H alpha emission and warm-start planets predicted from accretion shocks

¶ 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 sets the forming planet’s structure and thus its observable post-formation luminosity, and the…

H alpha emission and warm-start planets predicted from accretion shocks

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¶ 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 sets the forming planet’s structure and thus its observable post-formation luminosity, and the radiative feedback can affect the circumplanetary and circumstellar discs thermally and chemically. Also, there has been recent direct evidence for ongoing accretion at PDS 70 b and c and Delorme 1 (AB)b, and searches with SPHERE, MUSE, MagAO-X, etc. should soon reveal more forming planets.

¶ Using the non-LTE, chemical-kinetics code of Aoyama et al. (2018), we present the first predictions of spectrally-resolved line emission from the planetary-surface accretion shock (Aoyama, Marleau et al., in prep.). We focus on the brightest accretion tracers (H α, H β, Br γ, etc.). The line shape is sensitive to the planet mass and accretion rate and geometry. In several cases, molecular absorption in the accretion flow leaves an imprint visible at high resolution (R ~ 15’000), providing signatures of the accretion geometry (Marleau, Aoyama et al., subm.). We also introduce a relation between L_Hα and L_acc appropriate for accreting planets, which can be used instead of extrapolations from the stellar regime.

¶ Time permitting, we discuss the results of dedicated radiation-hydrodynamical simulations of the planetary accretion shock, using non-equilibrium radiation transport with up-to-date opacities (Marleau et al. 2017, 2019b). We derive shock properties for a large grid in accretion rate, planet mass, and planet radius. We compare these results to original semi-analytical derivations. We find that the fraction of the total accretion energy that is brought into the planet is significant compared to the internal luminosity. This supports strongly the warm-start scenario and suggests that young planets are luminous.

Talk given on 28.07.2020 at Exo III in “Heidelberg”

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