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

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 per-band sensitivity in relative brightness changes across the 0.9–2.4 μ m 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 μ m, 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 μ m 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 μ m. 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.