EVOLVED STARS IN OMEGA CENTAURI .1. RADIAL-DISTRIBUTION OF BLUE SUBDWARFS

We have obtained CCD photometry of ten fields in omega-Centauri, providing full coverage of one quarter of the cluster at radial distances between 3 and 14 arcmin. We present a U - V color-magnitude diagram containing over 5000 stars at or brighter than the main-sequence turnoff. By adding and reducing artificial stars in the CCD frames, we obtain completeness corrections as a continuous function of magnitude and radial distance (as opposed to traditional binning techniques). Using this completeness correction, we show that the blue subdwarfs are centrally concentrated with respect to subgiants and stars on the horizontal branch proper. The chance probability of this result is less than 1%. Since the blue subdwarfs are likely to consist of helium burning cores of about 0.5 M. surrounded by a thin hydrogen envelope, mass segregation could not produce this result if these stars have evolved singly. Therefore we suggest that these stars are the result of binary evolution. Two kinds of possible precursor systems are considered: pairs of degenerate dwarfs which subsequently merge to form helium burning stars, and moderately wide binaries in which mass transfer is initiated shortly before the helium flash is ignited. The latter mechanism seems particularly promising for the blue subdwarfs in omega-Cen, although it may not apply in other clusters which contain them. The study of such blue underluminous "horizontal-branch" stars, if indeed they are produced from binary systems, might provide information on the initial frequency of binary systems and the dynamics of globular clusters.