UBV and vbyβ stellar photometry in the eastern part of M31

Abstract

Abstract. We present UBV and the first vbyβ Strömgren stellar photometry in a field centered on the stellar association OB31 in the Eastern part of the Andromeda galaxy. The ages of the stellar populations are determined from the colormagnitude diagrams using the latest theoretical isochrones. HR diagram for all stars in the area is constructed. The mass of the brightest blue star is about 60M and for the brightest red star is about 20M . The slope of the Stellar Initial Mass Function (IMF) for blue stars is −2.0 ± 0.1 which is similar to that of other nearby galaxies. We investigate the stellar content of three van den Bergh associations as well as that of some new OB associations determined by cluster analysis. Ages of the van den Bergh associations are 4Myr for OB31 and 6Myr for OB32, OB34 and OB35. Eight blue supergiant candidates are isolated from the rest of the stars by using suitable color-color diagrams in Strömgren system. The results from UBV and Strömgren uvby photometry are compared.