2026-04-23T18:23:19Zivo://CDS.VizieR/J/ApJ/958/942026-02-26T00:56:23ZJ.ApJIVOAKeck/NIRES spectrum for CWISE J105512.11+544328.3Y dwarfs, the coolest known spectral class of brown dwarfs, overlap in mass and temperature with giant exoplanets, providing unique laboratories for studying low-temperature atmospheres. However, only a fraction of Y dwarf candidates have been spectroscopically confirmed. We present Keck/NIRES near-infrared spectroscopy of the nearby (d~6-8pc) brown dwarf CWISE J105512.11+544328.3. Although its near-infrared spectrum aligns best with the Y0 standard in the J band, no standard matches well across the full YJHK wavelength range. The CWISE J105512.11+544328.3 NH3-H=0.427+/-0.0012 and CH4-J=0.0385+/-0.0007 absorption indices and absolute Spitzer [4.5] magnitude of 15.18+/-0.22 are also indicative of an early-Y dwarf rather than a late-T dwarf. CWISE J105512.11+544328.3 additionally exhibits the bluest Spitzer [3.6]-[4.5] color among all spectroscopically confirmed Y dwarfs. Despite this anomalously blue Spitzer color given its low luminosity, CWISE J105512.11+544328.3 does not show other clear kinematic or spectral indications of low metallicity. Atmospheric model comparisons yield a log(g)<=4.5 and Teff~500+/-150K for this source. We classify CWISE J105512.11+544328.3 as a Y0 (pec) dwarf, adding to the remarkable diversity of the Y-type population. JWST spectroscopy would be crucial to understanding the origin of this Y dwarf's unusual preference for low-gravity models and blue 3-5{mu}m color.y-dwarfsbrown-dwarfsinfrared-astronomyspectroscopystellar-astronomyobservational-astronomyinterdisciplinary-astronomyhttps://dc.g-vo.org/rr/q/lp/custom/CDS.VizieR/J/ApJ/958/94https://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJ/958/942023ApJ...958...94Rhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/ApJ/958/94Robbins G.Meisner A.M.Schneider A.C.Burgasser A.J.,Kirkpatrick J.D.Gagne J.Hsu C.-C.Moranta L.Casewell S.,Marocco F.Gerasimov R.Faherty J.K.Kuchner M.J.Caselden D.,Cushing M.C.Alejandro S.The Backyard Worlds: Planet 9 Collaboration,The Backyard Worlds: Cool Neighbors CollaborationCDS2026https://cds.unistra.fr/vizier-org/licences_vizier.htmlCDS support team <cds-question@unistra.fr>DatasetAstroObjectsastronomy