X-ray Satellites Monitor the Clashing Winds of a Colossal Binary
The hottest and 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.
Artist's rendering of a colliding wind binary.
One of the nearest and richest OB associations in our galaxy is Cygnus OB2, which is located about 4,700 light-years away and 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.
Two of these stars can be found in the intriguing binary system known as Cygnus OB2 #9. In 2011, NASA's Swift satellite, the European Space Agency's XMM-Newton observatory and several ground-based facilities took part in a campaign to monitor the system as the giant stars raced toward their closest approach.
Now, a paper published in the October issue of the journal Astronomy and Astrophysics provides the campaign's first results and gives a more detailed picture of the stars, their orbits and the interaction of their stellar winds.
An O-type star is so luminous that the pressure of its starlight actually drives material from its surface, creating particle outflows with speeds of several million miles an hour. Put two of these humongous stars in the same system and their winds can collide during all or part of the orbit, creating both radio emission and X-rays.
In 2008, research showed that Cygnus OB2 #9 emitted radio signals that varied every 2.355 years. In parallel, Yael Nazé, an astronomer at the University of Liège in Belgium, detected for the first time a signature in the system's optical spectrum that indicated the presence of two stars. The binary nature of Cygnus OB2 #9 provided a natural explanation for the periodic radio changes.
To maximize their chances of catching X-rays from colliding winds, the researchers needed to monitor the system as the stars raced toward their closest approach, or periastron.
Two of these stars can be found in the intriguing binary system known as Cygnus OB2 #9. In 2011, NASA's Swift satellite, the European Space Agency's XMM-Newton observatory and several ground-based facilities took part in a campaign to monitor the system as the giant stars raced toward their closest approach.
Now, a paper published in the October issue of the journal Astronomy and Astrophysics provides the campaign's first results and gives a more detailed picture of the stars, their orbits and the interaction of their stellar winds.
An O-type star is so luminous that the pressure of its starlight actually drives material from its surface, creating particle outflows with speeds of several million miles an hour. Put two of these humongous stars in the same system and their winds can collide during all or part of the orbit, creating both radio emission and X-rays.
In 2008, research showed that Cygnus OB2 #9 emitted radio signals that varied every 2.355 years. In parallel, Yael Nazé, an astronomer at the University of Liège in Belgium, detected for the first time a signature in the system's optical spectrum that indicated the presence of two stars. The binary nature of Cygnus OB2 #9 provided a natural explanation for the periodic radio changes.
To maximize their chances of catching X-rays from colliding winds, the researchers needed to monitor the system as the stars raced toward their closest approach, or periastron.
Source: NASA
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