The Diversity of Cold Worlds: Age and Characterization of the Exoplanet COCONUTS-2b

TL;DR

This work analyzes COCONUTS-2b, a cold planetary-mass companion in a wide binary with an M3 star, to benchmark substellar atmospheres at $T_{ m eff}\approx 493\,\mathrm{K}$ and $M\approx 7.5\,M_{\rm Jup}$. It combines JWST/NIRSpec G395H spectroscopy (2.87–5.13 μm) and MIRI photometry with Gemini FLAMINGOS-2 data and updated astrometry to establish CUMA membership and an age of $414\pm23$ Myr, while constraining metallicity and a near-solar C/O ratio. Forward-modeling with multiple atmospheric grids (notably Sonora Elf Owl) yields a best-fit, sub-solar metallicity and C/O near solar, though degeneracies with $T_{ m eff}$, $\log g$, and $K_{zz}$ persist; an unconstrained fit suggests $T_{ m eff} \approx 5.17\times10^{2}$ K and $\log g \approx 3.25$ dex, illustrating parameter degeneracies in cold atmospheres. The results imply a star-like formation pathway for COCONUTS-2b and highlight the value of benchmark brown dwarfs in decoding atmospheric chemistry and vertical mixing, especially when contrasted with the lower-mass, colder Y-dwarf 0825+2805 within the same moving group.

Abstract

Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2, is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra (R~2700, 2.87-5.13 um) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major (414+/-23 Myr) using BANYAN Sigma and its full kinematics. We also re-estimate the astrometry of COCONUTS-2b using the MIRI data. We support the membership with a comparison of rotation period, metallicity and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2b is a planetary mass object (7.5+/-0.4 MJup) which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7+/-0.2 MJup). We identify absorption feature differences which could indicate that 0825+2805 has stronger vertical mixing.

Figures (11)

• Figure 1: Top panel shows the JWST spectrum of COCONUTS-2b in the filter G395H with horizontal lines indicating the molecular features which are recognizable in the spectrum. We also included the uncertainty in the flux in light-gray. Bottom panel shows the opacities for each identified molecule which we obtained from the DACE database Barber2006Rothman2010Yurchenko2011Azzam2016Hargreaves2020Grimm2021Tennyson2024. See Section \ref{['sec:atmosphere']} for a detailed description and discussion.
• Figure 2: Astrometric and photometric plots for the members of CUMA. The top row shows right ascension (ra), declination (dec), the Gaia color-magnitude diagram, proper motion ra ($\mu _{\rm ra}$), proper motion dec ($\mu _{\rm dec}$), radial velocity (RV) and parallax ($\pi$). The bottom row shows the 3D galactic positions (X, Y and Z) and the 3D galactic velocities ($V_{\rm X}$, $V_{\rm Y}$ and $V_{\rm Z}$). We show in light-purple the candidate members (CUMA C) and in purple the high-likelihood members (CUMA HM). We obtained this classification from the MOCA database. We also include in orange the members of the UMA cluster from Capistrant2024. In a yellow star we show COCONUTS-2A, and in green COCONUTS-2b. We find that the components of COCONUTS-2 agree well with the cluster in 3D velocity and position, while staying closer to each other, confirming that they are a binary.
• Figure 3: Rotation period in days as a function of the Gaia$({G_\mathrm{BP}}-{G_\mathrm{RP}})$ color for CUMA. The candidates and high-likelihood members are shown in light-purple points. We included the members of the UMA cluster from Capistrant2024 in orange. In addition we show extra known young clusters with measured rotation periods, color-coded by age. We included the clusters compiled by Curtis2020: Pleiades Rebull2016, Praesepe Douglas2017Douglas2019, NGC 6811 Curtis2019b, NGC 752 Agueros2018, NGC 6819 Meibom2015 and Ruprecht 147 Curtis2020. We also included COCONUTS-2A as a yellow five-point star, which has a measured rotation period. We found that the members of CUMA agree with the rotation period sequence of UMA, confirming that the two groups have the same age. We also found that the rotation period of COCONUTS-2A agrees with the locus of the CUMA group, providing further support for its membership in the group.
• Figure 4: [Fe/H] and [M/H] measurements for the candidate members of CUMA collected from the catalogs APOGEE-DR17, GALAH-DR3, RAVE-DR6 and LAMOST DR7. We included in a black horizontal line, the value of [Fe/H] for the UMA cluster Boesgaard1988. We also included as a vertical line, the value of [Fe/H] for COCONUTS-2A and [M/H] for COCONUTS-2b (See Section \ref{['subsection:forwardmodeling']}). The dotted vertical lines show the uncertainty in these two values. These measurements agree with the distribution of the moving group within uncertainty, supporting the membership of COCONUTS-2 to the group.
• Figure 5: [Fe/H] and [M/H] measurements for the candidate members of CUMA collected from the catalogs APOGEE-DR17, GALAH-DR3, RAVE-DR6 and LAMOST DR7. In the left panel we show the metallicity as a function of the probability of belonging to the CUMA group, and in the right panel is the distribution of values. We included the value of [Fe/H] for COCONUTS-2A and [M/H] for COCONUTS-2b as five-point stars in the left panel and as horizontal lines in the right panel. The dotted horizontal lines show the uncertainty in these two values. We also included in a black horizontal line, the value of [Fe/H] for the UMA cluster Boesgaard1988.