Observations of Luhman 16AB: A Brown Dwarf Binary at 2 pc

Early in March 2013, Kevin Luhman announced his discovery of a pair of brown dwarfs only 2 pc (6 light-years) from the Sun, the 3rd closest system to us after the α/Proxima Centauri system and Barnard’s Star. This remarkable find was buried in survey data going back 35 years, but elucidated with the mid-infrared sensitivity of the Wide-field Infrared Survey Explorer (WISE) and the object’s very high proper motion (2.8 arcseconds/year, or just under 0.1 degrees/century).  Using optical spectroscopy, Luhman found that the brighter of the two components had a late-L spectral type, suggesting that the system might straddle the transition between L dwarf and T dwarf spectral classes.  Knowing home much we like this really cool transition, we jumped into action.

Screen shot 2013-04-22 at 12.16.12 PMOur colleague Scott Sheppard from Carnegie Institute of Department of Terrestrial Magnetism was using the FIRE spectrograph on the 6.5m Magellan Baade Telescope on 12 March 2013, and was to obtained low-resolution, near-infrared spectra of both brown dwarfs.  Analyzing these data, we were able to confirm that the source observed by Luhman was an late-L dwarf, while the other brown dwarf showed weak absorption features arising from methane gas in its atmosphere, the hallmark of an early-T dwarf.  Luhman 16AB is definitely an L/T transition binary!

We next requested engineering time with the SpeX instrument on NASA’s 3m InfraRed Telescope Facility (IRTF) on 15 March 2013 (yes, the Ides of March!). This was a difficult observation given that Luhman 16AB never gets higher than about 17° above horizon on Mauna Kea (an airmass of 3.4), rising just slightly over the summit of Mauna Loa to the south.  We also weren’t helped by the clouds and bad seeing, but fortunately Luhman 16AB is a booming bright near-infrared source and we were able to obtain images in three near-infrared bands and a near-infrared spectrum.

Screen shot 2013-04-22 at 12.15.53 PMThe images proved to be the most interesting aspect of this investigation. You can see that the northwestern source (upper right) is brighter in the J-band (about 1.2 µm), while the southeastern source is brighter in the K-band (about 2.2 µm).  They flip in brightness!  The northwestern source is the T dwarf, and despite it being a cooler brown dwarf (based on the presence of methane in its atmosphere), it is brighter in the 1.0-1.25 µm range.  Remarkable!

This kind of behavior has been seen before, and appears to be due to a change in the cloud structure of brown dwarfs at the transition between the L dwarf and T dwarf classes.  The visible atmospheres of L dwarfs have thick clouds of minerals and metals, while those of T dwarfs are relatively cloud-free.  The change between these conditions seems to be quite sudden, and when the clouds disappear we see more deeply into the hotter inner atmosphere of a brown dwarf and it gets brighter.  We still don’t understand  why this happens in detail; it appears that Luhman 16AB might be the best laboratory to study this phenomena.

We’ve reported our results in a submitted paper to Astrophysical Journal; three other papers have also been submitted around this time reporting more details on the photometry and astrometry of the source, optical spectroscopy of both components, and the detection of remarkable variability.  We also have a monitoring campaign planned for 22-26 April 2013.  Stay tuned!

 

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  1. […] towers over have atmospheres that are quite similar to the hot gas giant exoplanets we’ve discovered. By studying brown dwarfs, which are […]

  2. […] binary Luhman 16AB, in a paper published in the Astrophysical Journal.  Our results confirm the earlier “flux reversal” seen in FIRE spectroscopy, and allow us to make the first constraints on the cloud coverage fraction and the temperatures of […]

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