Hydrogen-line profiles from accreting gas giants and their CPDs

Far fewer gas giants have been caught in their accretion phase than mature ones are known. Extremely Large Telescope (ELT) instruments will have a higher sensitivity and a smaller inner working angle than instruments up to now, which should allow more productive searches and detailed characterisation. We study the observability of accreting gas giants with METIS, the first-generation ELT spectrograph. We focus on the accretion-tracing hydrogen recombination lines accessible at a resolution R=1e5, mainly Brackett alpha and Pfund-series lines. Our approach is general but we take PDS 70 b as a fiducial case. To calculate high-resolution line profiles, we combine a semianalytical multidimensional description of the flow onto an accreting planet and its circumplanetary disc (CPD) with local non-equilibrium shock-emission models. We assume the limiting scenario of no extinction appropriate for gas giants in gaps and negligible contribution from magnetospheric accretion columns. We use simulated detector sensitivities to compute required observing times. Both the planet surface and the CPD surface shocks contribute to the total line profile, which is non-Gaussian and much narrower than the free-fall velocity. For the adopted baseline accretion rate onto PDS 70 b, the Br alpha line peak is equal to the photospheric continuum modulated mostly by water features. However, the rotation of the planet broadens the features, helping the shock excess stand out. At Br alpha, already the continuum of PDS 70 b should yield SNR=12 in 4 h. The peak excess should require only about 15 min to reach SNR=3. Br alpha is a potent planet formation tracer accessible to METIS in little integration time. Resolved line profiles will place independent constraints especially on the mass and radius of an accreting planet, and help identify the accretion mechanism(s) at work.

💡 Research Summary

This paper investigates the detectability of hydrogen recombination lines emitted by accreting gas‑giant planets and their circumplanetary disks (CPDs) using the first‑generation ELT spectrograph METIS. The authors focus on the Br α line (4.052 µm) and several Pfund lines, which are accessible at a spectral resolution of R≈10⁵ in the L‑ and M‑band windows of METIS. Using PDS 70 b as a fiducial example, they combine a semi‑analytical, multidimensional model of the gas flow within the planet’s Hill sphere with local non‑equilibrium shock‑emission calculations (Aoyama et al. 2018).

The flow model, an extension of the ballistic infall framework of Marleau (2025), assumes a “super‑thermal” planet that has opened a deep gap in the protoplanetary disk. Gas streams from the Hill sphere onto the planet and onto the CPD surface, with most of the mass landing on the CPD (parameterized by a centrifugal radius fraction f_cent≈0.03). The CPD is treated as a thin, rotationally supported structure; its thickness is described by a polar angle θ_CPD, which sets the aspect ratio h_CPD=tan(90°−θ_CPD). The authors assume azimuthal symmetry and neglect extinction (appropriate for a cleared gap) and any contribution from magnetospheric accretion columns.

Shock emission is calculated with the 1‑D radiation‑hydrodynamic models of Aoyama et al., which depend primarily on the preshock density n₀ and velocity v₀ (≈100–150 km s⁻¹ for Jupiter‑mass planets). The models predict that line widths are a modest fraction (10–130 %) of the free‑fall velocity and that emission drops sharply below v₀≈25–30 km s⁻¹, implying a lower mass limit of ~1–2 M_J for detectable lines. Each line is represented as a sum of Gaussians from the post‑shock cooling layer, but at R≈10⁵ the intrinsic line shape remains essentially non‑Gaussian and much narrower than the free‑fall speed.

Integrating the local emission over the visible portions of the planet and CPD surfaces yields total line profiles. Both the planetary surface shock and the CPD surface shock contribute, but the bulk of the Br α luminosity originates near the planetary pole. The resulting profiles are sharply peaked and non‑Gaussian. Planetary rotation (≈10 km s⁻¹) broadens the line enough to make the shock excess stand out against the photospheric continuum, which is modulated by water absorption features.

Observational feasibility is assessed using simulated METIS detector sensitivities. For the adopted baseline accretion rate onto PDS 70 b (Ṁ≈10⁻⁷ M_⊕ yr⁻¹), the photospheric continuum alone yields a signal‑to‑noise ratio (S/N) of ~12 in a 4‑hour exposure. The shock‑induced Br α peak excess reaches S/N≈3 in only ~15 minutes, demonstrating that METIS can detect the accretion signature with very modest integration times. Br α is therefore identified as a potent tracer of planet formation, with Pf β serving as a useful secondary line.

The authors discuss several caveats. The assumption of negligible extinction may break down for systems with higher dust column densities. Magnetospheric accretion columns, omitted here, could add additional line flux or alter the profile shape. The CPD thickness and viscosity are simplified into a single geometric parameter, whereas real disks may exhibit complex temperature and density gradients that affect shock conditions. Despite these simplifications, the study provides a robust framework for interpreting high‑resolution infrared spectra of forming planets.

In conclusion, the paper shows that METIS will be capable of detecting and resolving hydrogen recombination lines from accreting gas giants in a few minutes of integration. Resolved line profiles will independently constrain planetary mass, radius, and rotation, and will help discriminate between direct surface shocks and magnetospheric accretion as the dominant accretion mechanism. The work paves the way for systematic surveys of planet formation with ELT‑class facilities and highlights the need for future models that incorporate extinction, magnetic fields, and more detailed CPD physics.