The Retrieved Atmospheric Properties of the Sub-stellar Object VHS 1256 b from Analyzing HST, VLT and JWST Spectra

TL;DR

This study analyzes multi-epoch spectra of the sub-stellar object VHS 1256 b using HST, VLT, and JWST data, revealing consistent atmospheric properties despite observed flux variability, and highlights the limitations of current model grids.

Contribution

It demonstrates the effectiveness of Bayesian and machine-learning retrievals on multi-instrument spectra and discusses the invariance of atmospheric properties over time despite flux variations.

Findings

Water abundance profiles vary with altitude.

Model grids poorly fit HST spectra and are unreliable for temporal variability.

Retrieved atmospheric properties are consistent across different instruments and epochs.

Abstract

Motivated by the observed ~30% variations in flux from the L7 dwarf VHS 1256 b, we subjected its time-resolved Hubble Space Telescope (HST) WFC3 spectra (measured in two epochs in 2018 and 2020), as well as medium-resolution Very Large Telescope (VLT) X-shooter and Early Release Science James Webb Space Telescope (JWST) spectra to a suite of both standard Bayesian (nested sampling) and machine-learning (random forest) retrievals. We find that both HST and VLT data require vertically varying abundance profiles of water in order to model the spectra accurately. Despite the large flux variations observed in the HST data, the temporal variability cannot be attributed to a single varying atmospheric property. The retrieved atmospheric quantities are consistent with being invariant across time. However, we find that model grids provide generally poor fits to the measured HST spectra and are unsuitable for quantifying the temporal variability of atmospheric properties. Additionally, our analysis of JWST spectra using model grids indicates consistency in retrieved properties across different wavelength channels. Despite the temporal variability in flux, the retrieved properties between HST and VLT, as well as between HST and JWST, are consistent within the respective posterior uncertainties. Such an outcome bodes well for future retrieval analyses of exoplanetary atmospheres, which are expected to exhibit weaker flux variations.