NASA’s Chandra X-ray Observatory has confirmed that close encounters
between
stars form X-ray emitting, double-star systems in dense globular star
clusters. These X-ray binaries have a different birth process than their
cousins outside globular clusters, and should have a profound influence
on
the cluster’s evolution.
A team of scientists led by David Pooley of the Massachusetts Institute
of
Technology in Cambridge took advantage of Chandra’s unique ability to
precisely locate and resolve individual sources to determine the number
of
X-ray sources in 12 globular clusters in our Galaxy. Most of the
sources
are binary systems containing a collapsed star such as a neutron star
or a
white dwarf star that is pulling matter off normal, Sun-like companion
star.
“We found that the number of X-ray binaries is closely correlated with
the
rate of encounters between stars in the clusters,” said Pooley. “Our
conclusion is that the binaries are formed as a consequence of these
encounters. It is a case of nurture not nature.”
A similar study led by Craig Heinke of the Harvard-Smithsonian Center
for
Astrophysics in Cambridge, Mass. confirmed this conclusion, and showed
that
roughly 10 percent of these X-ray binary systems contain neutron stars.
Most of these neutron stars are usually quiet, spending less than 10% of
their time actively feeding from their companion.
A globular cluster is a spherical collection of hundreds of thousands or
even millions of stars buzzing around each other in a
gravitationally-bound
stellar beehive that is about a hundred light years in diameter. The
stars
in a globular cluster are often only about a tenth of a light year
apart.
For comparison, the nearest star to the Sun, Proxima Centauri, is 4.2
light
years away.
With so many stars moving so close together, interactions between stars
occur frequently in globular clusters. The stars, while rarely
colliding, do
get close enough to form binary star systems or cause binary stars to
exchange partners in intricate dances. The data suggest that X-ray
binary
systems are formed in dense clusters known as globular clusters about
once a
day somewhere in the universe.
Observations by NASA’s Uhuru X-ray satellite in the 1970’s showed that
globular clusters seemed to contain a disproportionately large number of
X-ray binary sources compared to the Galaxy as a whole. Normally only
one in
a billion stars is a member of an X-ray binary system containing a
neutron
star, whereas in globular clusters, the fraction is more like one in a
million.
The present research confirms earlier suggestions that the chance of
forming
an X-ray binary system is dramatically increased by the congestion in a
globular cluster. Under these conditions two processes, known as
three-star
exchange collisions, and tidal captures, can lead to a thousandfold
increase
in the number of X-ray sources in globular clusters.
In an exchange collision, a lone neutron star encounters a pair of
ordinary
stars. The intense gravity of the neutron star can induce the most
massive
ordinary star to “change partners,” and pair up with the neutron star
while
ejecting the lighter star.
A neutron star could also make a grazing collision with a single normal
star, and the intense gravity of the neutron star could distort the
gravity
of the normal star in the process. The energy lost in the distortion,
could
prevent the normal star from escaping from the neutron star, leading to
what
is called tidal capture.
“In addition to solving a long-standing mystery, Chandra data offer an
opportunity for a deeper understanding of globular cluster evolution,”
said
Heinke. “For example, the energy released in the formation of close
binary
systems could keep the central parts of the cluster from collapsing to
form
a massive black hole.”
NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra
program for the Office of Space Science, NASA Headquarters, Washington.
Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the
prime
development contractor for the observatory. The Smithsonian
Astrophysical
Observatory controls science and flight operations from the Chandra
X-ray
Center in Cambridge, Mass.
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