Brown Dwarfs

Redshifting the Study of Cold Brown Dwarfs and Exoplanets: the Mid-Infrared Wavelength Region as an Indicator of Surface Gravity and Mass

By Keith Cowing

Status Report

astro-ph.SR

November 18, 2024

Filed under astro-ph.SR, ATMO 2020++, atmosphere, brown dwarf, COCONUTS-2b, exoplanet, imaging, JWST, Spectroscopy, surface gravity, WISEPA J182831.08+265037.8, y dwarf

Redshifting the Study of Cold Brown Dwarfs and Exoplanets: the Mid-Infrared Wavelength Region as an Indicator of Surface Gravity and Mass — The top panel shows the synthetic spectrum for a Y dwarf at 10 pc with the atmospheric parameters given in the legend. WISE and JWST filter bandpasses are illustrated as red and yellow lines respectively. The middle and lower panels show the ratio of this spectrum divided by a spectrum where the surface gravity has been reduced, and by a spectrum where the metallicity is reduced, respectively. The total energy emitted by the lower gravity dwarf is greater because the radius is larger (Marley et al. 2021), hence the average value in the middle panel is < 1. Note that all three models have similar flux levels averaged over the λ ≈ 4.5 µm flux peak, for example through an imaging filter, although the strength of the absorption features at these wavelengths (primarily due to CO and CO2) are sensitive to gravity. See Section 3 for further discussion. -- astro-ph.SR

JWST is opening many avenues for exploration. For cold brown dwarfs and exoplanets, JWST has opened the door to the mid-infrared wavelength region, where such objects emit significant energy.

For the first time, astronomers have access to mid-infrared spectroscopy for objects colder than 600 K. The first spectra appear to validate the model suite known as ATMO 2020++: atmospheres which include disequilibrium chemistry and have a non-adiabatic pressure-temperature relationship. Preliminary fits to JWST spectroscopy of Y dwarfs show that the slope of the energy distribution from lambda = 4.5 um to lambda = 10 um is very sensitive to gravity.

We explore this phenomenon using PH3-free ATMO 2020++ models and updated WISE W2 – W3 colors. We find that an absolute 4.5 um flux measurement constrains temperature, and the ratio of the 4.5 um flux to the 10 – 15 um flux is sensitive to gravity and less sensitive to metallicity.

We identify 10 T dwarfs with red W2 – W3 colors which are likely to be very low gravity, young, few-Jupiter-mass objects; one of these is the previously known COCONUTS-2b. The unusual Y dwarf WISEPA J182831.08+265037.8 is blue in W2 – W3 and we find that the 4 to 18 um JWST spectrum is well reproduced if the system is a pair of high gravity 400 K dwarfs.

Recently published JWST colors and luminosity-based effective temperatures for late-T and Y dwarfs further corroborate the ATMO 2020++ models, demonstrating the potential for significant improvement in our understanding of cold very low-mass bodies in the solar neighborhood.

S. K. Leggett, Pascal Tremblin

Comments: Accepted for publication in ApJ, 4 November 2024
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2411.03549 [astro-ph.SR] (or arXiv:2411.03549v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2411.03549
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Submission history
From: Sandy Leggett
[v1] Tue, 5 Nov 2024 23:06:53 UTC (666 KB)
https://arxiv.org/abs/2411.03549

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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