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

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 MJ yr−1, gas extinction is negligible for spherical or polar inflow and at most AH α ≲ 0.5 mag for magnetospheric accretion. Up to Ṁ ≈ 3 × 10−4 MJ yr−1, the gas contributes AH 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 LH α –Ṁ relationship, which becomes non-monotonic with a maximum luminosity LH α ~ 10−4 L⊙ towards Ṁ ≈ 10−4 MJ yr−1 for a planet mass ~10 MJ. 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.

Improved Orbital Constraints and Hα Photometric Monitoring of the Directly Imaged Protoplanet Analog HD 142527 B

“…They include the (disputed) candidate LkCa 15b: M p < 5 − 10M J , M M M 10 yr companions have been directly imaged at Hα, with recent surveys returning no new detections (Cugno et al 2019;Zurlo et al 2020). This may be a low Strehl selection effect (Close 2020) or a reflection of differences in accretion physics at planetary masses (Aoyama et al 2021;Marleau et al 2022).…”

Near-infrared Accretion Signatures from the Circumbinary Planetary-mass Companion Delorme 1 (AB)b*

“…While the preshock velocity is consistent with measured M  's and assumed mass (and radius; see Figure 13 of Aoyama et al 2020), the number density is higher than expected for the measured M  assuming a pure planetaryshock model. This could be explained by shock emission with a low filling factor resulting from a magnetospheric accretion flow, absorption in the postshock region (Hashimoto et al 2020), strong accretion column extinction (Marleau et al 2022, though they found that the M  is too low for absorption by either gas or dust in the accretion flow), or circumplanetary disk extinction in the line of sight (Aoyama et al 2020). High-resolution (R ∼ 10,000) spectra will help disentangle the accretion flow geometry and shed light on the nature of the accretion shock, as resolved line profiles can distinguish between geometries (Aoyama et al 2020;Marleau et al 2022).…”

HST/WFC3 Hα Direct-imaging Detection of a Pointlike Source in the Disk Cavity of AB Aur

“…Adopting a line-of-sight extinction of = 0.5 mag (the same as the star, Garcia Lopez et al 2006) and assuming no additional extinction opacity, we find a H line luminosity of H = 2.2 ± 0.7 × 10 −5 ⊙ . At this H , gas in the accretion flow should not lead to significant absorption (Marleau et al 2022). The H value corresponds to a total accretion luminosity of log( acc ∕ ⊙ ) = −2.8 ± 0.3 by assuming the planetary surface shock model (Aoyama et al 2018;Aoyama et al 2021) or log( acc ∕ ⊙ ) = −3.5 ± 0.3 using the empirical relation for classic T Tauri stars (Alcalá et al 2017), respectively.…”