Resolved near-UV hydrogen emission lines at 40-Myr super-Jovian protoplanet Delorme 1 (AB)b

Ringqvist, Simon C.; Viswanath, Gayathri; Aoyama, Yuhiko; Janson, Markus; Marleau, Gabriel-Dominique; Brandeker, Alexis (2023). Resolved near-UV hydrogen emission lines at 40-Myr super-Jovian protoplanet Delorme 1 (AB)b. Astronomy and astrophysics, 669, L12. EDP Sciences 10.1051/0004-6361/202245424

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Context. Accretion at planetary-mass companions (PMCs) suggests the presence of a protoplanetary disc in the system, likely accom-
panied 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 Rλ ≈ 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 ˙M = 2 × 10−8 MJ yr−1. The lines are asym-
metric 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 MP = 13 ± 5 MJ, 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 accre-
tion 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.

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