JWST-TST High Contrast: Medium-resolution spectroscopy reveals a carbon-rich circumplanetary disk around the young accreting exoplanet Delorme 1 AB b

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Context. Young accreting planetary-mass objects are thought to draw material from a circumplanetary disk composed of gas and dust. While the gas within the disk is expected to disperse within the first million years, strong accretion has nonetheless been detected in older systems, including the 30-45 Myr-old planetary-mass companion Delorme 1 AB b. Aims. We conducted spectroscopic observations with the James Webb Space Telescope’s Mid-Infrared Instrument ( JWST /MIRI) to investigate the presence of circumplanetary material around this young accreting planet and to characterize the planet’s atmospheric properties and composition. Methods. We performed forward modeling using atmospheric models to characterize the planet’s atmosphere, combining our MIRI observations with archival ground-based near-infrared data. We used slab models to analyze the circumplanetary gas and investigated H 2 emission. Results. We derived the atmospheric parameters of Delorme 1 AB b, finding an effective temperature of T eff = 1725 ± 134 K. To achieve a satisfactory fit to the observed spectrum, a secondary component is required, consistent with dust emission from a circumplanetary disk (CPD), characterized by a blackbody temperature of T bb = 295 ± 27 K and an effective radius of R bb = 18.8 ± 2.7 R Jup . Beyond 10 μm, the spectral energy distribution (SED) becomes dominated by this circumplanetary disk rather than the planet itself. We detected strong emission from HCN and C 2 H 2 , along with tentative evidence of the isotopologue 13 CCH 2 , while no O-bearing species such as CO, CO 2 , or H 2 O are observed in the CPD spectrum. This suggests that the gas in the CPD has an elevated C/O. We also identified spatially extended H 2 emission around the planet, tracing warm gas, with indications that it may be at a higher temperature than the non-extended component. Conclusions. The mid-infrared spectrum of the planetary-mass companion Delorme 1 AB b reveals the first detection of bright C-bearing species in a CPD together with an outflow traced by H 2 extended emission, which could be interpreted as a disk wind. The hot dust continuum emission suggests an inner cavity in the CPD. The presence of warm gas in the CPD provides constraints on the disk’s chemical composition and physical conditions, opening up new avenues for disk studies. The study of these long-lived “Peter Pan” disks will enhance our understanding of how accretion persists in evolved low-mass systems and shed light on their formation, longevity, and evolutionary pathways in planetary systems.

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