Accreting protoplanets: Spectral signatures and magnitude of gas and dust extinction at Hα

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 should originate from the postshock region, making them particularly susceptible to extinction by the accreting material. Also, high-resolution (R>50,000) spectrographs operating at Hα will soon enable studying 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 Hα luminosity and accretion rate.Methods. We compute the high-resolution radiative transfer of the Hα line using a one-dimensional model of the velocity-density-temperature structure of the inflowing matter for three representative accretion geometries: spherical symmetry, polar inflow, and magnetospheric accretion. For each, we explore the wide relevant parameter space in accretion rate and mass. We use detailed gas opacities, including resonant absorption, and critically assess what appropriate dust opacity values might be.Results. At accretion rates Mdot ≲ 3×10^-6 Mjup yr^-1, gas extinction is negligible for spherical or polar inflow and at most A_Hα ≲ 0.5 mag for magnetospheric accretion. Up to Mdot ≈ 3 × 10^−4 Mjup yr^−1, the gas contributes A_Hα ≲ 4 mag, decreasing with mass. We estimate realistic dust opacities at Hα to be κ ∼ 0.01–10 cm^2 g^−1, 10–10^4 times lower than in the interstellar medium. Extinction flattens the L_Hα–Mdot relationship, which becomes non-monotonic with a maximum luminosity L_Hα ∼ 10 L_⊙ towards Mdot ≈ 10^-4 Mjup yr^-1 for a planet mass Mp ∼ 10 Mjup. 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 mild, simplifying the interpretation of observations, especially for planets in gaps. At high Mdot, strong absorption reduces the Hα flux, and some Hα fluxes can be interpreted as two Mdot values. Highly-resolved line profiles (R ∼ 10^5) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.