Probing the Early History of the Milky Way with New Models of Metal-poor Brown Dwarfs

Ultracool Dwarfs (UCDs) are stars and brown dwarfs with surface temperatures ≲ 3000K and masses ≲ 0.1 solar. The atmospheres of UCDs are dominated by molecular opacity and chemistry, rendering their spectra highly sensitive to element abundances. In combination with appropriate stellar models with non-solar chemistry, UCDs may be used as chemical tracers of stellar populations.

Metal-poor UCDs are particularly interesting, as they represent the poorly understood early phases of the chemical evolution of the Milky Way. However, few metal-poor UCD stellar models are available in the literature. We present a new grid of evolutionary models and model atmospheres for metal-poor UCDs. Our models were calculated using the PHOENIX code, version 15, and include the Allard & Homeier treatment of clouds with gravitational settling. The atmosphere grid extends down to 700 K in effective temperature, and spans metallicities ([Fe/H]) from -2.4 to 0.3 dex for multiple values of α-enhancement. The corresponding evolutionary models and synthetic isochrones were calculated with the MESA code, using our model atmospheres as atmosphere-interior boundary conditions.

These new models will support studies of metal-poor UCDs discovered by deep surveys with the James Webb Space Telescope, the Nancy Grace Roman Space Telescope, and the Vera Rubin Observatory, as well as new photometric surveys of UCDs in globular clusters.