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Cannibal
stars like their food hot, XMM-Newton reveals
23 March 2006
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Artist's
impression of a vast cloud of superheated gas whirling around
an asteroid-sized cannibal star, part of a low-mass X-ray
binary star system.
The clouds,
discovered by ESA's XMM-Newton space observatory, are
composed of iron vapour and other chemicals at many millions
of degrees and are located where the 'river' of matter from
the companion star strikes the disc. The clouds periodically
block the X-ray emission from the cannibal star and cause an
X-ray 'blinking'.
Credits: ESA
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ESA’s XMM-Newton has
seen vast clouds of superheated gas, whirling around miniature
stars and escaping from being devoured by the stars’
enormous gravitational fields - giving a new insight into the
eating habits of the galaxy’s ‘cannibal’
stars.
The clouds of gas range in size from a few
hundred thousand kilometres to a few million kilometres, ten to
one hundred times larger than the Earth. They are composed of
iron vapour and other chemicals at temperatures of many millions
of degrees.
"This gas is extremely
hot, much hotter than the outer atmosphere of the Sun," said
Maria Díaz Trigo of ESA’s European Science and
Technology Research Centre (ESTEC), who led the research.
ESA’s XMM-Newton x-ray
observatory made the discovery when it observed six so-called
‘low-mass X-ray binary’ stars (LMXBs). The LMXBs are
pairs of stars in which one is the tiny core of a dead star.
Measuring just 15–20
kilometres across and comparable in size to an asteroid, each
dead star is a tightly packed mass of neutrons containing more
than 1.4 times the mass of the Sun.
Its extreme density generates a
powerful gravitational field that rips gas from its ‘living’
companion star. The gas spirals around the neutron star, forming
a disc, before being sucked down and crushed onto its surface, a
process known as ‘accretion’.
The newly discovered clouds sit
where the river of matter from the companion star strikes the
disc. The extreme temperatures have ripped almost all of the
electrons from the iron atoms, leaving them carrying extreme
electrical charges. This process is known as ‘ionisation’.
The
discovery solves a puzzle that has dogged astronomers for several
decades. Certain LMXBs appear to blink on and off at X-ray
wavelengths. These are ‘edge-on’ systems, in which
the orbit of each gaseous disc lines up with Earth.
In previous attempts to
simulate the blinking, clouds of low-temperature gas were
postulated to be orbiting the neutron star, periodically blocking
the X-rays. However, these models never reproduced the observed
behaviour well enough.
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Artist's
impression of XMM-Newton, ESA's Earth-orbiting x-ray
observatory.
Credits: ESA
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XMM-Newton solves this by
revealing the ionised iron. "It means that these clouds are
much hotter than we anticipated," said Díaz. With
high-temperature clouds, the computer models now simulate much
better the dipping behaviour.
Some 100 known LMXBs populate
our galaxy, the Milky Way. Each one is a stellar furnace, pumping
X-rays into space. They represent a small-scale model of the
accretion thought to be taking place in the very heart of some
galaxies. One in every ten galaxies shows some kind of intense
activity at its centre.
This activity is thought to be
coming from a gigantic black hole, pulling stars to pieces and
devouring their remains. Being much closer to Earth, the LMXBs
are easier to study than the active galaxies.
"Accretion processes are
still not well understood. The more we understand about the
LMXBs, the more useful they will be as analogues to help us
understand the active galactic nuclei," says Díaz.
Source
/ Credit: ESA
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