Far ultraviolet photometry of globular clusters with ANS. II - Energy distributions of 27 clusters

Far ultraviolet fluxes in five wavebands ranging from 1550 to 3300 A have been measured for 27 globular clusters with the ultraviolet experiment onboard the Astronomical Netherlands Satellite (ANS). The observations are described, and reddening-free color indices are presented. The dip in the spectra at 2200 A correlates well with the color excess E(B-V). Clusters with blue horizontal branches form two separate groups in the diagram. This dichotomy is related to the heavy element abundance of the clusters. The redder of these two blue groups contains the very metal-poor clusters, the bluer one the intermediate-metallicity clusters with a blue horizontal branch. From deep HR diagrams for two clusters in this group (NGC6752 and M13) it is concluded that the excess ultraviolet radiation is caused by a separate clump of very blue stars on an extension of the horizontal branch.