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Astron. Astrophys. 347, 455-472 (1999)

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1. Introduction

The globular cluster system of the inner Milky Way is still not well understood. This is particularly true for the clusters' classification with respect of the galactic population structure. Because reliably determined parameters, e.g. metallicity, reddening, distance and age, are the basic requirement of any discussion, we present new photometry in (V, I) of the metal-rich globular clusters (GC's) NGC 5927, 6316, 6342, 6441 and 6760. We also re-discuss NGC 6528 and NGC 6553, where the data have already been published (Richtler et al. 1998, Sagar et al. 1998).

As there has been evidence for a correlation between metallicity and spatial distribution of the GC's since the late 1950's, Zinn (1985) classified the clusters via their kinematics, spatial distribution and metallicity into two subsystems: The disk-system with clusters of metallicity [FORMULA] dex and the halo-system with [FORMULA] dex. The disk-system shows a high rotational velocity and a small velocity dispersion, the halo-system vice versa. Armandroff (1989) derived a scale height of 1.1 kpc for the disk-system, which he identified with the galactic thick disk via rotational velocities and velocity dispersions. By comparing the metal-rich GC's of the inner 3 kpc with the underlying stellar population, Minniti (1995) assigned these objects to the bulge rather than to the disk. Burkert & Smith (1997) used kinematical arguments and the masses of the clusters to divide the metal-rich subsystem of Zinn (1985) into a bulge, a bar and a disk-group.

The thing these subdivisions have in common is, that they refer to the entire system of clusters and they try to formulate their criteria by identifying subsystems within the whole system. For the other way around, i.e. to classify observed objects with any of these subgroups, accurate parameters are needed. The halo clusters are well discernible from any other subsystem, but the metal-rich clusters near the galactic center are not. The determination of their parameters encounters observational difficulties, as their low galactic latitudes lead to strong contamination with field stars and to strong (differential) reddening. These effects have to be taken care of.

There is a variety of photometry existing for the program clusters. Recent studies on NGC 5927 were done by Fullton et al. (1996) and Samus et al. (1996). Armandroff (1988) presented and discussed CMDs including NGC 6316, 6342 and 6760. There is a photometry of NGC 6441 in (B,V) of Hesser & Hartwick (1976) and a more recent one in Rich et al. (1997). CMDs of NGC 6528 have been discussed by Ortolani et al. (1990) and Richtler et al. (1998). Guarnieri et al. (1998) as well as Sagar et al. (1998) present (V,I)-photometry of NGC 6553. Zinn (1985), Armandroff (1989), Richtler et al. (1994), Minniti (1995) and Burkert & Smith (1997) discuss the subdivision of the GC-system into a halo, disk and/or bulge component.

As the data and their reduction shall be published in a forthcoming paper, Sect. 2 deals only briefly with this subject. In Sects. 3 and 4, the derived CMDs are presented and the effects of differential reddening are discussed and removed. Sect. 5 contains the methods and results of the parameter determination, and in Sect. 6 we will discuss the resulting classification and its problems.

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999