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

Context. Accreting planetary-mass objects have been detected at H alpha, 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 H alpha 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 H alpha flux from the shock at the planetary surface and how they affect the line shape. We also study the absorption-modified relationship between the H alpha luminosity and accretion rate. Methods. We computed the high-resolution radiative transfer of the H alpha 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) over dot less than or similar to 3 x 10(-6) M(J)yr(-1), gas extinction is negligible for spherical or polar inflow and at most A(H alpha) less than or similar to 0.5 mag for magnetospheric accretion. Up to (M) over dot approximate to 3 x 10(-4) M(J)yr(-1), the gas contributes A(H alpha) less than or similar to 4 mag. This contribution decreases with mass. We estimate realistic dust opacities at H alpha to be k similar to 0.01-10 cm(2) g(-1) , which is 10-10(4) times lower than in the interstellar medium. Extinction flattens the L-H alpha-(M) over dot relationship, which becomes non-monotonic with a maximum luminosity L-H alpha similar to 10(-4) L-circle dot towards (M) over dot approximate to 10(-4) M(J)yr(-1) for a planet mass similar to 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) over dot strong absorption reduces the H alpha flux, and some measurements can be interpreted as two (M) over dot values. Highly resolved line profiles (R similar to 10(5)) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.