While great progress has been made over the last 20 years in determining the fundamental properties of brown dwarfs, the coolest brown dwarfs with Teff < 500 K have proven difficult to characterize because of their intrinsic faintness and the fact that their spectral energy distributions peak at mid-infrared wavelengths. In this talk, I will report on a Cycle 1 James Webb Space Telescope program to determine the bolometric luminosities, effective temperatures, and compositions of 22 cool brown dwarfs using 1-22 micron spectral energy distributions obtained with NIRSpec and MIRI. We find bolometric luminosities (log L/Lsun) ranging from -5.1 to -6.8 and effective temperatures from 1000 to 350 K (with uncertainties of +-20 K). Using atmospheric retrieval techniques, we constrain the atmospheric abundances of all the major carbon-, nitrogen-, and oxygen-bearing species (H_2O, CH_4, CO, CO_2, NH_3). Strong positive correlations between H_2O–CH_4 and CO–CO_2 abundances are present, which are consistent with thermochemical equilibrium expectations. Using the retrieved chemical abundances, we calculate [M/H]_atm for these objects, which are independently positively correlated with their H_2O and CH_4 abundances confirming they trace the overall bulk oxygen and carbon content, respectively. Finally, we report tentative PH_3 detections in roughly half the sample, which would more than quadruple the number of cold brown dwarfs with PH_3 detections if confirmed with future observations.

Speaker: Mike Cushing (The University of Toledo)

The 2025 Fall Colloquium talks are held on Wednesdays at 3:00 PM. This colloquium is hosted by STScI and will be held as an in-person and virtual event.

You may join in person at STScI’s John N. Bahcall Auditorium or virtually on the STScI Research YouTube channel.

Please direct questions or comments to contact above. The 2025 Fall Colloquium Committee members are: Nimisha Kumari (STScI), Elena Manjavacas (STScI), Jack Neustadt (JHU), Kevin Schlaufman (JHU), Adam Smercina (STScI), Ethan Vishniac (JHU).