CLOUD STRUCTURE OF THE NEAREST BROWN DWARFS: SPECTROSCOPIC VARIABILITY OF LUHMAN 16AB FROM THE HUBBLE SPACE TELESCOPE
- Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg (Germany)
- Los Alamos National Laboratory, Mail Stop F663, Los Alamos, NM 87545 (United States)
- NASA Ames Research Center, MS-245-3, Moffett Field, CA 94035 (United States)
- Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)
- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
- INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova (Italy)
The binary brown dwarf WISE J104915.57–531906.1 (also Luhman 16AB), composed of a late-L and early-T dwarf, is a prototypical L/T transition flux reversal binary located at a distance of only 2 pc. Luhman 16B is a known variable whose light curves evolve rapidly. We present a spatially resolved spectroscopic time-series of Luhman 16A and B covering 6.5 hr using the Hubble Space Telescope/WFC3 at 1.1-1.66 μm. The small, count-dependent variability of Luhman 16A at the beginning of the observations likely stems from instrumental systematics; Luhman 16A appears non-variable above ≈0.4%. Its spectrum is well fit by a single cloud layer with intermediate cloud thickness (f {sub sed} = 2, T {sub eff} = 1200 K). Luhman 16B varies at all wavelengths with peak-to-valley amplitudes of 7%-11%. The amplitude and light curve shape changes over only one rotation period. The lowest relative amplitude is found in the deep water absorption band at 1.4 μm, otherwise it mostly decreases gradually from the blue to the red edge of the spectrum. This is very similar to the other two known highly variable early-T dwarfs. A two-component cloud model accounts for most of the variability, although small deviations are seen in the water absorption band. We fit the mean spectrum and relative amplitudes with a linear combination of two models of a warm, thinner cloud (T {sub eff} = 1300 K, f {sub sed} = 3) and a cooler, thicker cloud (T {sub eff} = 1000-1100 K, f {sub sed} = 1), assuming out-of-equilibrium atmospheric chemistry. A model with parameters as for Luhman 16A except for the addition of cloud holes cannot reproduce the variability of Luhman 16B, indicating more complex cloud evolution through the L/T transition. The projected separation of the binary has decreased by ≈0.''3 in eight months.
- OSTI ID:
- 22364609
- Journal Information:
- Astrophysical Journal, Vol. 798, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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