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

Abstract

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 ($L_{\rm bol}$), and combine $T_{\mathrm{eff}}$ and radii with evolutionary tracks to estimate masses, ages, and evolutionary $\log g$. For sources at 1700--2500~K, spectroscopic $\log g$ is systematically lower than evolutionary $\log g$, with a median offset of $\sim$1.05~dex, likely driven primarily by cloud-related model uncertainties. We further construct an empirical atlas by binning the measurements in $T_{\mathrm{eff}}$ and evolutionary $\log g$, producing 57 spectrophotometric templates spanning $T_{\mathrm{eff}}\simeq700$--3000~K. Molecular indices trace a coherent atmospheric sequence: H$_2$O and CH$_4$ indices strengthen toward lower $T_{\mathrm{eff}}$, while CO and CO$_2$ indices increase below $\sim$1500~K and turn over near $\sim$1000~K. Model comparisons indicate that CO$_2$ index is more sensitive to metallicity than gravity at $T_{\mathrm{eff}}\sim800$--1200~K. SUDA provides a reference sample linking 0.75--5~$\mu$m spectrophotometric morphology to atmospheric and evolutionary trends in ultracool dwarfs.