Far ultraviolet photometry of globular clusters with ANS. I. Two different groups of blue horizontal branch clusters.

Photometric observations in the far ultraviolet with ANS of 12 globular clusters with blue horizontal branches show that the colour indices of these clusters form two separate groups. This dichotomy can be understood in terms of differences in their horizontal branch morphologies, i.e., presence or absence of very blue stars on the horizontal branch, and is related to differences in heavy element abundance. Preliminary synthesis of the integrated colours by modelling the contributions of horizontal branch and turn-off stars indicates that the UV colours are sensitive to the properties of the turn-off region and can be used to derive ages for individual clusters.