High-Precision Atmospheric Constraints for a Cool T Dwarf from JWST Spectroscopy

We present observations of the T8 dwarf 2MASS 0415-0935 with JWST's NIRSpec spectrograph using the G395H grating ($\sim$ 2.87 - 5.14 $\mu$m). We perform the first atmospheric retrieval analysis at the maximum spectral resolution of NIRSpec (R$\sim$2700) and combine the spectrum with previous observations to study the 0.9-20 $\mu$m spectral energy distribution. We obtain precise constraints on chemical abundances ($\sim$0.02 dex) for a number of species which complicate our understanding of disequilibrium chemistry, particularly for CO$_{2}$ and PH$_{3}$. Furthermore, we measure a $^{12}$CO/$^{13}$CO ratio of $\sim 97^{+9}_{-8}$, making 2MASS 0415-0935 the coldest ($\sim 760$ K) substellar object outside of our solar system with a measured $^{12}$CO/$^{13}$CO ratio. This work shows promise for similar observations with JWST to provide precise abundances of major chemical species as well as isotopologues, allowing for new tests of our understanding of the formation and atmospheres of substellar objects.