Close Encounters of the Stellar Kind (Forwarded)

Steve Roy
Media Relations Dept.
Marshall Space Flight Center, Huntsville, AL

Phone: 256-544-6535

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, MA

Phone: 617-496-7998

Science Contacts:
Dave Pooley, , 617-253-7457
Craig Heinke, , 617-495-2536

July 30, 2003

CXC Press Release: 07-01

Close Encounters of the Stellar Kind

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 a 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.

The image and additional information are available at:

http://chandra.harvard.edu/photo/2003/ngc6266/
and
http://www1.msfc.nasa.gov/NEWSROOM/n...tos03-130.html