MINDS

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

¹ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
³ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
⁶ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁷ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁸ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁹ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, A-1180 Vienna, Austria
¹⁰ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France
¹² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹³ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
¹⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹⁵ Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington DC 20015, USA
¹⁶ Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
¹⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042 Graz, Austria
¹⁸ TU Graz, Fakultät für Mathematik, Physik und Geodäsie, Petersgasse 16, 8010 Graz, Austria
¹⁹ Niels Bohr Institute, University of Copenhagen, NBB BA2, Jagtvej 155A, 2200 Copenhagen, Denmark
²⁰ Department of Astronomy, Oskar Klein Centre; Stockholm University;, SE-106 91 Stockholm, Sweden

^⋆ Corresponding author: mariamc@cab.inta-csic.es

Received: 21 May 2025
Accepted: 28 July 2025

Abstract

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.