Brown Dwarfs

The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres

By Keith Cowing
Status Report
October 1, 2023
Filed under , , , , ,
The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres
The black line is the observed spectrum of WISE 035934.06-540154.6 from Beiler et al. (2023). Black points represent their photometric measurements. The pink line and dots are the best fit Sonora model spectrum and photometry, from Beiler et al. The blue line and data points are the spectrum and photometry from the best fit model of the ATMO2020++ grid. The green line is the same model where PH3 has been removed from the atmospheric composition. The ATMO2020++ model is scaled by the known distance to WISE 0359 (Kirkpatrick et al. 2021) and the radius inferred from evolutionary models (Marley et al. 2021), and has not been scaled to fit. Principal opacity species are indicated. The lower panel shows the difference between the observed spectrum and the spectra generated by the three models (note the difference in y-axis scale in the upper and lower panels). The inset shows the near-infrared flux region. See Section 3 for discussion. — astro-ph.SR

The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest known brown dwarfs, with effective temperatures T_eff <= 475 K.

The first published observations provide important information: not only is the atmospheric chemistry out of equilibrium, as previously known, but the pressure-temperature profile is not in the standard adiabatic form. The rapid rotation of these Jupiter-size, isolated, brown dwarfs dominates the atmospheric dynamics, and thermal and compositional changes disrupt convection.

These processes produce a colder lower atmosphere, and a warmer upper atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021) presented empirical models where the pressure-temperature profile was adjusted so that synthetic spectra reproduced the 1 <= lambda um <= 20 spectral energy distributions of brown dwarfs with 260 <= T_eff K <= 540.

We show that spectra generated by these models fit the first JWST Y dwarf spectrum better than standard-adiabat models. Unexpectedly, there is no 4.3 um PH_3 feature in the JWST spectrum and atmospheres without phosphorus better reproduce the 4 um flux peak.

Our analysis of new JWST photometry indicates that the recently discovered faint secondary of the WISE J033605.05-014350AB system (Calissendorff et al. 2023) has T_eff = 295 K, making it the first dwarf in the significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is publicly available. Photometric color transformations are provided in an Appendix.

S. K. Leggett, Pascal Tremblin

Comments: Accepted for publication in ApJ on 25 September 2023
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2309.14567 [astro-ph.SR] (or arXiv:2309.14567v1 [astro-ph.SR] for this version)
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Submission history
From: Sandy Leggett
[v1] Mon, 25 Sep 2023 22:40:11 UTC (730 KB)
Astrobiology, Astrochemistry

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