Clouds and Chemistry Across the Brown Dwarf T-Y Sequence: Insights from JWST Atmospheric Retrievals

Abstract

The James Webb Space Telescope (JWST) offers exceptional spectral resolution and wavelength coverage, which are essential for studying the coldest brown dwarfs, particularly Y dwarfs. These objects are at the cold end of the sub-stellar sequence and exhibit atmospheric phenomena such as cloud formation, chemical disequilibrium, and radiative-convective coupling. We examine a curated sample of 22 late-T to Y dwarfs through Bayesian atmospheric retrieval (nested sampling) and supervised machine learning (random forests). Bayesian model comparison indicates that cloud-free models are generally favored for the hottest objects in the sample (T6-T8). Conversely, later-type dwarfs exhibit varying preferences, with both gray-cloud and cloud-free models providing comparable fits. The atmospheric parameters retrieved are consistent across the applied methodologies. Evidence of vertical mixing and disequilibrium chemistry is found in several objects; notably, the Y1 dwarf WISEPAJ1541-22 favors a gray cloud model and shows elevated abundances of both CO and CO2 compared to equilibrium chemistry calculations. As anticipated, the abundances of H2O, CH4, and NH3 increase with decreasing effective temperature over the T-Y sequence.

Figures (11)

• Figure 1: Spectral sample of 22 T and Y dwarfs originally published in Beiler2024ApJ...973..107B. All spectra are normalized at their maximum flux value and offset by a constant.
• Figure 2: Feature importance across wavelengths for all parameters included in our generated model grid for case study Y1-dwarf WISEPAJ1541-22, as well as their joint retrieval. The significance of a feature is quantified by the normalized decrease in variance it generates throughout the training process. Specifically, it denotes the total reduction in variance achieved whenever that feature is employed for partitioning nodes in a decision tree within the ensemble. For visual guidance, the rescaled spectrum of WISEPAJ1541-22 is overlaid as solid black lines.
• Figure 3: Joint posterior distributions from the free-chemistry retrieval analyses of the case study Y1-dwarf WISEPAJ1541-22, using a gray-cloud model. The vertical dashed lines within the histograms indicate the median parameter values and their 1-$\sigma$ uncertainties. Accompanying the montage of joint posterior distributions is the retrieved median temperature–pressure profile along with its associated 1-$\sigma$ uncertainties. The adiabatic profile (black dashed line) is indicated for comparison.
• Figure 4: Posterior retrieval median fit $F$ (orange line) and residuals $\Delta$F (black line) associated with the free-chemistry retrieval analyses of the case study Y1-dwarf WISEPAJ1541-22, using a gray-cloud model. The retrieved error inflation is represented by a gray horizontal bar. The JWST NIRSpec and MIRI data are shown as blue dots with associated uncertainties.
• Figure 5: Normalized carbon-to-oxygen (C/O) ratio probabilities, expressed relative to the solar value of $0.55 \pm 0.09$Asplund2009ARAA..47..481A, for case study Y1-dwarf WISEPAJ1541-22. Prior (red) and posterior (orange) probability distributions are derived from combinations of molecular gas abundance distributions. The solar value is indicated as dashed blue line and shaded region.