Characterizing The Cloud Decks of Luhman 16AB with Medium-Resolution Spectroscopic Monitoring

TL;DR

This study uses medium-resolution spectroscopy to analyze cloud variability in Luhman 16B, revealing that patchy clouds in the lower atmosphere cause observed brightness changes and spectral variations, with implications for understanding brown dwarf atmospheres.

Contribution

It provides the first detailed spectroscopic monitoring of Luhman 16AB's cloud variability, linking spectral changes to atmospheric pressure levels and cloud patchiness, and models the spectral states with composite templates.

Findings

Variability amplitude decreases with atmospheric pressure in Luhman 16B.

K I absorption is stronger in the faint state, suggesting changes in cloud composition or temperature-pressure profile.

Spectral states can be modeled by combinations of warmer and cooler spectral templates.

Abstract

We present results from a two-night R~4000 0.9-2.5 micron spectroscopic monitoring campaign of Luhman 16AB (L7.5 + T0.5). We assess the variability amplitude as a function of pressure level in the atmosphere of Luhman 16B: the more variable of the two components. The amplitude decreases monotonically with decreasing pressure, indicating that the source of variability - most likely patchy clouds - lies in the lower atmosphere. An unexpected result is that the strength of the K I absorption is higher in the faint state of Luhman 16B and lower in the bright state. We conclude that either the abundance of K I increases when the clouds roll in, potentially because of additional K I in the cloud itself, or that the temperature-pressure profile changes. We reproduce the change in K I absorption strengths with combinations of spectral templates to represent the bright and the faint variability states. These are dominated by a warmer L8 or L9 component, with a smaller contribution from a cooler T1 or T2 component. The success of this approach argues that the mechanism responsible for brown dwarf variability is also behind the diverse spectral morphology across the L-to-T transition. We further suggest that the L9-T1 part of the sequence represents a narrow but random ordering of effective temperatures and cloud fractions, obscured by the monotonic progression in methane absorption strength.