FOUR'S A CROWD: A RARE QUARTET OF STARS MAY UNLOCK SECRETS OF STELLAR EVOLUTION

Astronomers using telescopes on Mauna Kea have found an extremely rare quartet of stars that orbit each other within a region smaller than Jupiter's orbit round the Sun. The quartet appears as a speck of light even when viewed with the world's most powerful telescopes but its spectrum reveals not one, but four distinct stars arranged in two pairs. Astronomers are now struggling to work out whether they could have been born that way, or were forced together by a dense disk of gas in their youth.

This discovery that the star called "BD -22 5866" is really a very rare system of four closely orbiting stars was presented by Dr. Evgenya Shkolnik of the University of Hawaii's Institute for Astronomy and the NASA Astrobiology Institute this week at the annual meeting of the Canadian Astronomical Society in Victoria, BC. She and collaborators Dr. Michael C. Liu, also of the University of Hawaii, and Dr. I. Neill Reid of the Space Telescope Science Institute in Baltimore were monitoring several hundred nearby low-mass stars when one observation caught their attention because it was unlike anything they had seen before.

At the time of the observations, two of the stars were orbiting each other at 133 km/s (300,000 mph, a speed that would get you from Honolulu to New York in less than a minute), while the second pair moved at a more modest speed of 52 km/s (120,000 mph). Using these speeds and the stars' masses, astronomers were able to determine the maximum sizes of their oddly tight orbits. Less than 1 in 2,000 stars observed might be such tightly bound systems.

What makes this system even more rare (and more informative) is that the first pair of stars eclipse one another as they orbit each other in only 2.21 days at a distance of only 0.07 AU apart (where 1 AU is the distance from Earth to the sun). With the ability to measure how much light the eclipses block, astronomers are able to precisely determine the masses and radii of each star. The two stars appear identical, each being about 60% the size of the sun (0.59 solar masses and 0.61 solar radii). The fact the stars eclipse each other was discovered by Scottish colleagues Drs. Leslie Hebb and Andrew Collier Cameron of the University of St. Andrews during their search for extrasolar transiting planets using a small 20-cm SuperWASP telescope.

The second pair of stars in the quartet orbits with a period of less than 55 days and a maximum radius of 0.26 AU. The two pairs are orbiting each other with a maximum radius of only 6.1 AU (= orbital period of less than 10 years), about the same as Jupiter's distance from the sun.

"The extraordinarily tight configuration of this stellar system tells us that there may have been a single gaseous disk that forced them into such small orbits within the first 100,000 years of their evolution, as the stars could not have formed so close to one another. This is the first evidence of a disk completely encompassing four stars," says Dr. Shkolnik. "It is remarkable how much a single stellar spectrum can tell us about both the present and the past of these stars."

The data were acquired at the Keck I 10-m (33-foot) telescope and on the Canada-France-Hawaii 3.6-m (12-foot) telescope, both located on the summit of Mauna Kea, a 14,000-foot-high dormant volcano on the Big Island of Hawaii. Each telescope is equipped with a high-resolution spectrograph, an instrument capable of breaking up the star's light into different colours (or wavelengths), known as a spectrum.

The stellar system is 51 pc (or 166 light-years) away from the Sun and lies just south of the constellation Aquarius (The Water Bearer). Though BD -22º5866 cannot be seen without a telescope, it is relatively bright and will be carefully monitored to map the orbits in more detail. Since most stars form as part of a binary- or multiple-star system, the enormous potential of this quadruple system to give us previously unavailable physical information makes it a key to unlocking a few mysteries of stellar evolution.

This work has been accepted for publication to the Astrophysical Journal and was funded by the NASA Astrobiology Institute and the University of Hawaii.


FOR MORE INFORMATION:

Dr. Evgenya Shkolnik
University of Hawaii
Phone: (808) 292-9088
Email: shkolnik@ifa.hawaii.edu

EDITORS: High resolution images accompanying this press release are available at:

http://www.ifa.hawaii.edu/info/press-releases/AAS-Jan08/Shkolnik-AAS-1-08.html