HST observations of Terzan 1 : a second parameter globular cluster in the galactic bulge

Abstract

Hubble Space Telescope observations in the F555W and F814W bands are presented of the bulge globular cluster Terzan 1. This highly obscured cluster has the smallest projected distance to the Galactic center among all known globulars. Its colour-magnitude diagram shows a red horizontal branch, which is typical of metal-rich clusters, combined to a very steep red giant branch, which instead is typical of metal-poor clusters. These features make it similar to the second parameter halo cluster NGC 362. Terzan 1 is therefore another second parameter cluster in the Galactic bulge, and the first one in the bulge combining a steep red giant branch with a red horizontal branch. As in the case of NGC 362, these features of the colour-magnitude diagram can be accounted for by the cluster being 2 Gyr younger than other clusters with the same metallicity but showing a blue horizontal branch. These findings demonstrate that second parameter effects are not confined to the Galactic halo, but are actually encountered at all galactocentric distances. Despite the unusual sequences in the colour-magnitude diagram, the derivation of reddening and distance is straightforward, and we obtained E(B − V ) = 2:48 0.1, d = 5:2 0.5 kpc, thus placing Terzan 1 within the bulge.