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Brown dwarf in tight orbit around young Sun-like star
Posted: 30 July 2010

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A very young brown dwarf star has been found in a tight orbit around a nearby Sun-like star.

Brown dwarfs are also known as failed stars, since although they are larger than gas giants they don't possess the mass to sustain the nuclear fusion that fully-fledged stars boast.

The relative size of a brown dwarf star compared with Jupiter, the Sun and Earth. Image: Jon Lomberg/Gemini Observatory.

Using the Near-Infrared Coronagraphic Imager (NICI) on the eight-metre Gemini-South Telescope in Chile, an international team of astronomers made a rare find: a very young brown dwarf in orbit around a 12 million year old Sun-like star, around 160 light years away. The star is so young that the astronomers say it still sports a belt of cold circumstellar gas.

But the most exciting part of the discovery is that the 36 Jupiter-mass brown dwarf, PZ Tel B, and its primary star PZ Tel A are separated by just 18 AU a similar distance that separates Uranus and the Sun. Most young brown dwarfs and planetary companions discovered through direct imaging are separated by over 50 AUs.

"Because PZ Tel A is a rare star being both close and very young, it had been imaged several times in the past," says Laird Close of the University of Arizona. "So we were quite surprised to see a new companion around what was thought to be a single star."

PZ Tel B resides just 18 AU from its parent star. Image: Beth Biller and the Gemini NICI Planet-Finding Campaign).

An image taken in 2003 of the system showed that PZ Tel B had been obscured by the glare from its parent star, suggesting that the star is on a more elliptical orbit than previously thought. "PZ Tel B travels on a particularly eccentric orbit in the last 10 years, we have literally watched it careen through its inner Solar System," says Beth Biller, lead author of the paper discussing the results, which will appear in Astrophysical Journal Letters. "This can best be explained by a highly eccentric, or oval-shaped, orbit."

The system is an important case study for the early stages of Solar System formation, and will help astronomers determine the types of planets that can form around PZ Tel A. It will also add to the ever-growing inventory of solar system configurations found outside our own.

The images were achieved by using an adaptive optics system coupled to a coronagraph to block out the excess starlight, combined with analysis techniques to reveal the orbital motion of PZ Tel B. NICI is particularly adept at this, and is currently being used to carry out a 300-star survey that is set to reveal more closely separated objects.