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The shocking behaviour of massive stars in Cygnus OB2

posted Oct 26, 2015, 10:02 AM by Jeremy Drake
The amount of light a star puts out - its luminosity - depends strongly on its mass.  For massive stars, luminosity depends on mass to the power 3.5 so that a 20 solar mass star is about 3000 times more luminous than a 2 solar mass star. For the very rare stars with masses more than about fifteen times that of the Sun, dubbed "O-type" stars in the general scheme of stellar classification, the luminosity is so great that the light exerts more outward force on gas in its outer atmosphere than the inward force of gravity.  The result is a strong radiation-driven wind of stellar material into space at speeds of 1000 km per second or more. 

Such radiation-driven winds pushed out against gravity are generally not stable and are probably very clumpy. Clumps of gas can be moving at different speeds and inevitably run into each other.  The shocks of such collisions heat the gas to millions of degrees and generate X-rays that we can observe with facilities like the Chandra X-ray Observatory.  As part of a larger project to study massive star formation, we observed the Cygnus OB2 Association with Chandra and examined the X-ray output of the 50 or so massive O-type stars known in the cluster.  

We found a well-defined correlation between the X-ray output and the stellar luminosity, with the X-ray strength being about sixteen million times less.  A handful of stars though were up to ten times more X-ray luminous, with gas temperatures up to 10 million degrees.  Most are known binary stars in which the winds from each star collide in a terrific shock between them.  The relations we found are similar to those of earlier studies, indicating that the wind energetics depend on little but the stellar luminosity.  This work was lead by Belgian astronomer Gregor Rauw, and was published in the 2015 November issue of the Astrophysical Journal Supplement Series..