Cloud structure of the nearest brown dwarfs: Spectroscopic variability of Luhman 16AB from the Hubble space telescope

Esther Buenzli, Didier Saumon, Mark S. Marley, Dániel Apai, Jacqueline Radigan, Luigi R. Bedin, I. Neill Reid, Caroline V. Morley

Research output: Contribution to journalArticle

37 Scopus citations

Abstract

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 sed = 2, T 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 eff = 1300 K, f sed = 3) and a cooler, thicker cloud (T eff = 1000-1100 K, f 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.

Original languageEnglish (US)
Article number127
JournalAstrophysical Journal
Volume798
Issue number2
DOIs
StatePublished - Jan 10 2015

Keywords

  • binaries: visual
  • brown dwarfs
  • stars: atmospheres
  • stars: individual (WISE J104915.57-531906.1, Luhman 16AB)
  • stars: variables: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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