X-raying stellar winds in a high-speed collision

X-raying stellar winds in a high-speed collision
XMM-Newton observation of the core of the very massive cluster Cyg OB2 located in the constellation of Cygnus, 4700 light-years from Earth. The cluster contains massive O-type stars that shine brightly in X-rays. Cyg OB2 #9 is seen as the bright star just below the central star, and in 2008 was determined to be a binary system. ESA’s XMM-Newton and NASA’s Swift space telescopes have now found evidence for the winds of these hot, massive stars colliding at the closest points of their orbits, resulting in a four-fold increase in X-ray emission. Hundreds of lower mass stars also fill the field of view, which spans nearly 22 x 16 arcminutes. Credits: ESA/G. Rauw

(Phys.org)—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 .

In others, they may blast the clouds away before they have the chance to get started.

X-ray satellites monitor the clashing winds of a colossal binary
This is an artist's rendering of a colliding wind binary. Credit: NASA/C. Reed

Now, XMM-Newton and Swift have found a '' for such winds in a binary system known as Cyg OB2 #9, located in the Cygnus star-forming region, where the winds from two orbiting around each other collide at high speeds.

This computer simulation shows the gas density around a colliding wind binary named WR 22. The star locations are marked by yellow dots, and decreasing gas density is indicated by the sequence of red, white and blue colours. In this simulation, a 26-solar-mass O-type star orbits the hotter and more massive (72 solar mass) Wolf-Rayet star WR 22. Because WR 22 possesses the stronger stellar wind, a bow shock and wake is formed by the O star as it orbits. The stellar orbits are eccentric, so the separation contracts and expands. Consequently, the conditions in the shocked gas around the O star changes; as the stars near closest approach, the gas cools so effectively that it becomes very clumpy. During closest approach, WR 22's stellar wind is so intense that the region where the winds collide is actually forced back onto the O star. Such a collapse of the wind region does not occur with Cygnus OB2 #9, which makes it a well-behaved system for exploring wind-wind interactions. Simulations like this are being applied to Cygnus OB2 #9 in support of the recent observational campaign to help unravel the properties of its wind-wind collision. Copyright: Australian National University/E.R. Parkin, University of Liège/E. Gosset

Cyg OB2 #9 remained a puzzle for many years. Its peculiar 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.

The Cygnus OB2 #9 binary system consists of two nearly equal-mass O stars that follow an eccentric orbit around their common centre of gravity. Observations with ESA's XMM-Newton and NASA's Swift observatories showed a quadrupling of X-ray flux around the time of periastron (June/July 2011) - when the stars were close together in their orbits. This has provided the first concrete evidence that a wind-wind collision is taking place in this system. Previous observations with XMM-Newton had hinted at the presence of colliding stellar winds but those observations were carried out when the two stars were far from each other. Copyright: NASA/Goddard Space Flight Center

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.

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 .

"This is the first time that we have found 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."

O-type stars are among the most massive and hottest known, pounding their surroundings with intense ultraviolet light and powerful outflows called stellar winds. NASA's Swift and ESA's XMM-Newton X-ray observatories took part in a 2011 campaign to monitor the interaction of two O stars bound together in the same binary system: Cygnus OB2 #9. Credit: NASA's Goddard Space Flight Center

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 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 . "

"This particular 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.


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More information: "The 2.35 years itch of Cyg OB2 #9 I. Optical and X-ray monitoring" by Y. Nazé et al., is accepted for publication in Astronomy & Astrophysics. Preprint: arxiv.org/abs/1209.5622v2
Journal information: Astronomy & Astrophysics

Citation: X-raying stellar winds in a high-speed collision (2012, October 12) retrieved 24 November 2020 from https://phys.org/news/2012-10-x-raying-stellar-high-speed-collision.html
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