New X-Ray observations made with ESA’s XMM-Newton and NASA’s Swift space telescopes have provided evidence for the interaction of fierce stellar winds in a huge binary star system about 4,700 light-years away.
Artist’s rendering of a colliding wind binary (NASA / C. Reed)
The most massive stars don’t live long enough to disperse throughout the galaxy. Instead, they can be found near the clouds of gas and dust where they formed – and where they will explode as supernovae after a few million years. They huddle in tight clusters with other young stars or in looser groupings called OB associations, a name reflecting their impressive populations of rare O- and B-type stars.
One of the nearest and richest associations of such stars in our galaxy is Cygnus OB2. The cluster hosts some 3,000 hot stars, including about 100 in the O-class. Weighing in at more than a dozen times the Sun’s mass and sporting surface temperatures five to 10 times hotter, these ginormous blue-white stars blast their surroundings with intense ultraviolet light and powerful outflows called stellar winds.
A binary system known as Cyg OB2 #9, which is located in the Cygnus OB2, 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 kilometers per hour, generating hot plasma at a million degrees which then shone brightly in X-rays. 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 image shows the massive star cluster Cyg OB2 (ESA / G. Rauw)
“This is the first time that we have found clear evidence for colliding winds in this system,” said Dr Yael Nazé of the Université de Liège, Belgium, and lead author of a paper describing the results reported in the journal Astronomy & Astrophysics (arXiv.org version). “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,” Dr Nazé said. “But in the Cyg OB2 #9 system 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,” said Dr Norbert Schartel, ESA’s XMM-Newton project scientist.
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Bibliographic information: Y. Nazé et al. 2012. The 2.35 years itch of Cyg OB2 #9 I. Optical and X-ray monitoring. Astronomy & Astrophysics 546, A37; doi: 10.1051/0004-6361/201219442
