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Celestial odd couple discovered

Astronomers are puzzled by the "Hang-loose Binary" — a pair of low-mass stars with an extreme orbital separation.
Provided by Gemini Observatory, Hilo, Hawaii
The low-mass binary pair is tethered by a weak gravitational link that results in an orbital dance so slow that it takes about 500,000 years to complete a single revolution.
Gemini Observatory/Cerro Tololo/2MASS/DSS
April 11, 2007
Astronomers have serendipitously discovered a record-breaking pair of low-mass stars with an extreme orbital separation. The petite objects, each of which has a mass less than 100 times that of Jupiter, are separated by more than 5,000 times the distance between the Sun and Earth — a value that breaks the previous record by a factor of three, and leaves the duration of their future together uncertain.

The celestial duo is tethered by a weak gravitational link that results in an orbital dance so slow that it takes about 500,000 years to complete a single revolution. Scaled down, this system would be like two baseballs orbiting each other about 300 kilometers (200 miles) apart.

The characterization of the system was made using near-infrared spectroscopic data taken with the Gemini South telescope, in conjunction with earlier discovery and confirmation observations made at the Cerro Tololo Inter-American Observatory 1.5-meter telescope, operated by the Small and Moderate Aperture Research Telescope System (SMARTS), and archival data from the 2-Micron All-Sky Survey (2MASS) and the Digital Sky Survey (DSS). The result was published in the April 10, 2007 issue of the Astrophysical Journal Letters, by lead author Étienne Artigau, a Science Fellow at Gemini Observatory, and a team that includes astronomers from the Université de Montréal, and the Canada-France-Hawaii Telescope.

The discovery came as a surprise because the only other known binaries that have similar or greater separations are significantly more massive systems. Since mass determines how strongly objects pull on each other, the more massive stars in the known systems have strong gravitational attractions. On the other hand, the stars in the newly discovered system have extremely low masses, and thus, low gravitational attraction. How this occurs is a real mystery.

Equally intriguing is how the discovery came about. "The technique we used to make this discovery was born over a nice dinner and a couple of drinks," said Artigau, who first thought of it when he was a graduate student at the Université de Montréal. "The next morning, the technique didn't seem so crazy after all, and, in fact, it led to this discovery."

Artigau and his colleagues identified the two stars, which they nicknamed the "Hang-loose Binary," in the southern constellation of Phoenix, in 2006. The actual catalog names are 2MASS J012655.49-502238.8, and 2MASS J012702.83-502321.1. The team made the discovery by comparing their positions in the 2MASS point source catalog with those in the DSS. Since the 2MASS and DSS data were obtained 16 years apart, the stars had moved significantly between the two observations. The Cerro Tololo 1.5-meter telescope confirmed that the two stars were indeed moving through space together.

To discern the nature of the new binary system, the researchers obtained the infrared "spectral fingerprint" of each member using the Gemini Near Infrared Spectrograph (GNIRS) on Gemini South. The data revealed that both stars are likely red dwarfs, with temperatures of 2,200º Celsius (4,000º Fahrenheit) and a probable age of about a billion years.

Interestingly, the pair is seen juxtaposed against a group of stars called the Tucana/Horologium (TH) association, which presents the tantalizing possibility that the binary is part of this group. If this is true, then the stars would be significantly younger than the one-billion-year estimate and could be categorized as even less massive brown dwarfs.

The members of the TH association are a well understood, loose grouping of 30-million-year-old stars that lie some 120 to 215 light-years away. The newly discovered binary pair has an estimated distance of 200 light-years, which seems to support the idea that the pair is part of the group. Furthermore, the newly discovered binary system appears to be keeping pace with the TH association as it moves through our galaxy.

"If the new binary system truly belongs to the TH association, and is not a chance alignment," said team member David Lafrenière of the Université de Montréal, "then the stars are not 1-billion-year-old red dwarfs, but are much younger brown dwarfs of the same age as the association. Unlike red dwarfs, these brown dwarfs wouldn't have enough mass to ignite hydrogen into helium at their cores, so they are destined to loosen their weak embrace more quickly, slowly cool, and fade away."

However, if they are not members of the TH association, these stars would indeed be more-massive red dwarfs and could stay embraced for perhaps a billion years or more. To resolve their nature, observations are being proposed to use Gemini to look for lithium in the stars' atmospheres, which will help better constrain their ages and masses. Until this is determined, the future of this celestial pair remains uncertain.
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