Spectral Variability of Two Rapidly Rotating Brown Dwarfs: 2MASS J08354256-0819237 and 2MASS J18212815+1414010

TL;DR

This study uses high-precision near-infrared spectral monitoring to analyze the variability of two rapidly rotating brown dwarfs, revealing differences in their cloud properties and variability patterns.

Contribution

It provides the first detailed spectral variability analysis of these brown dwarfs, linking observed variability to dust particle size distributions and atmospheric structure.

Findings

2MASS J0835-0819 shows marginal variability with no spectral dependence.

2MASS J1821+1414 exhibits wavelength-dependent variability declining at longer wavelengths.

Variability in J1821+1414 can be modeled by dust particles with a median radius of 0.24 um.

Abstract

L dwarfs exhibit low-level, rotationally-modulated photometric variability generally associated with heterogeneous, cloud-covered atmospheres. The spectral character of these variations yields insight into the particle sizes and vertical structure of the clouds. Here we present the results of a high precision, ground-based, near-infrared, spectral monitoring study of two mid-type L dwarfs that have variability reported in the literature, 2MASS J08354256-0819237 and 2MASS J18212815+1414010, using the SpeX instrument on the Infrared Telescope Facility. By simultaneously observing a nearby reference star, we achieve <0.15% per-band sensitivity in relative brightness changes across the 0.9--2.4um bandwidth. We find that 2MASS J0835-0819 exhibits marginal (< ~0.5% per band) variability with no clear spectral dependence, while 2MASS J1821+1414 varies by up to +/-1.5% at 0.9 um, with the variability amplitude declining toward longer wavelengths. The latter result extends the variability trend observed in prior HST/WFC3 spectral monitoring of 2MASS J1821+1414, and we show that the full 0.9-2.4 um variability amplitude spectrum can be reproduced by Mie extinction from dust particles with a log-normal particle size distribution with a median radius of 0.24 um. We do not detect statistically significant phase variations with wavelength. The different variability behavior of 2MASS J0835-0819 and 2MASS J1821+1414 suggests dependencies on viewing angle and/or overall cloud content, underlying factors that can be examined through a broader survey.