X-raying
stellar winds in a high-speed collision
Two
massive stars racing in orbit around each other have had their colliding stellar
winds X-rayed for the first time, thanks to the combined efforts of ESA’s
XMM-Newton and NASA’s Swift space telescopes. Stellar winds, pushed away from a
massive star’s surface by its intense light, can have a profound influence on
their environment.
In
some locations, they may trigger the collapse of surrounding clouds of gas and
dust to form new stars. In others, they may blast the clouds away before they
have the chance to get started. Now, XMM-Newton and Swift have found a ‘Rosetta
stone’ for such winds in a binary system known as Cyg OB2 #9, located in the
Cygnus star-forming region, where the winds from two massive stars orbiting
around each other collide at high speeds.
Colliding
winds at Cyg OB2 #9
Cyg
OB2 #9 remained a puzzle for many years. Its peculiar radio emission could only
be explained if the object was not a single star but two, a hypothesis that was
confirmed in 2008.At the time of the discovery, however, there was
no direct evidence for the winds from the two stars colliding, even though the
X-ray signature of such a phenomenon was expected.
This
signature could only be found by tracking the stars as they neared the closest
point on their 2.4-year orbit around each other, an opportunity that presented
itself between June and July 2011.As the space telescopes looked on, the fierce
stellar winds slammed together at speeds of several million kilometres per hour,
generating hot plasma at a million degrees which then shone brightly in X-rays.
Colliding
winds at WR 22
The
telescopes recorded a four-fold increase in energy compared with the normal
X-ray emission seen when the stars were further apart on their elliptical
orbit.“This is the first time that we have found clear evidence for
colliding winds in this system,” says Yael Nazé of the Université de Liège,
Belgium, and lead author of the paper describing the results reported in
Astronomy & Astrophysics.
“We
only have a few other examples of winds in binary systems crashing together, but
this one example can really be considered an archetype for this
phenomenon.”Unlike the handful of other colliding wind systems, the style of the
collision in Cyg OB2 #9 remains the same throughout the stars’ orbit, despite
the increase in intensity as the two winds meet.
“In
other examples the collision is turbulent; the winds of one star might crash
onto the other when they are at their closest, causing a sudden drop in X-ray
emission,” says Dr Nazé.“But in the Cyg OB2 #9 systems there is no
such observation, so we can consider it the first ‘simple’ example that has been
discovered – that really is the key to developing better models to help
understand the characteristics of these powerful stellar winds.
”
“This
particular binary system represents an important stepping stone in our
understanding of stellar wind collisions and their associated emissions, and
could only be achieved by tracking the two stars orbiting around each other with
X-ray telescopes,” adds ESA’s XMM-Newton project scientist Norbert
Schartel.
For
further information visit: http://www.esa.int/esaCP/ SEM3H93S18H_index_0.html
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