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Astron. Astrophys. 348, 98-112 (1999)

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1. Introduction

The new Basel RGU high-latitude survey provides a three-color data base which is suitable for the determination of the larger-scale density and metallicity distributions of the major stellar population components of the Milky Way Galaxy. In Paper I, we have developed a least-squares algorithm for the statistical analysis of the data in terms of four-component parameterized Galactic models including a young and an old exponential thin disk, an exponential thick disk, and an oblate spheroidal halo component. Using the available photometric G and G-R star count and color data, the algorithm has been extensively tested by deriving optimized parameter values for both individual fields and the combined survey of seven fields through systematic exploration of parameter space. These preliminary test results have shown the data to be highly homogeneous and describable by a global model of the Galaxy whose structural parameter values are in general agreement with the canonical literature. Finally, we have shown that, based on the structural model thus derived, it is possible in principle to exploit the metallicity-sensitive U-G data for determining the larger-scale metallicity distribution of the Galactic thick disk as well.

In practice, however, failure of our initial attempt to prove or disprove in our data the existence of a metallicity gradient of the thick disk - which is crucial for discriminating between possible formation scenarios of this component - soon convinced us that our exploration and control of the structural models in general and of the metallicity effects in particular need to be advanced to significantly higher levels for a successful match to the quality of the available data.

Thus in this paper, we reexamine and upgrade our preliminary model (Buser & Rong 1995, 1996a,b, and Paper I) by extending the analysis (1) beyond the critical structural parameter ranges found so far, (2) to include more recent data on the component-specific luminosity functions, and (3) to account for improved photometric transformations used in calculating star counts and color distributions. In Sect. 2, we provide a more detailed discussion of the thick-disk local density and structural parameter values resulting from the model calculations described in Paper I. In particular, we show that models including only a weak (or even zero) thick-disk component can be plainly ruled out by the present data. However, we also show that the variation ranges of the thick-disk parameters explored in Paper I must be expanded in order to better account for field-to-field variations in the observed star counts and color distributions and to improve the determination of optimized parameter values and constraints. To this same end, we also describe new model calculations implementing (1) new input for the luminosity functions as derived from HIPPARCOS and from globular cluster data, and (2) more adequate UBV-RGU transformations for the red stars. In Sect. 3, results obtained from the [FORMULA]-analysis of these new model calculations are presented, and optimized thick-disk parameters hence derived are briefly discussed. In Sect. 4, we compare these results to other work and summarize our main conclusions. Finally, we briefly mention the prospects for future work on this survey project.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999
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