Eclipsing Star System May Be Slowing Down by Ejecting Gas

(AAS Meeting, Toronto, Canada) -- Regular eclipses in an exotic binary star system that scientists have been studying for more than a decade may be occurring less frequently because gas is being ejected from one of the stars at an unprecedented rate. Reporting the results of their research to the American Astronomical Society (AAS) meeting, Drs. Paul Hertz and Kent S. Wood of the Naval Research Laboratory (NRL) in Washington, DC, and Professor Lynn Cominsky of Sonoma State University in Rohnert Park, California, note that this system is of special interest because it is apparently evolving faster than any other known eclipsing star system. Accurate eclipse observations are important to astronomers for measuring changes in star systems.

The binary star system known as EXO0748-676 was discovered in 1985 when it suddenly began emitting X-rays. Located more than 10 thousand light years away in the southern constellation Vulpecula ("Little Fox"), it contains two stars orbiting each other every 3 hours 49 minutes 27 seconds (3.8 hours). The X rays disappear for 500 seconds during an eclipse that occurs during every orbit when one star passes in front of the other.

Last summer, Dr. Hertz and his collaborators used the Rossi X-ray Timing Explorer, a NASA satellite launched in December 1995 to study X rays from nearby stellar systems and distant galaxies, to obtain the most accurate eclipse timings ever measured for this star system. Combining timing measurements of eclipses obtained from four other satellites over the last decade, the researchers show that the eclipses are arriving later and later. The time between eclipses has been increasing by 0.1 microseconds with every eclipse. After 11 years, the eclipses are arriving two minutes later than in 1985. One possible explanation requires gas to be streaming out of one of the stars in a stellar wind so that the star is getting lighter and lighter. As the lightened star drifts further from its companion to maintain a stable orbit, the time it takes for the stars to orbit each other increases, and the eclipses are delayed.

Dr. Hertz's team, however, notes two problems with this theory. First, this star is losing mass 100 times faster than other stars do. Second, the time between eclipses, which might be expected to remain stable or increase at a smooth rate, is instead showing many little irregularities.

The researchers suggest that large amounts of matter are being ejected from the star at irregular intervals. Instead of the star losing 100 trillionths (10^-10) of itself yearly through a stellar wind, which is typical for stars of this type, the star in EXO0748-676 may be losing as much as 100 times more, or 10 billionths (10^-8) of its material, every year through a series of hot gas ejections. "If the stars keep moving apart at this rate, then eventually gas won't flow from one star to the other and we won't see any X-rays, " said Dr. Hertz, an astrophysicist in NRL's Space Science Division. "An evolution this rapid can help us understand how X-ray binaries form and what their ultimate fate is."

The star that blocks the X-rays is a normal star with about half the size and weight of the
Sun. The star radiating the X-rays is an exotic type of collapsed star called a neutron star. A neutron star weighs 50% more than the Sun, but is only 20 km (12 miles) across --- about the size of a city like Washington, DC. Neutron stars result when stars 3-10 times larger than the Sun use up all of their fuel and burn out. Without the heat and pressure to puff them up, they collapse under their own gravity into a tiny sphere.

In order to orbit each other in less than 4 hours, the stars must be very close together. Their centers are only 1 million km (600,000 miles) apart, so the entire binary system could fit inside the Sun. The stars are so close together that the gravitational force of the neutron star pulls gas from the atmosphere of the normal star onto itself. The gas swirls about the neutron star as it falls, like water flowing down a bathtub drain, and forms a disk of hot gas which makes the neutron star look somewhat like Saturn with its rings. When the gas flows onto the surface of the neutron star, it is falling so fast and has been heated so hot that X-rays are emitted.

For the last 11 years, astronomers have noted that the time between eclipses is changing. In 1991, a team of American and Japanese astronomers proposed the existence of a third star in the system about as far from the binary as Jupiter is from the Sun. They theorized that the binary orbited the third star, causing the time between eclipses to grow and shrink every 7 years as the binary moved further from and then closer to the Earth. The new data reported in Toronto are inconsistent with the earlier theory and require a new explanation. Drs. Hertz, Wood, and Cominsky believe that matter ejection from the normal star can explain the rapidly changing eclipse times in EXO0748 676.

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