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

TL;DR

The SUDA spectral atlas provides a comprehensive analysis of 1675 ultracool dwarfs, deriving atmospheric parameters and creating an empirical spectral sequence to understand their atmospheric and evolutionary properties.

Contribution

This work offers the first homogeneous spectral atlas of ultracool dwarfs with derived atmospheric parameters and an empirical spectral sequence linking spectral features to physical properties.

Findings

Systematic offset between spectral and evolutionary surface gravities (~1.1 dex).

Molecular indices reveal atmospheric trends across temperature range.

CO$_2$ index serves as an empirical metallicity tracer at certain temperatures.

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.