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Astron. Astrophys. 362, 959-967 (2000)

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1. Introduction

Galaxy disks constitute important subsystems in many galaxies, in addition to halos and bulges, which tell us about the chemical evolution history of such galaxies. In the case of the Milky Way, the disk plays an important role in the Galaxy history, connecting its formation to the present during approximately 2/3 of the Galaxy life. In this framework, the Galactic open cluster system provides a valuable tool to investigate the dynamical and chemical evolution of the Galactic disk, since open clusters are found throughout the whole disk and roughly spread over its full age range. In addition, open clusters can supply accurate ages, distances and metallicities in a more expeditive way than individual stars. Colour-magnitude diagrams (CMDs) have long been used to derive their fundamental parameters as well as to put new observational constraints on the stellar evolutionary models. In particular, open clusters located towards the Galactic centre are of great interest as they offer the possibility to explore in detail the structure and evolution of the inner disk more precisely.

The advent of CCD detectors and computing facilities has allowed one to build up deeper and more populous CMDs of relatively small angular size objects. Indeed, the success of CCD technique results are particularly evident in studies of compact star clusters. This is why during the last decade numerous works of faint clusters projected in obscure and/or crowded fields have appeared. CMDs of such clusters, however, often suffer from high interstellar absorption, field star contamination, or both effects combined, which need to be corrected before deriving any cluster parameter. At times CMDs contain two star sequences - one of them being the genuine cluster sequence - or even only one sequence of field stars corresponding on the sky to a chance grouping. The presence of an apparent stellar concentration in the sky is not at all a sufficient condition for inferring that we are dealing with a physical cluster. This criterion is probably valid in the case of globular clusters or very concentrated open clusters. But, for most of the apparent stellar concentrations it is necessary to supplement the photometric data with spectroscopic and, if possible, also kinematical data (proper motion and/or radial velocities) in order to confirm their real physical existence. However, since the number of open clusters with available spectroscopic and kinematical data is much smaller than those which have photometric data, frequently the latter is the only set of data from which the existence of an open cluster may be inferred from. Differences in the star density in some region of the sky can be caused either by the presence of a genuine open cluster, by a random fluctuation of the star density in that region, by the presence of interstellar material inhomogeneously distributed, or by some combination of these three effects. We recall that more than half of the 1200 presently catalogued open clusters have been poorly studied or not studied yet. Therefore, the confirmation of the physical reality of cluster candidates alone, is a valuable contribution to our knowledge of the open cluster system.

The present study is part of an ongoing long-term program which consists in obtaining CCD photometric and spectroscopic data of southern open cluster candidates preferably projected towards the inner Galactic disk (galactocentric distances [FORMULA] 8 kpc). The candidates were selected taking into account the following criteria: (i) first, objects from early cataloguing studies for which little or no information is available, together with some control clusters. (ii) The angular sizes of the clusters were basically limited to 4´, in order to facilitate the observation with CCD detectors covering relatively small fields, and also to allow one to obtain long slit integrated spectra with background sampling (some objects with larger angular sizes were also added) and (iii) according to the stellar density and morphology of the candidates they were expected to be genuine clusters.

In this work we present the results obtained for three of the selected open clusters, namely: Ruprecht 119 (OCl-963, [FORMULA]=16h28m15s, [FORMULA]=-51o30´14"; l =333o.27, b=-1o.88), NGC 6318 (OCl-1004, [FORMULA]=17h16m12s, [FORMULA]=-39o25´30"; l =347o.90, b=-0o.69), and BH 245 ([FORMULA]=17h46m15s, [FORMULA]=-29o42´01"; l =359o.41, b=-0o.51). Ruprecht 119 is located in Norma, NGC 6318 in Scorpius and BH 245 in Sagittarius, all projected close to the Galactic centre. Notice that the original coordinates of NGC 6318 appearing in early catalogs are off, the correct ones appeared in van den Bergh & Hagen (1975, hereinafter vdBH75), therein as BH 218, and in Lauberts (1982, therein indicated both as NGC 6318 and ESO333-SC1). Only Ruprech 119 has some information available. Moffat & Vogt (1973, hereinafter MV73) observed 13 stars in the field of Ruprecht 119 using UBV photoelectric photometry and concluded that the object is not a cluster. On the other hand, vdBH75 identified for the first time a questionable object visible in red plates and not visible in blue plates, which they catalogued as a probable very poor cluster (BH 245).

This paper is structured as follows: in Sect. 2 we present the observations and briefly describe the reduction procedures. In Sect. 3 we present the results obtained from the photometric and spectroscopic data, respectively, and discuss the adopted cluster parameters. Finally, Sect. 4 summarizes the main conclusions of this work.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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