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

• Main Authors: Rowland, Melanie, Morley, Caroline, Line, Michael
• Format: Journal Article
• Language: English
• Published: 19-01-2023
• Subjects: Physics - Earth and Planetary Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2301.08258

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.