MINDS: Cha Hα 1, a brown dwarf with a hydrocarbon-rich disk

Context. The chemistry of disks around brown dwarfs (BDs) remains largely unexplored due to their faintness. Despite the efforts performed with Spitzer, we have far less understanding of planet formation, chemical composition, disk structure, and evolution in disks around BDs compared to their more massive counterparts (T Tauri and Herbig Ae/Be stars), which are more readily studied due to their greater brightness. Recent JWST observations, with up to an order of magnitude improvement in both spectral and spatial resolution, have shown that these systems are chemically rich, offering valuable insights into giant planet formation. Aims. As part of the MIRI mid-INfrared Disk Survey (MINDS) JWST guaranteed time program, we aim to characterize the gas and dust composition of the disk around the brown dwarf [NC98] Cha HA 1, hereafter Cha Hα 1, in the mid-infrared. Methods. We obtained data from the MIRI Medium Resolution Spectrometer (MRS) from 4.9 to 28 μm (R ∼ 1500–3500; FWHM ∼ 0.2″–1.2″). We used the dust fitting tool DuCK to investigate the dust composition and grain sizes, while we identified and fit molecular emission in the spectrum using slab models. Results. Compared with disks around very low mass stars, clear silicate emission features are seen in this BD disk. In addition, JWST reveals a plethora of hydrocarbons, including C₂H₂ ¹³CCH₂, CH₃, CH₄, C₂H₄, C₄H₂, C₃H₄, C₂H₆, and C₆H₆ which suggest a disk with a gas C/O > 1. Additionally, we detected CO₂ ¹³CO₂, HCN, H₂, and H₂O. Notably, CO and OH are absent from the spectrum. The dust is dominated by large ∼4 μm size amorphous silicates (MgSiO₃). We inferred a small dust mass fraction (> 10%) of 5 μm size crystalline forsterite. We did not detect any polycyclic aromatic hydrocarbons. Conclusions. The mid-infrared spectrum of Cha Hα 1 shows the most diverse chemistry seen to date in a BD protoplanetary disk, consisting of a strong dust feature, 12 carbon-bearing molecules plus H₂, and water. The diverse molecular environment offers a unique opportunity to test our understanding of BD disk chemistry and how it affects the possible planets forming in them.