Resolved near-UV hydrogen emission lines at 40-Myr super-Jovian protoplanet Delorme 1 (AB)b. Indications of magnetospheric accretion

Context. Accretion at planetary-mass companions (PMCs) suggests the presence of a protoplanetary disc in the system, likely accompanied by a circumplanetary disc. High-resolution spectroscopy of accreting PMCs is very difficult due to their proximity to bright host stars. For well-separated companions, however, such spectra are feasible and they are unique windows into accretion.
Aims: We have followed up on our observations of the 40-Myr, and still accreting, circumbinary PMC Delorme 1 (AB)b. We used high-resolution spectroscopy to characterise the accretion process further by accessing the wealth of emission lines in the near-UV.
Methods: We have used the UVES spectrograph on the ESO VLT/UT2 to obtain R_λ ≈ 50 000 spectroscopy, at 3300-4520 Å, of Delorme 1 (AB)b. After separating the emission of the companion from that of the M5 low-mass binary, we performed a detailed emission-line analysis, which included planetary accretion shock modelling.
Results: We reaffirm ongoing accretion in Delorme 1 (AB)b and report the first detections in a (super-Jovian) protoplanet of resolved hydrogen line emission in the near-UV (Hγ, Hδ, Hϵ, H8, and H9). We tentatively detect H11, H12, He I, and Ca II H/K. The analysis strongly favours a planetary accretion shock with a line-luminosity-based accretion rate of Ṁ = 2 × 10⁻⁸ M_J yr⁻¹. The lines are asymmetric and are well described by sums of narrow and broad components with different velocity shifts. The overall line shapes are best explained by a pre-shock velocity of v₀ = 170 ± 30 km s⁻¹, implying a planetary mass of M_P = 13 ± 5 M_J, and number densities of n₀ ≳ 10¹³ cm⁻³ or n₀ ∼ 10¹¹ cm⁻³. The higher density implies a small line-emitting area of ∼1% relative to the planetary surface. This favours magnetospheric accretion, a case potentially strengthened by the presence of blueshifted emission in the line profiles.
Conclusions: High-resolution spectroscopy offers the opportunity to resolve line profiles, which are crucial for studying the accretion process in depth. The super-Jovian protoplanet Delorme 1 (AB)b is still accreting at ∼40 Myr. Thus, Delorme 1 belongs to the growing family of `Peter Pan disc' systems with (a) protoplanetary and/or circumplanetary disc(s) far beyond the typically assumed disc lifetimes. Further observations of this benchmark companion and its presumed disc(s) will help answer key questions about the accretion geometry in PMCs.

Based on observations collected at the European Southern Observatory under ESO programme 0108.C-0655(A).