Cloudless atmospheres for young low-gravity substellar objects

TL;DR

This study demonstrates that cloudless atmospheric models with reduced temperature gradients due to fingering convection accurately reproduce the near-infrared spectra and colors of young, low-gravity brown dwarfs, challenging cloud-based explanations.

Contribution

It introduces a cloudless atmospheric modeling approach incorporating fingering convection, providing better spectral fits and radius estimates for young brown dwarfs compared to previous cloud models.

Findings

Cloudless models match observed NIR spectra of young brown dwarfs.

Predicted radii align with evolutionary models for low-gravity objects.

Future JWST observations will test the role of fingering convection and cloud presence.

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.