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Astron. Astrophys. 350, 840-846 (1999)

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

S. Ortolani 1,2, B. Barbuy 3, E. Bica 4, A. Renzini 2, G. Marconi 5 and R. Gilmozzi 2

1 Università di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 5, 35122 Padova, Italy (ortolani@pd.astro.it)
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany (sortolan,arenzini,rgilmozz@eso.org)
3 Universidade de São Paulo, CP 3386, São Paulo 01060-970, Brazil (barbuy@orion.iagusp.usp.br)
4 Universidade Federal do Rio Grande do Sul, Dept. de Astronomia, CP 15051, Porto Alegre 91501-970, Brazil (bica@if.ufrgs.br)
5 Osservatorio Astronomico di Roma, Via dell'Osservatorio 2, 00040 Monteporzio, Italy (marconi@mporzio.astro.it)

Received 1 July 1999 / Accepted 3 September 1999


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 [FORMULA] 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 [FORMULA]0.1, [FORMULA]0.5 kpc, thus placing Terzan 1 within the bulge.

Key words: stars: Hertzsprung – Russel (HR) and C-M diagrams – Galaxy: globular clusters: individual: Terzan 1

* Observations obtained at the Space Telescope Science Institute, with the NASA/ESA Hubble Space Telescope, operated by AURA Inc. under contract to NASA

Send offprint requests to: B. Barbuy

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© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999