Towards Robust Atmospheric Retrieval on Cloudy L Dwarfs: The Impact of Thermal and Abundance Profile Assumptions

TL;DR

This study evaluates methods for atmospheric retrieval of L dwarf spectra, emphasizing the importance of thermal and abundance profile assumptions, especially in cloud-free models, to improve accuracy in characterizing these objects.

Contribution

The paper introduces a systematic testing framework using synthetic data to identify optimal retrieval strategies for L dwarf spectra, highlighting the need for nonuniform abundance profiles and comprehensive pressure layer coverage.

Findings

Parameterizing PT profiles biases results for hotter L dwarfs.

Probing many atmospheric layers improves retrieval accuracy.

Nonuniform FeH profiles are essential for early- to mid-L dwarfs.

Abstract

Constraining L dwarf properties from their spectra is challenging. Near-infrared spectra probe a limited range of pressures, while many species condense within their photospheres. Condensation creates two complexities: gas-phase species "rain out" (decreasing in abundances by many orders of magnitude) and clouds form. We designed tests using synthetic data to determine the best approach for retrieving L dwarf spectra, isolating the challenges in the absence of cloud opacity. We conducted atmospheric retrievals on synthetic cloud-free L dwarf spectra derived from the Sonora Bobcat models at SpeX resolution using a variety of thermal and chemical abundance profile parameterizations. For objects hotter than L5 (T$_{eff}$ ~ 1700 K), the limited pressure layers probed in the near-IR are mostly convective; parameterized PT profiles bias results and free, unsmoothed profiles should be used. Only when many layers both above and below the radiative-convective boundary are probed can parameterized profiles provide accurate results. Furthermore, a nonuniform abundance profile for iron hydride (FeH) is needed to accurately retrieve bulk properties of early- to mid- L dwarfs. Nonuniform prescriptions for other gases in near-IR retrievals may also be warranted near the L/T transition (CH$_{4}$) and early Y dwarfs (Na and K). We demonstrate the utility of using realistic self-consistent models to benchmark retrievals and suggest how they can be used in the future.