Water depletion and ¹⁵NH₃ in the atmosphere of the coldest brown dwarf observed with JWST/MIRI

¹ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191, Gif-sur-Yvette, France
⁴ Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA
⁵ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
⁶ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁸ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁰ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium
¹² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹³ School of Physics & Astronomy, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester LE4 5SP, UK
¹⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
¹⁶ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin, D02 XF86, Ireland
¹⁷ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

^★ Corresponding author; kuehnleh@ethz.ch

Received: 9 October 2024
Accepted: 19 January 2025

Abstract

Context. With a temperature of ∼285 K, WISEJ0855–0714 (hereafter, WISE 0855) is the coldest brown dwarf observed thus far. Studying such cold gas giants allows us to probe the atmospheric physics and chemistry of evolved objects that resemble Solar System gas giants.

Aims. Using James Webb Space Telescope (JWST), we obtained observations to characterize WISE 0855’s atmosphere, focusing on vertical variation in the water steam abundance, measuring trace gas abundances, and obtaining the bulk parameters for this cold object.

Methods. We observed the ultra-cool dwarf WISE 0855 using the Mid-Infrared Instrument Medium Resolution Spectrometer (MIRI/MRS) on board JWST at a spectral resolution of up to 3750. We combined the observation with published data from the Near-Infrared Spectrograph (NIRSpec) G395M and PRISM modes, yielding a spectrum ranging from 0.8 to 22 µm. We applied atmospheric retrievals using <mono>petitRADTRANS</mono> to measure the atmospheric abundances, pressure-temperature structure, radius, and gravity of the brown dwarf. We also employed publicly available clear and cloudy self-consistent grid models to estimate the bulk properties of the atmosphere such as the effective temperature, radius, gravity, and metallicity.

Results. Atmospheric retrievals have constrained a variable water abundance profile in the atmosphere, as predicted by equilibrium chemistry. We detected the ¹⁵NH₃ isotopolog and inferred a ratio of volume fraction of ¹⁴NH₃/¹⁵ NH₃ = 349₋₄₁⁺⁵³ for the clear retrieval. We measured the bolometric luminosity by integrating the presented spectrum, obtaining a value of log(L/L_⊙) = −7.291 ± 0.008.

Conclusions. The detected water depletion indicates that water condenses out in the upper atmosphere due to the very low effective temperature of WISE 0855. The height in the atmosphere where this occurs is covered by the MIRI/MRS data, thereby demonstrating the potential of MIRI to characterize the atmospheres of cold gas giants. After comparing the data to retrievals and self-consistent grid models, we did not detect any signs of water ice clouds, although their spectral features have been predicted in previous studies.