The Limiting Effects of Dust in Brown Dwarf Model Atmospheres

TLDR

The study presents opacity‑sampling model atmospheres, spectra, and colors for brown dwarfs and very low‑mass stars under two dust‑settling regimes: inefficient settling following chemical equilibrium and efficient settling where dust depletes refractory elements but does not alter the thermal structure. The models incorporate more than 600 gas‑phase species, 1,000 liquid and crystal phases, and opacities for 30 grain types, covering effective temperatures from 3,000 to 100 K, surface gravities 2.5–6.0, and solar metallicity. Silicate dust grains form abundantly in the outer atmospheres of late‑M and brown dwarfs, producing a greenhouse effect that explains their unusually red spectra, while dust‑free models reproduce methane brown dwarfs such as Gliese 229B; additionally, Na I and K I resonance doublets dominate the pseudocontinuum of T dwarfs from the I to Z band.

Abstract

We present opacity sampling model atmospheres, synthetic spectra, and colors for brown dwarfs and very low mass stars in the following two limiting cases of dust grain formation: (1) Inefficient gravitational settling (i.e., the dust is distributed according to the chemical equilibrium predictions) and (2) efficient gravitational settling (i.e., the dust forms and depletes refractory elements from the gas, but their opacity does not affect the thermal structure). The models include the formation of over 600 gas-phase species and 1000 liquids and crystals and the opacities of 30 different types of grains including corundum (Al2O3), the magnesium aluminum spinel MgAl2O4, iron, enstatite (MgSiO3), forsterite (Mg2SiO4), amorphous carbon, SiC, and a number of calcium silicates. The models extend from the beginning of the grain formation regime well into the condensation regime of water ice (Teff = 3000-100 K) and encompass the range of log g = 2.5-6.0 at solar metallicity. We find that silicate dust grains can form abundantly in the outer atmospheric layers of red and brown dwarfs with a spectral type later than M8. The greenhouse effects of dust opacities provide a natural explanation for the peculiarly red spectroscopic distribution of the latest M dwarfs and young brown dwarfs. The grainless (cond) models, on the other hand, correspond closely to methane brown dwarfs such as Gliese 229B. We also discover that the λλ5891, 5897 Na I D and λλ7687, 7701 K I resonance doublets play a critical role in T dwarfs, in which their red wings define the pseudocontinuum from the I to the Z bandpass.