(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.![](https://webarchive.library.unt.edu/web/20161103080810im_/https://www.nrl.navy.mil/PressReleases/1997/1-97r.jpeg)
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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