# A Uniform Retrieval Analysis of Ultracool Dwarfs. III. Properties of Y-Dwarfs

## TL;DR

This study conducts a uniform atmospheric retrieval analysis on 14 Y and T-dwarfs, revealing insights into their temperature structures, molecular abundances, and alkali metal depletion, with implications for understanding substellar atmospheres and future JWST observations.

## Contribution

It provides the first comprehensive retrieval analysis of Y-dwarfs, confirming alkali metal depletion trends and linking them to atmospheric chemistry and color transitions.

## Key findings

- Temperature structures are consistent with radiative-convective equilibrium.
- Water and methane abundances match chemical equilibrium predictions.
- Alkali metals sodium and potassium are increasingly depleted with decreasing temperature.

## Abstract

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.