Many brown dwarfs exhibit photometric variability at levels from tenths to tens of percents. The photometric variability is related to magnetic activity or patchy cloud coverage, characteristic of brown dwarfs near the L-T transition. Time-resolved spectral monitoring of brown dwarfs provides diagnostics of cloud distribution and condensate properties. However, current time-resolved spectral studies of brown dwarfs are limited to low spectral resolution (R$\sim$100) with the exception of the study of Luhman 16 AB at resolution of 100,000 using the VLT$+$CRIRES. This work yielded the first map of brown dwarf surface inhomogeneity, highlighting the importance and unique contribution of high spectral resolution observations. Here, we report on the time-resolved high spectral resolution observations of a nearby brown dwarf binary, 2MASSW J0746425+200032AB. We find no coherent spectral variability that is modulated with rotation. Based on simulations we conclude that the coverage of a single spot on 2MASSW J0746425+200032AB is smaller than 1\% or 6.25\% if spot contrast is 50\% or 80\% of its surrounding flux, respectively. Future high spectral resolution observations aided by adaptive optics systems can put tighter constraints on the spectral variability of 2MASSW J0746425+200032AB and other nearby brown dwarfs.
J. Wang, L. Prato and D. Mawet
Tue, 28 Feb 17
Comments: 11 Pages, 8 figures, 1 table, accepted by ApJ