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

Abstract

We present the SPHEREx Ultracool Dwarf spectral Atlas (SUDA), a homogeneous sample of 1675 ultracool dwarfs with continuous 0.75--5 $\mu$m spectroscopy from SPHEREx QR2. Using the SAND and ATMO2020++ atmospheric model grids, we derive atmospheric parameters and calculate bolometric luminosities ($L_{\rm bol}$). We combine the inferred $T_{\mathrm{eff}}$ and radii with evolutionary tracks to estimate masses and ages for the full sample. Evolutionary surface gravities ($\log g$) are also reported for sources with parallaxes. In the 1700--2500~K range, the atmospheric $\log g$ from spectral fitting are systematically lower than the evolutionary $\log g$, with a median offset of about 1.1~dex, likely reflecting residual degeneracy between $\log g$ and metallicity in low resolution SPHEREx spectra. We also construct an empirical spectral atlas by grouping the spectra in the adopted parameter space, using the $T_{\mathrm{eff}}$ and the evolutionary $\log g$. The resulting atlas contains 52 parameter bins and spans $T_{\mathrm{eff}}\simeq 700$--$3000$~K. Molecular indices reveal a coherent atmospheric sequence across the sample. H$_2$O and CH$_4$ strengthen toward lower $T_{\mathrm{eff}}$, whereas CO and CO$_2$ rise below $\sim$1500~K and turn over near $\sim$1000~K. Comparison with model sequences shows that the CO$_2$ index is a useful empirical metallicity tracer at $T_{\mathrm{eff}}\sim 800$--1300~K. Together, these results establish SUDA as a reference sample for linking observed 0.75--5~$\mu$m spectral morphology to atmospheric and evolutionary trends in ultracool dwarfs.