For the
first time, astronomers have pinpointed the spot where the intense winds of two
massive stars in a binary system violently collide and detected the production
of high-energy X-rays there.
The
monstrously large Eta Carinae binary contains between 100 and 150 times the
mass of the sun and glows more brightly than four million suns together. The
so-called hypergiant contains two massive stars, the second of which was not discovered
until 2005.
Astronomers
have long suspected that the stellar pair should give off high-energy X-rays,
but until now, they didn't have the instruments to detect the radiation. But
the European Space Agency's Integral
telescope, launched to detect some of the most violent events in the
universe, has conclusively detected such X-rays emanating from Eta Carinae,
more or less as astronomers had thought.
"The
intensity of the X-rays is a little lower than we expected, but given that this
is the first-ever conclusive observation, that's OK," said Jean-Christophe
Leyder of the University of Liege in Belgium, one of the astronomers who made
the discovery.
Stellar
wind shockwave
The intense
X-rays are generated by the collision of the massive stars' stellar winds,
flows of charged gas ejected from the stars' upper atmospheres.
The light
and other radiation that "blows" off these particles is so strong
that the stellar winds of Eta Carinae
can reach speeds of 900 to 1200 miles per second (1,500 to 2,000 kilometers per
second). Because the two stars are in such close proximity, the winds collide
in a ferocious shockwave where temperatures reach several thousand million
degrees Kelvin.
"It's
a very tough environment," Leyder said.
Electrons
get caught in the magnetic environment of the shockwaves and are bounced back
and forth, accelerating to huge energies. When they finally burst out of the
shockwave, they collide with low-frequency photons and give them an energy
boost, creating the high-energy X-ray emissions spied by Integral.
Search
for other emitters
Finding
other examples of colliding-wind binaries, as astronomers call them, is tough
because massive stars are rare; finding two in a binary system is rarer still.
"In
our galaxy, there are probably only 30 to 50 colliding-wind binaries that
display a clear signature of wind-wind collision," Leyder said. This
meager number of stars is just a tiny fraction of the galaxy's stellar
population. So to have an example such as Eta Carinae on our cosmic doorstep is
a stroke of luck.
Astronomers
did detect X-rays emitted from another colliding-wind binary, HD 5980, in our
galactic neighbor the Small
Magellanic Cloud last year.
The X-rays
detected from Eta Carinae are of a much higher energy though. Astronomers estimate
that the Eta Carinae system loses one Earth mass per day of ejected material,
roughly 140 times higher than the mass loss rate of HD 5980.
Understanding
the X-ray emissions from stellar winds is important because stellar winds
affect the evolution of stars and the chemical evolution of the universe, while
also acting as a source of energy in the galaxy.
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