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

Stellar activity is typically attributed to magnetic field interactions, but in brown dwarfs this emission is more of a mystery.

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.

Discovery of the Nearest Young Binary

Three-color image of the sky around TWA 30, with the faint red companion TWA 30B indicated due north.

Earlier this year, U. Hawaii Graduate student Dagny Looper reported the discovery of a young, low-mass and highly active star in the TW Hydrae Association, TWA 30.  She has now found that that star is just one component of an unusual pair.

Only 80″ (0.02 degrees) north of TWA 30 lies a faint red star just barely visible in the image above.  This source is so red that it was worth a look with the MagE spectrograph at the Magellan Telescopes.  The resulting spectrum revealed a rich set of the same forbidden emission lines the we saw in the spectrum of TWA 30, indicating accretion onto the star from a circumstellar disk and  powerful magnetically-driven jets emanating from the star’s poles.  Moreover, we found two lines of forbidden neutral carbon emission, [C I], an important probe of the gas in the disk that surrounds this source, according to a 2009 study led by Dr. Barbara Ercolano.  This is the first time these lines have been seen in the spectrum of a young star.

Discovery of a Wacko Star

Jets, disks and accretion in nearby low mass star

U. Hawaii Graduate student Dagny Looper reports the discovery of a young, low-mass nearby star that is both unusually active and highly variable. The star, TWA 30, is a member of the TW Hydrae Association, roughly two dozen ~8 million year-old stars located about 50 pc away. TWA 30 is the newest member of this group, and one of the most intriguing. Its optical spectrum shows classical and forbidden emission lines, indicating that it is both accreting material and emitting high-speed jets of gas. The star’s near-infrared color also varies dramatically on week-long timescales, evidence that it periodically hides behind a nearly edge-on circumstellar disk. This makes TWA 30 one of the nearest T Tauri stars to the Sun.

The paper was published in the Astrophysical Journal.

May 2010