Hyperactive brown dwarf has a companion, and may be old and heavy to boot

For the most part, brown dwarfs don’t do very much; they just sit there and slowly cool off.  But a rare few are “hyperactive”, exhibiting emission lines that both vary and persist over long periods of time. The source of this activity has been a mystery for nearly a decade, but new observations we’ve conducted with the IRTF, Magellan and Keck Telescopes show that one of these hyperactive dwarfs has a very faint companion, and that it is hyperactive because it is very old and relatively massive.

The hyperactive dwarf in question is 2MASS J13153094-2649513 (2MASS 1315-2649 for short), an L-type dwarf independently discovered in 2002 by John Gizis and Pat Hall in the Two Micron All Sky Survey.  Both researchers were surprised to find very strong hydrogen alpha emission coming from this source, particularly since very few L dwarfs show any such emission.   As the emission was seen to persist, various ideas were proposed to explain it, such as accretion from a disk or a close companion, an unusually strong magnetic field, or other heating mechanisms. However, subsequent observations were unable to firmly confirm or refute these hypotheses.

Is this companion responsible for 2MASS 1315-2649′s hyperactivity?  Probably not, as the two sources are about 6.6 Astronomical Units (AU) away from each other, too far apart to really interact.  Yet the fact that the companion is so faint gives us another clue.  It turns out that most brown dwarf binaries are nearly equal in mass, a characteristic that might be related to how they form.  If that trend applies here, then the 2MASS 1315-2649 system would have to be very old – of order 5-10 billion years old – to give the two sources time to diverge in brightness.  That would make the brighter source in 2MASS 1315-2649 more massive, perhaps massive enough to be a hydrogen-burning star rather than a brown dwarf.  This turns out to be an important point, as a recent study by Ulrich Christensen finds that more massive brown dwarfs have stronger magnetic fields.  From this line of reasoning, it would appear that strong magnetic fields might the cause of 2MASS 1315-2649′s hyperactivity.

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