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

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
e-mail: gabriel.marleau@uni-tuebingen.de
² Physikalisches Institut, Universität Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Institute for Advanced Study, Tsinghua University, Beijing 100084, PR China
⁵ Department of Astronomy, Tsinghua University, Beijing 100084, PR China
⁶ Institut für Theoretische Astrophysik (ITA), Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁷ Physics and Astronomy Department, Amherst College, 25 East Drive, Amherst, MA 01002, USA
⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
¹⁰ Institute for Particle Physics and Astrophysics, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
¹¹ University of Arizona, 1200 E University Blvd., Tucson, AZ 85721, USA
¹² Centre for Space and Habitability, Universität Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
¹³ Institutionen för astronomi, AlbaNova universitetscentrum, Stockholms universitet, 106 91 Stockholm, Sweden
¹⁴ Department of Astronomy and Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85719, USA
¹⁵ Department of Physics and Astronomy, University of California, 4129 Frederick Reines Hall, Irvine, CA 92697-4575, USA
¹⁶ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁷ NASA Ames Research Center, Moffett Blvd, Mountain View, CA 94035, USA
¹⁸ SETI Institute, 189 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
¹⁹ Observatoire astronomique de l’Université de Genève, 51 ch. des Maillettes, 1290 Versoix, Switzerland

Received: 13 January 2020
Accepted: 17 November 2021

Abstract

Context. Accreting planetary-mass objects have been detected at H α, 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 α 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 α 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 α luminosity and accretion rate.

Methods. We computed the high-resolution radiative transfer of the H α 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 Ṁ ≲ 3 × 10⁻⁶ M_J yr⁻¹, gas extinction is negligible for spherical or polar inflow and at most A_{H α} ≲ 0.5 mag for magnetospheric accretion. Up to Ṁ ≈ 3 × 10⁻⁴ M_J yr⁻¹, the gas contributes A_{H α} ≲ 4 mag. This contribution decreases with mass. We estimate realistic dust opacities at H α to be κ ~ 0.01–10 cm² g⁻¹, which is 10–10⁴ times lower than in the interstellar medium. Extinction flattens the L_{H α} –Ṁ relationship, which becomes non-monotonic with a maximum luminosity L_{H α} ~ 10⁻⁴ L_⊙ towards Ṁ ≈ 10⁻⁴ M_J yr⁻¹ 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 Ṁ, strong absorption reduces the H α flux, and some measurements can be interpreted as two Ṁ values. Highly resolved line profiles (R ~ 10⁵) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.