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Astron. Astrophys. 361, 73-84 (2000)

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1. Introduction

During the past years many efforts have been undertaken to gain information about the structure of the Milky Way. However, several unresolved questions still remain. A program has been recently started to study the structure of the Galaxy by means of deep photometry of stellar populations. Several fields in the direction of the Galactic center has been been analyzed with the aim of studying the Galactic structure; deriving the stellar extinction along the line of sight; obtaining information about the age and the metallicity of the stars in various Galactic components (halo, bulge and disc); deriving the past history of star formation and chemical enrichment.

In brief, Ng et al. (1995) studied the color magnitude diagram (CMD) and luminosity functions (LF) of the field #3 of the Palomar Groningen Survey (PG3) located at the periphery of the Galactic Bulge (l=0, b=-10) and set up the basic tool to deconvolve the contribution from many stellar population to highly composite CMDs. Bertelli et al. (1995) analyzed three regions of the Bulge located near the clusters NGC6603 (l=12.9 b=-2.8), Lynga 7 (l=328.8 b=-2.8) and Terzan 1 (l=357.6 b=1.0). First they show that the extinction law greatly varies with the direction towards the center. Second, they trace the position of a molecular ring between 3.5 and 4 kpc in the direction of NGC6603, of a stellar ring between 3.0 and 4.0 kpc, of the Sagittarius arm in the direction of NGC6603 (5.0 to 7.0 kpc) and Lynga 7 (4.2 to 7.0 kpc). Third, they recognize the presence near the Galactic Center of an old and metal rich population (see also the detailed study of the CMD of Terzan 1 by Bertelli et al. 1996 in which evidences for the existence of old, high metallicity stars in the so-called hot horizontal branch stage are found), and finally give a hint that the Galactic Bar points its nearest side toward positive galactic longitude.

This paper is devoted to the study of four low latitude fields in the Galactic disk in the direction of the Coalsack-Carina region. Studying the CMDs and luminosity functions it is possible to determine the age and star formation history of the thin disk components, the extinction along the line of sight derive hints about the scale height and length.

In Sect. 2 and in Sect. 3 the observations and the data reduction respectively are described. In Sect. 4 the CMDs are presented. In Sect. 6.1 the model and the input parameters are given. In Sect. 6 the results are discussed. The presence of a spiral arm at l[FORMULA] 292 and l[FORMULA] 305 is analyzed in Sect. 7. Finally, the conclusions are drawn in Sect. 8.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000