Astron. Astrophys. 324, 65-79 (1997)

1. Introduction

Close to the galactic midplane one is most sensitive to stars with a low or moderate scale height. Information about stellar populations from an extended or thick disc can be constrained with observations at high galactic latitudes. The star counts from the north galactic pole field are well suited to disentangle and to determine the scale heights for the various disc populations. The apparent scale height for each spectral type is the cumulative result from different populations with each their own characteristic age, metallicity and scale height. For the determination of the scale heights one should start with the youngest population with the lowest scale height and proceed backwards to older populations with larger scale heights.

Observational data for the north and south galactic pole (hereafter respectively NGP and SGP) are available from several studies: Weistrop (1972); Faber et al. (1976); Chiu (1980); Kron (1980); Koo & Kron (1982); Gilmore, Reid & Hewett (1985, hereafter GRH85); Murray et al. (1986), Yoshii, Ishida & Stobie (1987); Stobie & Ishida (1987, hereafter SI87); Reid (1990, hereafter RD90); Majewski (1992); Soubiran (1992); and Reid & Majewski (1993, hereafter RM93). In this study the data for the NGP are taken from SI87, RD90, and RM93 and the star counts for the SGP are taken from GRH85.

In the direction of the galactic anti-centre (hereafter GAC) data are available from various sources: Friel & Cudworth (1986) in the direction of , Fenkart & Esin-Yilmaz (1983) and Yamagata & Yoshii (1992, hereafter referred to as YY92) in the direction of , and Ojha et al. (1994, hereafter referred to as OBRM94) for the field towards . For the analysis we used the data sets from YY92 and OBRM94.

Star counts from Selected Areas (Blaauw & Elvius 1965) and well studied fields are analyzed with the HRD-GST (HR-Diagram Galactic Software Telescope, see Ng 1994 & 1996 and Ng et al. 1995 for details, the latter is hereafter referred to as Paper I). The aim is to generate synthetic CMDs (Colour-Magnitude Diagrams) from first principles, without putting à priori observed quantities in the model and to determine in a self-consistent way the structural parameters (scale height, flattening of bulge and halo) and the evolutionary status (age-metallicity) of the stellar populations from fields directed towards the outer part of our Galaxy. The results are used as input for an overall galactic model and are then applied to fields in the direction of the SGP and GAC. In Sect. 2 a description is given of the method used for the analysis. The results are presented in Sect. 3 and they are discussed in Sect. 4.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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