Disequilibrium Chemistry, Diabatic Thermal Structure, and Clouds in the Atmosphere of COCONUTS-2b
Located 10.888 pc from Earth, COCONUTS-2b is a planetary-mass companion to a young (150–800 Myr) M3 star, with a wide orbital separation (6471 au) and a low companion-to-host mass ratio (0.021±0.005). We have studied the atmospheric properties of COCONUTS-2b using newly acquired 1.0–2.5~μm spectroscopy from Gemini/Flamingos-2.
The spectral type of COCONUTS-2b is refined to T9.5±0.5 based on comparisons with T/Y dwarf spectral templates. We have conducted an extensive forward-modeling analysis, comparing the near-infrared spectrum and mid-infrared broadband photometry with sixteen state-of-the-art atmospheric model grids developed for brown dwarfs and self-luminous exoplanets near the T/Y transition.
The PH3-free ATMO2020++, ATMO2020++, and Exo-REMmodels best match the specific observations of COCONUTS-2b, regardless of variations in the input spectrophotometry. This analysis suggests the presence of disequilibrium chemistry, along with a diabatic thermal structure and/or clouds, in the atmosphere of COCONUTS-2b. All models predict fainter Y-band fluxes than observed, highlighting uncertainties in the alkali chemistry models and opacities.
We determine a bolometric luminosity of log(Lbol/L⊙)=−6.18 dex, with a 0.5 dex-wide range of [−6.43,−5.93] dex that accounts for various assumptions of models. Using thermal evolution models, we derive an effective temperature of Teff=483+44−53 K, a surface gravity of log(g)=4.19+0.18−0.13 dex, a radius of R=1.11+0.03−0.04 RJup, and a mass of M=8±2 MJup. Various atmospheric model grids consistently indicate that COCONUTS-2b’s atmosphere has sub- or near-solar metallicity and C/O.
These findings provide valuable insights into COCONUTS-2b’s formation history and the potential outward migration to its current wide orbit.
Masses (top) or mass ratios (bottom) of confirmed exoplanets (collected from the NASA Exoplanet Archive on June 21, 2024) as functions of their orbital separations (grey), with those having observed transmission and emission spectra highlighted in orange and blue, respectively. Substellar binaries with dynamical masses are shown in green (Dupuy et al. 2016, 2019, 2023; Dupuy & Liu 2017; Lazorenko & Sahlmann 2018; Chabrier et al. 2023). Solar system planets are plotted with sizes unscaled. Brown dashed lines highlight the planet mass of 13 MJup and the planet-to-host mass ratio of 0.04, as adopted by the IAU working definition of exoplanets. — astro-ph.EP
Top: The 1–25 µm SED of COCONUTS-2b, including our new Gemini/F2 spectrum (blue) and the WISE (green) and Spitzer (purple) photometry. This object’s W1 and W2 photometry are from CatWISE, with proper motion accounted for; the W4 photometry is an upper limit. The spectral flux uncertainties are shown as light shades. Bottom: The Gemini/F2 spectrum with key spectral features labeled. Fluxes in grey shades are masked for the subsequent forward-modeling analysis in Section 4. — astro-ph.EP
Zhoujian Zhang, Sagnick Mukherjee, Michael C. Liu, Jonathan J. Fortney, Emily Mader, William M. J. Best, Trent J. Dupuy, Sandy K. Leggett, Theodora Karalidi, Michael R. Line, Mark S. Marley, Caroline V. Morley, Mark W. Phillips, Robert J. Siverd, Joseph A. Zalesky
Comments: Re-submitted to AJ. Main text: Pages 1-25, Figures 1-11, Tables 1-4; Appendix: Pages 26-43, Figures 12-15
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2410.10939 [astro-ph.EP] (or arXiv:2410.10939v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2410.10939
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From: Zhoujian Zhang
[v1] Mon, 14 Oct 2024 18:00:00 UTC (11,694 KB)
https://arxiv.org/abs/2410.10939
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