Title: Cloudless atmospheres for young low-gravity substellar objects,
Abstract: Atmospheric modeling of low-gravity (VL-G) young brown dwarfs remains a
challenge. The presence of very thick clouds has been suggested because of
their extremely red near-infrared (NIR) spectra, but no cloud models provide a
good fit to the data with a radius compatible with evolutionary models for
these objects. We show that cloudless atmospheres assuming a temperature
gradient reduction caused by fingering convection provides a very good model to
match the observed VL-G NIR spectra. The sequence of extremely red colors in
the NIR for atmospheres with effective temperature from ~2000 K down to ~1200 K
is very well reproduced with predicted radii typical of young low-gravity
objects. Future observations with NIRSPEC and MIRI on the James Webb Space
Telescope (JWST) will provide more constrains in the mid-infrared, helping to
confirm/refute whether or not the NIR reddening is caused by fingering
convection. We suggest that the presence/absence of clouds will be directly
determined by the silicate absorption features that can be observed with MIRI.
JWST will therefore be able to better characterize the atmosphere of these hot
young brown dwarfs and their low-gravity exoplanet analogues.