2026A&A...706A..57D

Context. Accretion processes in the planetary-mass regime are still poorly constrained, yet they strongly impact the formation and evolution of planets and the composition of circumplanetary disks.
Aims. We investigate the resolved Balmer hydrogen emission-line profiles and their variability timescales in the ∼13 M_Jup, 30-45 Myr-old companion Delorme 1 (AB)b to derive constraints on the accretion mechanism at play.
Methods. With VLT/UVES, we collected 31 new epochs of high-resolution optical (330-680 nm) spectra of the companion at R = 50 000, probing variability on timescales of hours to years. We study the companion's H I emission line shape and flux variability and compare them to two proposed line origins: magnetospheric accretion funnel and localized accretion shock.
Results. We detect H I Balmer lines from Hα up to H10 (6564-3799 Å), as well as the UV continuum excess - signs of ongoing accretion. All lines and UV excess are variable. The H I lines can be decomposed into two static components that vary only by their flux. The broader component in velocity correlates strongly with the UV excess, and its profile is qualitatively reproduced by magnetospheric accretion funnel models but clearly not by shock models. With strong relative variability, this broad component almost entirely explains the variability in the shape of the line profiles. The second, narrower component correlates less with the UV excess and is best reproduced by shock-emission models. Its strong absolute variability makes it responsible for most of the line flux variability. Overall, the lines have low relative flux variability on hourly timescales, but up to ∼100% on weekly timescales and beyond, a behavior similar to T Tauri stars.
Conclusions. The properties of the broad component of the H I lines strongly support magnetospheric accretion. The narrow component could be due to an accretion shock as well as chromospheric activity. Higher-cadence observations could search for rotational modulations to constrain the object's rotational period and the exact geometry of the accretion flow.