Binary star system found right under our noses
By Brian Dodson
March 13, 2013
In a day when we have examined astronomical objects shining forth from a time shortly after the Big Bang, one would think astronomers have a pretty good handle on what is in the immediate vicinity of the Solar System. That's why the recent report of a binary star lying only 6.5 light-years away came as rather a surprise to the astronomical community. The pair, called WISE J1049-5319 A and B, are brown dwarf stars and only two star systems – the triple star Alpha Centauri, and Barnard's Star – lie closer to our Sun.
In December of 2009, NASA's Wide-field Infrared Survey Explorer (WISE) was launched into a polar orbit at an altitude of 525 km (326 miles) back in 2009. This small-scale space telescope was designed to perform a survey of the entire expanse of space in near- to mid-infrared wavelengths ranging from 3.4 to 22 microns. WISE examined the skies for over a year using extremely sensitive imaging sensors, in the process discovering room temperature stars, galaxies formed just after the Big Bang, tens of thousands of asteroids, protoplanetary disks, newborn stars, and more.
Brown dwarf stars are stars that have too little mass to fuse hydrogen (although deuterium is likely to fuse for a brief period). Interestingly enough, they are all about the size of Jupiter, regardless of their mass. Brown dwarfs are heated as they slowly collapse by conversion of their gravitational potential energy into thermal energy. The upper mass limit for a star to be a brown dwarf is about 0.07-0.08 solar masses, while the transition to being considered a large planet takes place by convention at 0.012 solar masses, where even deuterium fusion stops.
Prof. Kevin Luhman of Pennsylvania State University's department of Astronomy and Astrophysics took on the task of examining some of the 2.7 million photos taken by WISE for brown dwarfs (objects brighter in the mid-IR than in near-IR) that displayed large proper motion (movement relative to the more distant stars due to the relative velocity between the Sun and the object). Large proper motion implies the object is nearby, and intensity increasing toward longer infrared wavelengths implies cool (and therefore dim) objects that are even dimmer in visible light, and hence might have been overlooked.
WISE J1049-5319 was one of the objects selected by Luhman's search of the WISE database. It has a low galactic latitude (meaning that it appears against the bright background of the Milky Way), which may partially explain why it avoided detection until now. Once he had a location for the object, Prof. Luhman found it in earlier IR surveys and its parallax of was measured at 0.496 + 0.037 seconds of arc, corresponding to a distance of 2.016 parsecs (6.575 light years).
During a spectroscopic examination of WISE J1049-5319 using the Gemini 8.1 meter (26 foot) telescope, Prof. Luhman found that the object is actually a binary star system composed of two brown dwarfs currently separated by 1.5 seconds of arc. Their period of rotation around each other is about 25 years, which corresponds to a separation of about three astronomical units (approximately 449 million km). The spectrum revealed that the surface temperature of the stars is in the neighborhood of 1300 K (1850 F).
Beyond the simple pleasure in finding another very close star system and being able to study nearby brown dwarf stars, the discovery of WISE J1049-5319 could be ideal for developing methods for imaging exoplanets.
"It will be an excellent hunting ground for planets because it is very close to Earth, which makes it a lot easier to see any planets orbiting either of the brown dwarfs." Since it is the third-closest star system, in the distant future it might be one of the first destinations for manned expeditions outside our solar system," Luhman says.
In recent years, astronomy has taken enormous strides and continuously expands our understanding of the Universe, while at the same time alternately astonishing and tickling the fancy of the public. The astronomical bang for the buck is very large, and there is no sign that the surprises will damp down any time soon.
Sources: Pennsylvania State University