  Ultra-cool brown dwarfs offer a unique window into understanding substellar
atmospheric physics and chemistry. Their strong molecular absorption bands at
infrared wavelengths, Jupiter-like radii, cool temperatures, and lack of
complicating stellar irradiation, make them ideal test-beds for understanding
Jovian-like atmospheres. Here we report the findings of a uniform atmospheric
retrieval analysis on a set of 14 Y and T-dwarfs observed with the Hubble Space
Telescope Wide Field Camera 3 instrument. From our retrieval analysis, we find
the temperature-structures to be largely consistent with radiative-convective
equilibrium in most objects. We also determine the abundances of water,
methane, and ammonia and upper limits on the alkali metals sodium and
potassium. The constraints on water and methane are consistent with predictions
from chemical equilibrium models, while those of ammonia may be affected by
vertical disequilibrium mixing, consistent with previous works. Our key result
stems from the constraints on the alkali metal abundances where we find their
continued depletion with decreasing effective temperature, consistent with the
trend identified in a previous retrieval analysis on a sample of slightly
warmer late T-dwarfs in Line et al. (2017). These constraints show that the
previously observed Y-J color trend across the T/Y transition is most likely
due to the depletion of these metals in accordance with predictions from
equilibrium condensate rainout chemistry. Finally, we simulate future James
Webb Space Telescope observations of ultra-cool dwarfs and find that the
NIRSpec PRISM offers the best chance at developing high-precision constraints
on fundamental atmospheric characteristics.
