Hp 1 : a blue horizontal branch globular cluster in the bulge

Abstract

We present V, I colour-magnitude diagrams of the globular cluster HP 1 obtained at the European Southern Observatory New Technology Telescope under exceptional seeing conditions. The cluster shows blue horizontal branch (HB) and red giant branch morphologies very similar to clusters of intermediate metallicity such as NGC 6752; we therefore estimate [Fe/H] ~~ - 1.5 for HP 1. The blue HB morphology is extended, which is not unusual for a post-core-collapse cluster. We derive a reddening of E (B - V) = 1.19 and a distance d̛ =6.85 kpc. HP 1 might be a halo cluster crossing the bulge or belong to the low-metallicity tail of the bulge.