2000A&A...361...73V

We present deep V and I photometry of stellar fields in four previously unstudied low latitude regions of the Galactic disk. All observed fields are located at the western side of the Galactic Center in the direction of the Coalsack-Carina region. They are chosen on the large scale surface photometry of the Milky Way (Hoffmann et al., 1998A&AS..128..417H and Kimeswenger et al., 1993A&AS...97..517K) and corresponding term maps (Schlosser et al., 1995, Astr. Ges. Abstr. Ser. 11, 190) as being affected by low interstellar absorption and having integrated colours typical of a very young population. Two of them are suspected to cross the inner spiral arm. More than 10⁵ stars are detected in total, down to a magnitude of V∼23.5. The observational colour-magnitude diagrams (CMDs) and luminosity functions are analyzed using a revised version of the Padova software described in Bertelli et al. (1995A&A...301..381B). The interstellar extinction along the line of sight is derived and found to be in reasonable agreement with Mendez & van Altena (1998A&A...330..910M) maps. Due to the low galactic latitude of the studied fields, the scale length and mainly the scale height of the thick disk are not strongly constrained by the observations. However a thin disk scale height of about 250±60pc seems to be favored. The data are very sensitive to the star formation rate of the thin disk. A decreasing star formation rate is necessary to reproduce the distribution of the stars in the colour-magnitude diagrams as well as the luminosity functions. A constant or a strongly increasing star formation rate as derived using Hipparcos data for the solar neighborhood (Bertelli et al., 1999BaltA...8..271B) are marginally in agreement with the luminosity functions but they are at odds with the CMDs. The analysis of these data suggest that the solar neighborhood star formation rate cannot be considered as representative for the whole thin disk. To properly reproduce the luminosity functions a thick disk component having a local density of about 2-4% must be included. From the star-counts the local neighborhood mass density in stars more massive than 0.1M_☉ is found to be 0.036-0.02M_☉/pc³. Finally, the location of inner spiral arm is discussed. We find evidence of a population younger than 10⁸yr distributed in a spiral arm at a distance of 1.3±0.2 and 1.5 ±0.2Kpc in the directions l∼292 and l∼305 respectively. This result is consistent with the spiral arm pattern defined on the basis of pulsars and young associations (Taylor & Cordes (1993ApJ...411..674T); Humphreys (1976PASP...88..647H)). Due to the small field of view two of the studied fields do not set strong constrains on the scale height and lenght of the disk. A larger field of view, see e.g. the WFI at the 2.2m ESO telescope having 30'x30' would allow us to have good statistics down to faint magnitudes.