Atmospheric Retrievals of the Phase-resolved Spectra of Irradiated Brown Dwarfs WD-0137B and EPIC-2122B

TL;DR

This study performs atmospheric retrievals on phase-resolved spectra of two irradiated brown dwarfs, revealing temperature structures and opacity sources, and compares findings with hot Jupiter models to understand their atmospheric behaviors.

Contribution

First detailed atmospheric retrieval analysis of irradiated brown dwarfs WD-0137B and EPIC-2122B, highlighting their similarities and differences with hot Jupiter systems.

Findings

WD-0137B has a dayside with inverted temperature structure and H$^-$ opacity.

EPIC-2122B's atmosphere remains inverted across all observed longitudes.

Inclined viewing geometry affects the observed atmospheric spectra.

Abstract

We present an atmospheric retrieval analysis of HST/WFC3/G141 spectroscopic phase curve observations of two brown dwarfs, WD-0137B and EPIC-2122B, in ultra-short period orbits around white dwarf hosts. These systems are analogous to hot and ultra-hot Jupiter systems, enabling a unique and high-precision comparison to exoplanet systems. We use the PETRA retrieval suite to test various analysis setups, including joint-phase retrievals, multiple temperature structures, and non-uniform abundances. We find that WD-0137B has a dayside that closely resembles that of other ultra-hot Jupiters with inverted temperature structures and H$^-$ opacity, but quickly transitions to a mostly non-inverted temperature structure on the nightside. Meanwhile, EPIC-2122B's atmosphere remains inverted at all constrained longitudes, with dominant H$^-$ opacity. Retrievals with multiple temperature profiles and non-uniform vertical abundances were generally not statistically justified for this dataset, but retrievals with dayside-dilution factors were found to be justified. Retrieving all phases simultaneously with a linear combination of a dayside and nightside atmosphere was found to be an adequate representation of the entire phase-curve once a longitudinal temperature gradient free parameter was included in the retrieval. Comparing to global circulation models, we attribute behavior in the 1D retrievals to the inclined viewing geometry of the systems, which results in always-visible irradiated and inverted portions of the atmosphere "contaminating" spectra measured from the nightside hemisphere. This study sheds light on the similarities between these irradiated brown dwarf systems and hot and ultra-hot Jupiters, but also their unique differences, including the influence of the inclined viewing geometry.