Tuesday, March 12, 2013

A Nearby Pair of Brown Dwarfs

NASA’s Wide-field Infrared Survey Explorer (WISE) is a space telescope which performed an all-sky astronomical survey in the infrared-wavelength. One of its mission objectives was to detect a class of cool and dim objects known as brown dwarfs. In fact, data from WISE has allowed the discovery of a bonanza of such objects. A brown dwarf is more massive than a planet but is short of being a “true” star since it is not massive enough to fuse hydrogen in its core to produce energy. Brown dwarfs do not produce much visible light. Instead, they are warm and glow in the infrared. Because brown dwarfs are extremely faint, some of them may lie as close as the nearest stars and still remain undiscovered.

A paper published in the Astrophysical Journal Letters announced the discovery of a pair of brown dwarfs located at a mere 6.5 light years from the Sun. This binary brown dwarf system is known as WISE 1049-5319 and the discovery was made by Kevin Luhman, an associate professor of astronomy and astrophysics at Pennsylvania State University. WISE 1049-5319 is the third closest star system to us, after the Alpha Centauri triple-star system at about 4.3 light years away and Barnard’s Star at about 6 light years away.

 This diagram illustrates the locations of the star systems that are closest to the Sun. Credit: Janella Williams, Penn State University.

The closeness of WISE 1049-5319 was revealed by its large proper motion. Since proper motion is the observed change in position of an object over time, nearby stars have large proper motions while stars further away have smaller proper motions. This is like looking out from a car window and seeing nearby trees fly by while distant mountains appear almost motionless. The large proper motion of WISE 1049-5319 was revealed in older images taken between the years 1978 to 1999 from the Digitized Sky Survey (DSS), the Two Micron All-Sky Survey (2MASS) and the Deep Near-Infrared Survey of the Southern Sky (DENIS).

WISE 1049-5319 shows up as a single object in the WISE data and in older images. Its binary nature was only revealed from follow-up observations using the large Gemini telescope on the night of 23 February 2013. The sharper image from Gemini shows WISE 1049-5319 as two brown dwarfs separated from each other by approximately 3 times the Earth-Sun distance. It is estimated that the two brown dwarfs circle each other every 25 years or so.

Image of WISE 1049-5319 from the WISE satellite and the Gemini imagery (inset) that revealed it to be a binary system. Credit: NASA/JPL/Gemini Observatory/AURA/NSF

The close proximity of WISE 1049-5319 makes it a unique target for the detection of planets around it. Furthermore, the signature of a planet orbiting a brown dwarf is more easily detectable than for a planet orbiting a star because a brown dwarf is much fainter and less massive than a star. In the distant future, WISE 1049-5319 may turn out to be one of the first few destinations for interstellar exploratory missions beyond our solar system. The presence of planets around WISE 1049-5319 will make it an even more interesting target. Recall that in late 2012, an Earth-sized planet was discovered in a close-in orbit around one star in the Alpha Centauri triple-star system.

K. L. Luhman (2013), “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun”, arXiv:1303.2401 [astro-ph.GA]