Brown Dwarfs

Water Depletion and 15NH3 in the Atmosphere of the Coldest Brown Dwarf Observed With JWST/MIRI

By Keith Cowing

Status Report

astro-ph.EP

October 17, 2024

Filed under astro-ph.EP, astrochemistry, atmosphere, brown dwarf, Clouds, imaging, JWST, JWST-MIRI/MRS, metallicity, NIRSpec, petitRADTRANS, Spectroscopy, Water, WISE0855

Water Depletion and 15NH3 in the Atmosphere of the Coldest Brown Dwarf Observed With JWST/MIRI — The MIRI/MRS (this work), NIRspec/PRISM and NIRSpec/G395M Luhman et al. (2024) spectrum of WISE 0855 from 0.8 to 22 µm. Panel a) shows the NIRspec/PRISM data in red and panel from 0.8 to 2.2µm b) the NIRspec/G395M in green and MIRI/MRS data in blue from 2.2 to 22 µm. Insets c) and d) show the overlap between the three data sets from 3.5 to 5.4 µm and the ammonia feature from 8.0 to 13.0 µm, respectively. The most dominant absorbing species are presented above the spectrum in various colors. For better visualization he data was binned to a resolution of R = 1000 for MIRI/MRS and NIRSpec/G395M and R = 100 for NIRSpec/PRISM. — astro-ph.EP

With a temperature of ∼285 K WISE0855 is the coldest brown dwarf observed so far. Using the James Webb Space Telescope (JWST) we obtained observations that allow us to characterize WISE0855s atmosphere focusing on vertical variation in the water steam abundance, measuring trace gas abundances and receiving bulk parameters for this cold object.

We observed the ultra cool dwarf WISE0855 using the Mid-Infrared Instrument Medium Resolution Spectrometer (MIRI/MRS) onboard 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 um.

We apply atmospheric retrievals using petitRADTRANS to measure atmospheric abundances, the pressure-temperature structure, radius and gravity of the brown dwarf. We also employ publicly available clear and cloudy self-consistent grid models to estimate bulk properties of the atmosphere such as the effective temperature, radius, gravity and metallicity. Atmospheric retrievals constrain a variable water abundance profile in the atmosphere, as predicted by equilibrium chemistry. We detect the 15NH3 isotopologue and infer a ratio of mass fraction of ¹⁴NH₃/¹⁵NH₃ = 332+63-43 for the clear retrieval.

We measure the bolometric luminosity by integrating the presented spectrum and obtain a value of log(L/L_⊙) = -7.291+/-0.008. The detected water depletion indicates that water condenses out in the upper atmosphere due to the very low effective temperature of WISE0855.

The height in the atmosphere where this occurs is covered by the MIRI/MRS data, and thus demonstrates the potential of MIRI to characterize cold gas giants atmospheres. Comparing the data to retrievals and self-consistent grid models, we do not detect signs for water ice clouds, although their spectral features have been predicted in previous studies.

H. Kühnle, P. Patapis, P. Mollière, P. Tremblin, E. Matthews, A. M. Glauser, N. Whiteford, M. Vasist, O. Absil, D. Barrado, M. Min, P.-O. Lagage, L. B. F. M. Waters, M. Guedel, Th. Henning, B. Vandenbussche, P. Baudoz, L. Decin, J. P. Pye, P. Royer, E. F. van Dishoeck, G. Östlin, T. P. Ray, G. Wright

Comments: Submitted to A&A, 29 pages, 21 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2410.10933 [astro-ph.EP] (or arXiv:2410.10933v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2410.10933
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Submission history
From: Helena Kuehnle
[v1] Mon, 14 Oct 2024 17:50:51 UTC (4,946 KB)
https://arxiv.org/abs/2410.10933

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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