ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY)

III. Optical He I line profiles of the accreting super Jupiter Delorme 1 (AB)b

¹ Institutionen för astronomi, Stockholms universitet, AlbaNova universitetscentrum, 106 91 Stockholm, Sweden
² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Department of Astronomy, University of Virginia, 530 McCormick Rd, Charlottesville, VA 22904, USA
⁴ SUPA, School of Science and Engineering, University of Dundee, Nethergate, DD1 4HN Dundee, UK
⁵ Fakultät für Physik, Universität Duisburg–Essen, Lotharstraße 1, 47057 Duisburg, Germany
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Division of Space Research & Planetary Sciences, Physics Institute, University of Bern, Sidlerstr. 5, 3012 Bern, Switzerland
⁸ Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 December 2025
Accepted: 9 March 2026

Abstract

Context. Observations of helium emission lines from classical T Tauri stars at high resolution (R_λ > 10 000) offer great potential, showing distinct profile characteristics that help probe regions within the accretion geometry untapped by hydrogen lines. Parallel studies in the planetary-mass regime have not been explored.

Aims. We investigate helium line emission from the nearby (47 pc), wide orbit (∼84 au), ∼13 M_Jup, accreting circumbinary companion Delorme 1 (AB)b and analyse the resolved profile characteristics to infer clues to line origin.

Methods. We obtained high signal-to-noise spectra of the target over 33 exposures with VLT/UVES over near-ultraviolet to optical wavelengths at high resolution (R_λ ~ 50 000). We studied the helium line profiles in the spectra and compared them to helium emission recorded from both accreting and non-accreting young stellar objects.

Results. We detected seven neutral helium (He I) lines λλ3890, 4027, 4473, 4923, 5017, 5877, 6680 at high confidence (> 5σ), with notable flux variation between epochs. The line profiles of He I λλ5877,4923,4473,4027 show clear asymmetry, with a narrow component at ∼0 kms⁻¹ and a broad component redshifted by ∼15 kms⁻¹. The accretion luminosity (1.3_−0.7^+1.6 × 10⁻⁵ L_⊙) and mass accretion rate (0.7_−0.4^+0.9 × 10⁻⁸ M_Jup yr⁻¹) obtained from median He I line luminosities using empirical scaling relations from stars are comparable but slightly higher than from the target’s ultraviolet excess emission.

Conclusions. The protoplanet Delorme 1 (AB)b exhibits asymmetric He I lines similar to classical T Tauri stars, but with much smaller widths for the narrow and broad components. The triplet-singlet line ratio, a strong correlation with ultraviolet excess and the near-zero, redshifted velocities obtained for the narrow component suggest that it originates within the post-shock region, close to the planet surface. The persistent redshift of the broad component, its line width, and velocity correlation with the narrow component imply an origin within the shock structure, closer to the shock front. Emission seems to be dominated by accretion based on the obtained accretion luminosities, but a contribution from chromospheric activity may be present.