Spectral appearance of the planetary-surface accretion shock: Global spectra and hydrogen-line profiles and fluxes

Abstract: Hydrogen-line emission from an accretion shock has recently been observed at planetary-mass objects. Our previous work predicted the shock spectrum and luminosity for a shock on the circumplanetary disc. We extend this to the planet-surface shock. We calculate the global spectral energy distribution (SED) of accreting planets by combining our model emission spectra with photospheric SEDs, and predict the line-integrated flux for several hydrogen lines, especially H alpha, but also H beta, Pa alpha, Pa beta, Pa gamma, Br alpha, and Br gamma. We apply our non-equilibrium emission model to the surface accretion shock for a wide range of accretion rates Mdot and masses M_p . Fits to formation calculations provide radii and effective temperatures. Extinction by the surrounding material is neglected, which is arguably often relevant. We find that the line luminosity increases monotonically with Mdot and M_p , depending mostly on Mdot and weakly on M_p for the relevant range of parameters. The Lyman, Balmer, and Paschen continua can exceed the photosphere. The H beta line is fainter by 0 to 1 dex than H alpha, whereas other lines are weaker (by 1 to 3 dex). Shocks on the planet or the CPD surface are distinguishable at very high spectral resolution, but the planet surface shock likely dominates if both are present. Applied to recent non-detections of H alpha, our models imply looser constraints on the Mdot of putative planets than when extrapolating fits from the stellar regime. These hydrogen-line luminosity predictions are useful for interpreting (non-)detections of accreting planets.