ExoplaNeT accRetion mOnitoring sPectroscopic surveY (ENTROPY) III. Optical He I line profiles of the accreting super Jupiter Delorme 1 (AB)b

Abstract

High-resolution spectroscopic observations of helium emission lines provide a powerful probe of accretion geometry in classical T Tauri stars, revealing regions not well traced by hydrogen lines. Parallel studies in the planetary-mass regime are lacking. In this work, we investigate helium emission from the nearby (47 pc), wide-orbit (~84 au), ~13 $M_{Jup}$ accreting circumbinary companion Delorme 1 (AB)b and use resolved line profiles to constrain their origin. We analyse 33 high-S/N VLT/UVES spectra spanning near-ultraviolet to optical wavelengths at R~50,000. We detect seven He I lines at >5$\sigma$ confidence - 3890, 4027, 4473, 4923, 5017, 5877, and 6680 \.A - with significant epoch-to-epoch variability. The He I 5877, 4923, 4473, and 4027 \.A lines are asymmetric, showing a narrow component near 0 km/s and a broad component redshifted by ~15 km/s. The accretion luminosity ($1.3^{+1.6}_{-0.7}\times 10^{-5} L_{\odot}$) and mass accretion rate ($0.7^{+0.9}_{-0.4} \times 10^{-8} M_{Jup} yr^{-1}$) inferred from the median He I line luminosities are broadly consistent with, but slightly higher than, estimates from the ultraviolet excess. We conclude that protoplanet Delorme 1 (AB)b shows asymmetric He I profiles analogous to those of classical T Tauri stars, but with much smaller narrow- and broad-component widths. 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.