Battle of Giant Windy Stars
From NASA’s legendary Scientific Visualization Studio, here’s news of one of the nearest and richest stellar associations in our galaxy. Cygnus OB2, located about 4,700 light-years away, 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 ten 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. The observations are giving astronomers 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. The first opportunity arose in 2011.
NASA’s Swift made five sets of X-ray observations during the 10 months around the date of periastron, and XMM-Newton carried out one high-resolution observation near the predicted time of closest approach. The new data indicate that Cygnus OB2 #9 is a massive binary with components of similar mass and luminosity following long, highly eccentric orbits. The most massive star in the system has about 50 times the sun’s mass, and its companion is slightly smaller, with about 45 solar masses. At periastron, these stellar titans are separated by less than three times Earth’s average distance from the sun.
Two sets of measurements taken 5.5 days apart near the time of periastron — one in late June by XMM-Newton and one in early July by Swift — show that the X-ray flux increased by four times when the stars were closest together. This is compelling evidence for the interaction of fierce stellar winds.