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 <0.15% per-band sensitivity in relative brightness changes across the 0.9-2.4µm 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 µm, 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 µm 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 µm. 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.

Aerosols in Exoplanet Atmospheres

“…It is important to note, however, that these studies do not consider the kinetics of cloud formation, and thus how cloud opacity varies with gravity on directly imaged exoplanets is still uncertain. Furthermore, interpreting spectra of the reddest objects still requires the inclusion of high altitude submicron aerosols and/or highly vertically extended cloud layers (Allart et al., 2020; Burningham et al., 2017; Hiranaka et al., 2016; Kellogg et al., 2017; Lew et al., 2016; Manjavacas et al., 2018; Schlawin et al., 2017; Stone et al., 2020).…”

Cloud Atlas: High-precision HST/WFC3/IR Time-resolved Observations of Directly Imaged Exoplanet HD 106906b

“…From the astrophysical perspective, we can qualitatively evaluate the likelihood for HD106906b, an early L-type planetary-mass companion to be rotationally modulated only in the F127M band but not in the other two bands. Multiwavelength and time-resolved observations of ultra-cool dwarfs have found that the rotational modulations for the majority brown dwarfs and planetary-mass companions are wavelength-dependent and have higher amplitudes at shorter wavelengths than longer wavelengths (e.g., Apai et al 2013;Yang et al 2015;Zhou et al 2016;Schlawin et al 2017;Zhou et al 2019). These findings are consistent with a model prediction based on Mie-scattering calculation (Hiranaka et al 2016;Lew et al 2016;Schlawin et al 2017).…”

Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs

“…Multiple studies have used time-resolved ground-based photometry (e.g., Artigau et al 2009;Radigan et al 2012;Biller et al 2013;Radigan 2014), spectroscopy (e.g., Schlawin et al 2017), or space-based spectrophotometry (e.g., Buenzli et al 2012Buenzli et al , 2015Yang et al 2015Yang et al , 2016Karalidi et al 2016;Lew et al 2016) to explore the variations of the near-infrared (near-IR) colors of rotating brown dwarfs (from mid-L to T8 spectral types). All of these studies found gray, i.e., only weakly wavelength-dependent modulations in the near-IR, even for objects with large-amplitude (>10%) modulations.…”