Massive stars in CYG OB2
Abstract
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.