SPHEREx Ultracool Dwarf spectrophotometric Atlas (SUDA): Atmospheric and Fundamental Parameters of Ultracool Dwarfs

We present the SPHEREx Ultracool Dwarf spectrophotometric Atlas (SUDA), a homogeneous sample of 1675 field ultracool dwarfs with continuous 0.75--5 mu m spectrophotometry from SPHEREx Quick Release 2 (QR2). Using the SAND and ATMO2020++ atmospheric grids, we derive atmospheric parameters, compute bolometric luminosities (L bol), and combine T eff and radii with evolutionary tracks to estimate masses, ages, and evolutionary g. For sources at 1700--2500 K, spectroscopic g is systematically lower than evolutionary g, with a median offset of about 1.05 dex, likely driven primarily by cloud-related model uncertainties. We further construct an empirical atlas by binning the measurements in T eff and evolutionary g, producing 57 spectrophotometric templates spanning T eff 700 --3000 K. Molecular indices trace a coherent atmospheric sequence: H 2 O and CH 4 indices strengthen toward lower T eff, while CO and CO 2 indices increase below about 1500 K and turn over near about 1000 K. Model comparisons indicate that CO 2 index is more sensitive to metallicity than gravity at T eff about800 --1200 K. SUDA provides a reference sample linking 0.75--5 mu m spectrophotometric morphology to atmospheric and evolutionary trends in ultracool dwarfs.