Discovery of the Second Y+Y Dwarf Binary System: CWISEP J193518.59-154620.3

We present the discovery of a companion to the Y dwarf, CWISEP J193518.59–154620.3, the second Y-Y dwarf binary detected to date. Y dwarfs are the coldest known free-floating objects (<500 K) and, on average, represent the lowest mass objects directly formed through turbulent fragmentation of a molecular cloud. Studying their multiplicity allows us to place strong constraints on the ability to form multiple systems of planetary masses and approach the opacity limit of fragmentation. Due to their physical properties, Y dwarfs also serve as analogs to gas giant planets. CWISEP J193518.59–154620.3 has been shown to have a unique methane emission feature in its near-infrared spectrum at 3.326 μm, potentially indicative of auroral processes without a clear origin. CWISEP J193518.59–154620.3 was observed with JWST’s MIRI in the F1000W, F1280W, and F1800W filters. We applied a point-spread function (PSF) fitting algorithm using empirically derived PSF models and resolved a companion in the F1000W and F1280W filters separated by 172 mas and 2.48 au, assuming a distance of 14.43 pc. Using the ATMO2020 evolutionary models, we estimate a mass of 12–39 MJup for the primary and 7–24 MJup for the companion, assuming an age of 1–10 Gyr for a mass ratio of 0.55–0.62, resulting in an estimated period of 16–28 yr. It is unknown which component of this binary exhibits the methane emission feature. We also resolve known companions WISE J014656.66+423410.0B and WISE J171104.60+350036.8B using MIRI data and present their F1000W and F1280W photometry.