Massive stars in CYG OB2

The stellar content of Cyg OB2 has been investigated using CCD photometry and spectroscopy. The intrinsically bluest stars have been isolated for spectroscopy by using a reddening-free parameter. The spectroscopy has made it possible to determine an improved distance modulus to the association (11.2 + or - 0.1) and has provided the means of accurately placing stars in the H-R diagram. The H-R diagram shows a well-defined main sequence extending upward to 85 solar masses. There are a few slightly evolved B supergiants of lower mass, suggesting that star formation did not take place strictly in a coeval manner. The initial mass function of Cyg OB2 is found to have a slope of -0.1 + or - 0.1, much flatter than that generally found for massive stars in the Galaxy or Magellanic Clouds.

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