PHOTOMETRIC CLASSIFICATION OF STARS ON THE BLUE HORIZONTAL-BRANCH

CCD Stromgren uvby photometry has been done on a number of globular clusters at KPNO and CTIO. The first CCD observations were made with an RCA chip with effective dimensions of 322 x 511 pixels, giving a viewing area of 2.8 x 4 arcmin2 with a 0.9-m telescope. In M22, 12 blue horizontal-branch (BHB) stars were found in one field, but in some of the other clusters only 6 BHB stars were found and when these stars are broken into two different groups the statistics become very poor. The RCA chips were insensitive in the uv and no reliable u magnitudes could be obtained; thus no cl index could be calculated. In (b - y), mi diagrams the distribution of points representing BHB stars in different clusters showed common features. There was a tight distribution of points on a lower envelope and above this relation there were other points, 0.2 mag or brighter. The interpretation of this distribution is that the points falling on the lower relation represent stars that are on the zero-age horizontal branch, evolving to the blue. The points scattered above represent more evolved stars that are evolving back up towards the red asymptotic branch. Thus, uvby photometry can distinguish stars of different evolutionary stages on the horizontal branch.