HCN and C<sub>2</sub>H<sub>2</sub> in the Atmosphere of a T8.5+T9 Brown Dwarf Binary

Abstract T-type brown dwarfs present an opportunity to explore atmospheres teeming with molecules such as H 2 O, CH 4 , and NH 3 , which exhibit a wealth of absorption features in the mid-infrared. With JWST, we can finally explore this chemistry in detail, including for the coldest brown dwarfs that were not yet discovered in the Spitzer era. This allows precise derivations of the molecular abundances, which in turn inform our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets. This study presents the first JWST/MRS mid-IR spectrum ( R ∼ 1500–3000) of a T dwarf: the T8.5+T9 brown dwarf binary WISE J045853.90+643451.9. We fit the spectrum using a parameterized P – T profile and free molecular abundances (i.e., a retrieval analysis), treating the binary as unresolved. We find a good fit with a cloud-free atmosphere and identify H 2 O, CH 4 , and NH 3 features. Moreover, we make the first detections of HCN and C 2 H 2 (at 13.4 σ and 9.5 σ respectively) in any brown dwarf atmosphere. The detection of HCN suggests intense vertical mixing ( K zz ∼ 10 11 cm 2 s −1 ), challenging previous literature derivations of K zz values for T-type brown dwarfs. Even more surprising is the C 2 H 2 detection, which cannot be explained with existing atmospheric models for isolated objects. This result challenges model assumptions about vertical mixing and/or our understanding of the C 2 H 2 chemical network, or might hint towards more complex atmospheric processes such as magnetic fields driving aurorae or lightning driving ionization. These findings open a new frontier in studying carbon chemistry within brown dwarf atmospheres.

Authors

Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-03-07
DOI
https://doi.org/10.3847/2041-8213/adb4ec
Citations
6
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
6.93

Funders

Controls
|||
ALL TIME
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
article

HCN and C2H2 in the Atmosphere of a T8.5+T9 Brown Dwarf Binary

M. Guêdel, Kevin Zahnle, Adrian M. Glauser, Ioannis Argyriou et al.
6 citations
The Astrophysical Journal Letters
Stellar, planetary, and galactic studies
6.93
article

HCN and C2H2 in the Atmosphere of a T8.5+T9 Brown Dwarf Binary

M. Guêdel, Kevin Zahnle, Adrian M. Glauser, Ioannis Argyriou, Niall Whiteford, Rens Waters, Göran Östlin, E. F. van Dishoeck, T. P. Ray, Olivier Absil, Shang‐Min Tsai, J. P. Pye, D. Barrado, Polychronis Patapis, M. Samland, P. Mollière, Daniel Rouan, G. Olofsson, Thomas Henning, Elisabeth C. Matthews, P. Royer, B. Vandenbussche, Pierre-Olivier Lagage, H. Kühnle, A. Coulais
article en
6 citations

Abstract

Abstract T-type brown dwarfs present an opportunity to explore atmospheres teeming with molecules such as H 2 O, CH 4 , and NH 3 , which exhibit a wealth of absorption features in the mid-infrared. With JWST, we can finally explore this chemistry in detail, including for the coldest brown dwarfs that were not yet discovered in the Spitzer era. This allows precise derivations of the molecular abundances, which in turn inform our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets. This study presents the first JWST/MRS mid-IR spectrum ( R ∼ 1500–3000) of a T dwarf: the T8.5+T9 brown dwarf binary WISE J045853.90+643451.9. We fit the spectrum using a parameterized P – T profile and free molecular abundances (i.e., a retrieval analysis), treating the binary as unresolved. We find a good fit with a cloud-free atmosphere and identify H 2 O, CH 4 , and NH 3 features. Moreover, we make the first detections of HCN and C 2 H 2 (at 13.4 σ and 9.5 σ respectively) in any brown dwarf atmosphere. The detection of HCN suggests intense vertical mixing ( K zz ∼ 10 11 cm 2 s −1 ), challenging previous literature derivations of K zz values for T-type brown dwarfs. Even more surprising is the C 2 H 2 detection, which cannot be explained with existing atmospheric models for isolated objects. This result challenges model assumptions about vertical mixing and/or our understanding of the C 2 H 2 chemical network, or might hint towards more complex atmospheric processes such as magnetic fields driving aurorae or lightning driving ionization. These findings open a new frontier in studying carbon chemistry within brown dwarf atmospheres.

The Astrophysical Journal LettersVol. 981(2)
National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, European Space Agency, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Swedish National Space Agency, Fonds De La Recherche Scientifique - FNRS, Belgian Federal Science Policy Office, Centre National d’Etudes Spatiales, Nuclear Safety and Security Commission, Knut och Alice Wallenbergs Stiftelse, Science and Technology Facilities Council, Agencia Estatal de Investigación
Openalex Percentile: Top 4%
Stellar, planetary, and galactic studies
6.93
DOIOpen Access PDFOpenAlex
AI Navigator

Ask Laika to Summarize, Analyze, and Connect papers live on the map.

Summarize Papers & Methodologies

Extract key findings, datasets, and comparative methods across publications.

Benchmark Rankings & Visual Analytics

Rank top research institutions, authors, funders, topics, and journals by Field-Weighted Citation Impact (FWCI) and paper volume with instant charts.

Connect Distant Disciplines

Bridge topological clusters on the map to find hidden collaborative intersections.