenario of gas giant planets, most of the gas accreting onto a planet is likely processed through an accretion shock. This shock is key in setting the forming planet's structure and thus its observable post-formation luminosity, and the radiative feedback can change the thermal and chemical structure of the circumplanetary and local circumstellar disc. Also, direct evidence for ongoing accretion has been provided very recently for PDS 70 b and c, and more forming planets are expected in the near future thanks to ongoing and new searches with e.g. SPHERE or MUSE.

We present the first dedicated radiation-hydrodynamical simulations of the planetary accretion shock, using non-equilibrium radiation transport with up-to-date opacities (Marleau et al. 2017, 2019). We derive shock properties for a large grid of parameters. We find that usually, the temperature of the shock is given by the "free-streaming" limit. At very high accretion rates, the massive Rosseland opacity of the gas raises the shock temperature dramatically, an effect which has not been discussed explicitly before. We compare these results to original semi-analytical derivations. Additionally, we compute the fraction of the total accretion energy that is brought into the planet and find it is significant compared to the internal luminosity, supporting the hot-start scenario and suggesting that young planets are luminous.

Finally, using the non-LTE radiation-hydrodynamics code of Aoyama et al. (2018), we present the first predictions of hydrogen-line emission (H α, Pa beta, Br gamma, etc.) from the accretion shock on the surface of the planet (Aoyama, Marleau et al., in prep.). We compare with PDS 70 b and c and derive joint constraints on each planet's mass and accretion rate.