2022A&A...657A..38M

Context. Accreting planetary-mass objects have been detected at Hα, but targeted searches have mainly resulted in non-detections. Accretion tracers in the planetary-mass regime could originate from the shock itself, making them particularly susceptible to extinction by the accreting material. High-resolution (R > 50 000) spectrographs operating at Hα should soon enable one to study how the incoming material shapes the line profile. Aims. We calculate how much the gas and dust accreting onto a planet reduce the Hα flux from the shock at the planetary surface and how they affect the line shape. We also study the absorption-modified relationship between the Hα luminosity and accretion rate. Methods. We computed the high-resolution radiative transfer of the Hα line using a one-dimensional velocity-density-temperature structure for the inflowing matter in three representative accretion geometries: spherical symmetry, polar inflow, and magnetospheric accretion. For each, we explored the wide relevant ranges of the accretion rate and planet mass. We used detailed gas opacities and carefully estimated possible dust opacities. Results. At accretion rates of M ≥ 3 x 10^–6 M_J/yr, gas extinction is negligible for spherical or polar inflow and at most A_Hα ≥ 0.5 mag for magnetospheric accretion. Up to M ≃ 3 x 10^–4 M_J/yr^, the gas contributes A_Hα ≥ 4 mag. This contribution decreases with mass. We estimate realistic dust opacities at Hα to be κ ∼ 0.01-10 cm² g^–1, which is 10-10⁴ times lower than in the interstellar medium. Extinction flattens the L_Hα -M relationship, which becomes non-monotonic with a maximum luminosity L_Hα ∼ 10^–4 L_☉ towards M ≃ 10^–4 M_J/yr for a planet mass ∼10 M_J. In magnetospheric accretion, the gas can introduce features in the line profile, while the velocity gradient smears them out in other geometries. Conclusions. For a wide part of parameter space, extinction by the accreting matter should be negligible, simplifying the interpretation of observations, especially for planets in gaps. At high M, strong absorption reduces the Hα flux, and some measurements can be interpreted as two M values. Highly resolved line profiles (R ∼ 10⁵) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.