Weather on Other Worlds. VI. Optical Spectrophotometry of Luhman 16B Reveals Large-amplitude Variations in the Alkali Lines

Abstract

Using a novel wide-slit, multi-object approach with the GMOS spectrograph on the 8-meter Gemini South telescope, we have obtained precise time-series spectrophotometry of the binary brown dwarf Luhman 16 at optical wavelengths over two full nights. The B component of this binary system is known to be variable in the red optical and near-infrared with a period of 5 hr and an amplitude of 5--20%. Our observations probe its spectrally-resolved variability in the 6000--10000 Angstrom range. At wavelengths affected by the extremely strong, broadened spectral lines of the neutral alkali metals (the potassium doublet centered near 7682 Angstroms and the sodium doublet at 5893 Angstroms), we see photometric variations that differ strikingly from the those of the 8000--10000 Angstrom `red continuum' that dominates our detected flux. On UT 2014 February 24, these variations are anticorrelated with the red continuum, while on Feb 25 they have a large relative phase shift. The extent to which the wavelength-dependent photometric behavior diverges from that of the red continuum appears to correlate with the strength of the alkali absorption. We consider but ultimately reject models in which our observations are explained by lightning or auroral activity. A more likely cause is cloud-correlated, altitude-dependent variations in the gas-phase abundances of sodium and potassium, which are in chemical equilibrium with their chlorides in brown dwarf atmospheres. Clouds could influence these chemical equilibria by changing the atmospheric temperature profile and/or through cloud particles acting as chemical catalysts.

26 pages, 14 figures, accepted by ApJ

References in corpus (12)

• Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
• Strong Brightness Variations Signal Cloudy-to-Clear Transition of Brown Dwarfs
• Vertical Atmospheric Structure in a Variable Brown Dwarf: Pressure-dependent Phase Shifts in Simultaneous Hubble Space Telescope-Spitzer Light Curves
• Clouds, Gravity and Metallicity in Blue L dwarfs: The Case of 2MASS J11263991-5003550
• Cloud structure of the nearest brown dwarfs: Spectroscopic variability of Luhman 16AB from the Hubble Space Telescope
• TESS Observations of the Luhman 16AB Brown Dwarf System: Rotational Periods, Lightcurve Evolution, and Zonal Circulation
• Lightning climatology of exoplanets and brown dwarfs guided by Solar System data
• Rotational Light Curves of Jupiter from UV to Mid-Infrared and Implications for Brown Dwarfs and Exoplanets
• Weather on Other Worlds. III. A Survey for T Dwarfs with High Amplitude Optical Variability
• Hubble Space Telescope astrometry of the closest brown dwarf binary system -- I. Overview and improved orbit
• The Deepest Constraints on Radio and X-ray Magnetic Activity in Ultracool Dwarfs from WISE J104915.57-531906.1
• Characterizing The Cloud Decks of Luhman 16AB with Medium-Resolution Spectroscopic Monitoring

Cited by in corpus (4)

• The JWST Weather Report from the Nearest Brown Dwarfs I: multi-period JWST NIRSpec + MIRI monitoring of the benchmark binary brown dwarf WISE 1049AB
• A Unified Spectroscopic and Photometric Model to Infer Surface Inhomogeneity: Application to Luhman 16B
• Tracing the Top-of-the-atmosphere and Vertical Cloud Structure of a fast-rotating late T-dwarf
• A 1.48-2.48 $\mu$m R=28000 spectroscopic atlas of the L7.5 and T0.5 components of the nearest pair of brown dwarfs: Luhman 16AB