The Department of Astronomy 2024-2025 Colloquium Series Presents:

"Spatial Variations in Atmospheric Chemistry of the Coldest Brown Dwarf"

For two decades astronomers have been measuring weather on other worlds with the goal of understanding what atmospheric phenomena drive time-dependent brightness variations in brown dwarfs and gas giant exoplanets. Previous weather studies have been limited to broadband photometry or low resolution (R ∼100) spectroscopy. In the era of JWST, precise time-resolved medium-resolution spectroscopy of the coldest brown dwarfs is finally possible, allowing the effects of chemistry, temperature, and condensates to be disentangled. WISE 0855 (280K) is the coldest known brown dwarf and the best analog for studying processes that also occur on gas giant planets within our Solar System. We present high SNR (80 – 100), medium resolution (R ∼1000), time-series JWST/NIRSpec spectra of WISE 0855. Our observations span 11 hours with 15-minute pointings covering 2.87–5.27 microns. The dominant time-variable feature is carbon monoxide, with smaller amplitude changes from carbon dioxide and phosphine. Wavelengths impacted by methane, water vapor, and ammonia show relatively less variability. Outside of major molecular features, there are variations that may be interpreted as changes in deeper atmospheric heat. Using atmospheric and structural models, we investigate the potential impact of water clouds and convection on our observations. Lastly, I will discuss how these observations tie into the overall picture of this cold world and necessary steps for interpreting other time-series data sets.