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Astron. Astrophys. 331, 934-948 (1998)

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1. History and introduction

The Basel Palomar-Schmidt RGU-photographic survey has been a major project that pioneered the study of the Galactic field halo when it was begun in the mid-sixties (Becker 1965). With the growth of the plate material obtained in an increasing number of fields, the following two decades saw the gradual emergence of a large-scale halo component whose significantly flattened density distribution was in sharp contrast to the essentially spherical halo occupied by the globular clusters (Becker 1980, Buser & Kaeser 1985). However concurrently, new photometric, spectroscopic, and proper motion surveys accumulated increasing independent evidence of a thick disk population component with a density gradient that appeared to be similarly intermediate between the thin disk and the conventional spheroid as was observed in the Basel halo data (e.g., Reid & Gilmore 1982). Indeed, Bahcall et al. (1985) and Fenkart (1989a-d) subsequently showed that the Basel RGU high-latitude survey data could also be described with a model containing a thick disk component having a local density of 2 percent and a scale height of order 1 kpc. It thus became evident that, since the Basel RGU survey penetrates to faint enough magnitudes ([FORMULA] or [FORMULA]) for a significant sampling of this intermediate population component, advantage must be taken of its unique properties of providing three-color data - including the metallicity-sensitive [FORMULA] colors - and systematic directional coverage of a significant number of fields in order to derive more reliable estimates of the larger-scale structural parameters of the density, luminosity, and metallicity distributions of the Galactic thick disk stars than had been inferred from the previous limited analyses of the original survey data.

This task, however, could not be attempted using the available original catalogs, because detailed investigations of the data in a number of individual fields revealed the existence of inhomogeneities, systematic calibration errors, and color equations that could not be corrected by simple means a posteriori but that were shown to be traceable to the lack of an adequate standard of the RGU photometric system (Buser 1988). We thus determined to establish the required standard and calibrations using synthetic photometry techniques, and to construct a new homogeneous data base from the available plate material ab initio (Buser & Fenkart 1990).

Construction of the new data base was begun in 1990 and will be completed in 1997. The catalog will be published in Papers III and IV of this series (Buser et al. 1997a,b) and is briefly described in Sect. 2 below. In Sect. 3 we describe the basic model calculations that we have been using to analyse the catalog. The tools and procedures for the analysis are detailed in Sect. 4. A discussion of test results obtained from the first-half sample of the data for seven fields is then given in Sect. 5 to demonstrate the feasibility of the present approach. Finally, in the concluding Sect. 6 we give a brief outline of the work covered in the subsequent papers of this series.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998