The First JWST Spectral Energy Distribution of a Y dwarf

Abstract

We present the first JWST spectral energy distribution of a Y dwarf. This spectral energy distribution of the Y0 dwarf WISE J035934.06$-$540154.6 consists of low-resolution ($\hat{I}»$/$\hat{I}”\hat{I}»$ $\sim$ 100) spectroscopy from 1$-$12 $\hat{I}¼$m and three photometric points at 15, 18, and 21 $\hat{I}¼$m. The spectrum exhibits numerous fundamental, overtone, and combination rotational-vibrational bands of H${}_2$O, CH${}_4$, CO, CO${}_2$, and NH${}_3$, including the previously unidentified $\hat{I}{½}_3$ band of NH${}_3$ at 3 $\hat{I}¼$m. Using a Rayleigh-Jeans tail to account for the flux emerging at wavelengths greater than 21 $\hat{I}¼$m, we measure a bolometric luminosity of $1.523\pm 0.090\times 10^{20}$ W. We determine a semi-empirical effective temperature estimate of $467_{-18}^{+16}$ K using the bolometric luminosity and evolutionary models to estimate a radius. Finally, we compare the spectrum and photometry to a grid of atmospheric models and find reasonably good agreement with a model having $T_{\mathrm{eff}}$=450 K, log $g$=3.25 [cm s${}^{-2}$], [M/H]=$-0.3$. However, the low surface gravity implies an extremely low mass of 1 $M_{\mathrm{Jup}}$ and a very young age of 20 Myr, the latter of which is inconsistent with simulations of volume-limited samples of cool brown dwarfs.