Astron. Astrophys. 348, 783-788 (1999)

1. Introduction

Recently we have completed a CCD Colour-Magnitude Diagram (CMD) survey of the known globular clusters within 5^o of the Galactic center, deriving accurate parameters, and the results are gathered in Barbuy et al. (1998a), where we discuss the overall structure and other characteristics of the old population of the central regions of the Galaxy. In Barbuy et al. (1998a) the sample consisted of 16 clusters out of the 17 known clusters in the region; the missing object Terzan 9 is now studied (Ortolani et al. 1999b).

Our focus is now turned towards a wider region extending to a 20^o20^o box around the Galactic center. This region is fundamental since it contains the transition bulge-halo (e.g. Minniti 1995a).

CCD CMDs obtained for clusters in the ring 5^o r 20^o have been steadily increasing in the last years, such as: M80 (NGC 6093) (Brocato et al. 1998), NGC 6256, NGC 6717 (Palomar 9) (Ortolani et al. 1999a; Brocato et al. 1996), NGC 6287 (Stetson & West 1995), NGC 6316, NGC 6342, NGC 6496, NGC 6539, Palomar 8 (Armandroff 1988), NGC 6380 and Terzan 12 (Ortolani et al. 1998), NGC 6558 (Rich et al. 1998), Terzan 3 and IC 1276 (Palomar 7) (Barbuy et al. 1998b) and Tonantzintla 2 (Bica et al. 1996). Not all clusters projected on the studied ring will be within the bulge volume, such as the case of IC 1257 for which Harris et al. (1997) derived a distance of 24 kpc from the Sun, thus placing it in the halo on the other side of the Galaxy.

NGC 6401 is a globular cluster projected in the studied region, and its parameters are not well determined in the literature. In particular, no optical CMDs are available. Minniti et al. (1995) presented an infrared CMD for this cluster, detecting the red giant branch (RGB), but not clearly reaching the horizontal branch (HB) level.

NGC 6401, also named ESO520-SC11 and GCL1735-238, is located at = , = -23^o 54´ 32" (l = 3.45^o, b = 3.98^o). Zinn & West (1984) report [Fe/H] = -1.13 derived from integrated spectroscopy. Bica & Alloin (1986), by means of a visible integrated spectrum, have found that line strengths and continuum distributions are compatible with [Fe/H] = -1.1 and E(B-V) = 0.81. Hesser & Shawl (1985) report an integrated spectral type F9, suggesting that the cluster could be moderately metal-rich. Webbink (1985) and Harris (1996) compilations provide respectively the following parameters: [Fe/H] = -1.01 and -1.12, = 17.3 and 17.7, E(B-V) = 0.76 and 0.85, = 7.1 and 7.2 kpc. Minniti (1995b) derived [Fe/H] = -1.10 and E(B-V) = 0.59. NGC 6401 shows some concentration with c = 1.69 (Trager et al. 1995).

In Sect. 2 the observations are described. In Sect. 3 we discuss the Colour-Magnitude Diagram and determine cluster parameters. In Sect. 4 a discussion of bulge or halo membership is given in the frame of a cluster sample in the central parts of the Galaxy.

© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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