Scientists have gazed into an incredibly dense star cluster with
NASA’s Chandra X-ray Observatory and identified a surprising bonanza of
binary stars, including a large number of rapidly rotating neutron stars.
The discovery may help explain how one of the oldest structures in our
Galaxy evolved over its lifetime.
By combining Chandra, Hubble Space Telescope, and ground-based radio
data, the researchers conducted an important survey of the binary systems
that dominate the dynamics of 47 Tucanae, a globular cluster about 12
billion years old located in our Milky Way galaxy.
Most of the binaries in 47 Tucanae are systems in which a normal,
Sun-like companion orbits a collapsed star, either a white dwarf or a
neutron star. White dwarf stars are dense, burnt-out remnants of stars like
the Sun, while neutron stars are even denser remains of a more massive star.
When matter from a nearby star falls onto either a white dwarf or a neutron
star, as in the case with the binaries in 47 Tucanae, X-rays are produced.
“This Chandra image provides the first complete census of compact
binaries in the core of a globular cluster,” said Josh Grindlay of the
Harvard-Smithsonian Center for Astrophysics (CfA) and lead author of the
report that appears in the May 18 issue of Science. “The relative number of
neutron stars versus white dwarfs in these binaries tell us about the
development of the first stars in the cluster, and the binaries themselves
are key to the evolution of the entire cluster core.”
Many of the binaries in 47 Tucanae are exotic systems never before
seen in such large quantities. Perhaps the most intriguing are the
“millisecond pulsars”, which contain neutron stars that are rotating
extremely rapidly, between 100 to nearly 1000 times a second.”
“The Chandra data, in conjunction with radio observations, indicate
that there are many more millisecond pulsars than we would expect based on
the number of their likely progenitors we found,” said co-author Peter
Edmonds, also of the CfA. “While there is a general consensus on how some
of the millisecond pulsars form, these new data suggest that there need to
be other methods to create them.”
In addition to the millisecond pulsars, Chandra also detected other
Important populations of binary systems, including those with white dwarf
stars and normal stars, and others where pairs of normal stars undergo large
flares induced by their close proximity.
The Chandra data also indicate an apparent absence of a central
black hole. Stellar-sized mass black holes — those about five to ten times
as massive as the Sun – have apparently not coalesced to the center of the
star cluster. All or most stellar-sized black holes that formed over the
lifetime of the cluster have likely been ejected by their slingshot
encounters with binaries deep in the cluster core.
“These results show that binary star systems are a source of
gravitational energy which ejects stellar mass black holes and prevents the
collapse of the cluster’s core to a more massive, central black hole,” said
the CfA’s Craig Heinke. “In other words, binary systems – not black holes –
are the dynamical heat engines that drive the evolution of globular
clusters.”
Chandra observed 47 Tucanae on March 16-17, 2000, for a period of
74,000 seconds with the Advanced CCD Imaging Spectrometer (ACIS). The ACIS
X-ray camera was developed for NASA by Penn State and the Massachusetts
Institute of Technology. The High Energy Transmission Grating Spectrometer
was built by MIT. NASA’s Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program. TRW, Inc., Redondo Beach, Calif. is the prime
contractor for the spacecraft. The Smithsonian’s Chandra X-ray Center
controls science and flight operations from Cambridge, Mass.
Images associated with this release are available on the World Wide
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