  A large suite of 228 atmospheric retrievals is performed on a curated sample
of 19 brown dwarfs spanning the L0 to T8 spectral types using the open-source
Helios-r2 retrieval code, which implements the method of short characteristics
for radiative transfer and a finite-element description of the
temperature-pressure profile. Surprisingly, we find that cloud-free and cloudy
(both gray and non-gray) models are equally consistent with the archival SpeX
data from the perspective of Bayesian model comparison. Only upper limits for
cloud properties are inferred if log-uniform priors are assumed, but the cloud
optical depth becomes constrained if a uniform prior is used.
  Water is detected in all 19 objects and methane is detected in all of the T
dwarfs, but no obvious trend exists across effective temperature. As carbon
monoxide is only detected in a handful of objects, the inferred
carbon-to-oxygen ratios are unreliable. The retrieved radius generally
decreases with effective temperature, but the values inferred for some T dwarfs
are implausibly low and may indicate missing physics or chemistry in the
models. For the early L dwarfs, the retrieved surface gravity depends on
whether the gray or non-gray cloud model is preferred. Future data are
necessary for constraining cloud properties and the vertical variation of
chemical abundances, the latter of which is needed for distinguishing between
the chemical instability versus traditional cloud interpretation of the L-T
transition.
