The JWST Weather Report from the Nearest Brown Dwarfs I: multi-period JWST NIRSpec + MIRI monitoring of the benchmark binary brown dwarf WISE 1049AB

We report results from 8 hours of JWST/MIRI LRS spectroscopic monitoring directly followed by 7 hours of JWST/NIRSpec prism spectroscopic monitoring of the benchmark binary brown dwarf WISE 1049AB, the closest, brightest brown dwarfs known. We find water, methane, and CO absorption features in both components, including the 3.3 μm methane absorption feature and a tentative detection of small grain (< 1μm) silicate absorption at >8.5 μm in WISE 1049A. Both components vary significantly (>1%), with WISE 1049B displaying larger variations than WISE 1049A. Using K-means clustering, we find three main transition points in wavelength for both components of the binary: 1) change in behavior at ∼2.3 μm coincident with a CO absorption bandhead, 2) change in behavior at 4.2 μm, close to the CO fundamental band at λ > 4.4 μm, and 3) change in behavior at 8.3-8.5 μm, potentially corresponding to silicate absorption. We interpret the lightcurves observed with both NIRSpec and MIRI as likely stemming from 1) a deep pressure level driving the double-peaked variability seen in WISE 1049B at wavelengths <2.3 μm and >8.5 μm, 2) an intermediate pressure level shaping the lightcurve morphology between 2.3 and 4.2 μm, and 3) a higher-altitude pressure level producing single-peaked and plateaued lightcurve behavior between 4.2 and 8.5 μm.