A photometric survey of the rich OB association NGC 206 in M31

Abstract

A detailed UBV photometric survey of the stellar content of the rich OB association NGC 206 has been carried out with two high-quantum-efficiency area detectors. Weighted mean B band magnitudes are derived and used to construct a differential luminosity function, which is in turn transformed to an absolute scale using the estimated total absorption value of 1.15 + or - 0.13 and the geometric distance modulus of 24.07. The B band luminosity function for NGC 206 is transformed to a present day mass function. An initial mass function (IMF) is derived whose power law fit gives a mass-function index value of -2.0 + or - 0.3. This value is consistent with several independent determinations of the slope of the IMF for massive stars. The brightness of the most massive stars in NGC 206 is found to correlate most strongly with the integrated magnitude of the spiral arms and somewhat less with the integrated old-disk luminosity. This indicates that bulge luminosities should be considered in refining the use of the brightest blue stars as extragalactic distance indicators.