STELLAR PHOTOMETRY OF THE GLOBULAR-CLUSTER NGC-6229 .1. DATA REDUCTION AND MORPHOLOGY OF THE BRIGHTER PART OF THE CMD

BV CCD photometry of the central (1.5' X 2.0') part of the mildly conc entrated outer-halo globular cluster NGC 6229 is presented. The data r eduction in such a crowded field was based on a wavelet transform anal ysis. Our larger dataset extends the previous results by Carney et al. [AJ, 101, 1699, (1991)] for the outer and less crowded fields of the cluster, and confirms that NGC 6229 has a peculiar color-magnitude dia gram for its position in the Galaxy. In particular, NGC 6229's horizon tal branch (HB) presents several interesting features, among which sta nd out: a well populated and very extended blue tail; a rather blue ov erall morphology, with (B - R)/(B + V + R) = 0.24 +/- 0.02; a bimodal color distribution, resembling those found for NGC 1851 and NGC 2808; and gaps on the blue HB. NGC 6229 is the first bimodal-HB cluster to b e identified in the Galactic outer halo. A low value of the R paramete r is confirmed, suggestive of a low helium abundance or of the presenc e of a quite substantial population of extreme HB stars fainter than o ur photometric limit (similar or equal to 2.5 mag below the RR Lyrae l evel in V). Twelve new possible variable stars were found in the centr al part of the cluster. The morphology of the red giant branch (RGB) a lso seems to be peculiar. In particular, the RGB luminosity function ' 'bump'' is not a prominent feature and has only been tentatively ident ified, on the basis of a comparison with a previously reported detecti on for M3 (NGC 5272). We also find evidence that red HB stars are more centrally concentrated than blue HB stars and the RGB population. Fin ally, we compare the properties of NGC 6229 with those for other outer -halo globular clusters, and call attention to what appears to be a bi modal HB distribution for the outer-halo cluster population, where obj ects with very red or very blue HB types are much more frequently foun d than clusters with intermediate HB types. (C) 1997 The American Astr onomical Society.