TY - JOUR
T1 - The Diversity of Cold Worlds
T2 - Age and Characterization of the Exoplanet COCONUTS-2b
AU - Kiman, Rocio
AU - Beichman, Charles A.
AU - Diaz, Azul Ruiz
AU - Faherty, Jacqueline K.
AU - Lacy, Brianna
AU - Suarez, Genaro
AU - Marocco, Federico
AU - Kirkpatrick, J. Davy
AU - Gagne, Jonathan
AU - Copeland, Jessica
AU - Burningham, Ben
AU - Whiteford, Niall
AU - Rowland, Melanie J.
AU - Gagliuffi, Daniella C. Bardalez
AU - Vos, Johanna M.
AU - Schneider, Adam C.
AU - Gonzales, Eileen C.
AU - Merchan, Sherelyn Alejandro
AU - Rothermich, Austin
AU - Smart, Richard
AU - Costa, Edgardo
AU - Mendez, Rene A.
N1 - 22 pages, 11 figures, 4 tables. Accepted for publication The Astronomical Journal
PY - 2026/1/2
Y1 - 2026/1/2
N2 - 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.
AB - 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.
KW - astro-ph.EP
KW - astro-ph.SR
U2 - 10.3847/1538-3881/ae230f
DO - 10.3847/1538-3881/ae230f
M3 - Article
SN - 0004-6256
JO - The Astronomical Journal
JF - The Astronomical Journal
ER -