The re-emergence of the 0.99Â \mu m FeHÂ feature in brown dwarfs of early- to mid-T spectral type has been suggested as evidence for cloud disruption where flux from deep , hot regions below the Fe cloud deck can emerge .
The same mechanism could account for color changes at the L/TÂ transition and photometric variability .
We present the first observations of spectroscopic variability of brown dwarfs covering the 0.99Â \mu m FeH feature .
We observed the spatially resolved very nearby brown dwarf binary WISE J104915.57-531906.1 ( Luhman 16AB ) , a late-L and early-T dwarf , with HST/WFC3 in the G102 grism at 0.8-1.15 \mu m. We find significant variability at all wavelengths for both brown dwarfs , with peak-to-valley amplitudes of 9.3 % for Luhman 16B and 4.5 % for Luhman 16A .
This represents the first unambiguous detection of variability in Luhman 16A .
We estimate a rotational period between 4.5 and 5.5 h , very similar to Luhman 16B .
Variability in both components complicates the interpretation of spatially unresolved observations .
The probability for finding large amplitude variability in any two brown dwarfs is less than 10 % .
Our finding may suggest that a common but yet unknown feature of the binary is important for the occurrence of variability .
For both objects , the amplitude is nearly constant at all wavelengths except in the deep KÂ IÂ feature below 0.84Â \mu m. No variations are seen across the 0.99Â \mu m FeH feature .
The observations lend strong further support to cloud height variations rather than holes in the silicate clouds , but can not fully rule out holes in the iron clouds .
We re-evaluate the diagnostic potential of the FeHÂ feature as a tracer of cloud patchiness .