The First JWST Spectral Energy Distribution of a Y Dwarf

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 ( λ /Δ λ ∼100) spectroscopy from 1–12 μ m and three photometric points at 15, 18, and 21 μ 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 ν 3 band of NH 3 at 3 μ m. Using a Rayleigh–Jeans tail to account for the flux emerging at wavelengths greater than 21 μ m, we measure a bolometric luminosity of 1.523 ± 0.090 × 10 20 W. We determine a semiempirical 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 eff = 450 K, log g = 3.25 [cm s −2 ], and [M/H] = −0.3. However, the low surface gravity implies an extremely low mass of 1 M 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.