Wide-field multi-color photometry of the Galactic globular cluster NGC 1261^*,**

¹ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile e-mail: vkravtsov@ucn.cl
² Sternberg Astronomical Institute, University Avenue 13, 119899 Moscow, Russia
³ Isaac Newton Institute of Chile, Ministerio de Educación de Chile, Casilla 8-9, Correo 9, Santiago, Chile e-mail: inewton@terra.cl; falvarad@eso.org
⁴ ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: gmarconi@eso.org

Received: 12 October 2009
Accepted: 4 March 2010

Abstract

Aims. This work studies in more detail the stellar population, including its photometric properties and characteristics, in the rarely studied southern Galactic globular cluster NGC 1261. We focus on the brighter sequences of the cluster's color–magnitude diagram (CMD). Like in our previous works, we rely upon photometry in several passbands to achieve more reliable results and conclusions.

Methods. We carried out and analyzed new multi-color photometry of NGC 1261 in UBVI reaching below the turnoff point in all passbands in a fairly extended cluster field, about 14' × 14'.

Results. We found several signs of the inhomogeneity (“multiplicity”) in the stellar population. The most prominent of them are: (1) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, with brighter stars less centrally concentrated at the 99.9% level than their fainter counterparts; (2) the dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD on their radial distance from the cluster center, with the portion of stars bluer in the (U-B) color increasing towards the cluster outskirts. Additionally, the radial variation of the RGB luminosity function in the bump region is suspected. We assume that both the SGB stars brighter in the U and the RGB stars bluer in the (U-B) color are probably associated with blue horizontal branch stars, because of a similarity in their radial distribution in the cluster. We estimated the metalicity of NGC 1261 from the slope of the RGB in U-based CMDs and the location of the RGB bump on the branch. These metallicity indicators give [Fe/H]_ZW = -1.34 ± 0.16 dex and [Fe/H]_ZW = -1.41 ± 0.10 dex, respectively. We isolated 18 probable blue straggler candidates. They are more centrally concentrated than the lower red giants of comparable brightness at the 97.9% level. Their photometric characteristics imply that their majority is not consistent with the collisional origin. We also reliably isolated the asymptotic giant branch (AGB) clump and estimated the parameter = 1.01 ± 0.06, that is the difference between the V-levels of the zero age HB and the clump.