The Galactic disk: study of four low latitude Galactic fields

We present deep V and I photometry of stellar fields in four previously uns tudied low latitude regions of the Galactic disk. All observed fields are l ocated at the western side of the Galactic Center in the direction of the C oalsack-Carina region. They are chosen on the large scale surface photometr y of the Milky Way (Hoffmann et al., 1998 and Kimeswenger ct al., 1993) and corresponding term maps (Schlosser et al., 1995) as being affected by low interstellar absorption and having integrated colours typical of a very you ng population. Two of them are suspected to cross the inner spiral arm. Mor e than 10(5) stars are detected in total, down to a magnitude of V similar to 23.5. The observational colour-magnitude diagrams (CMDs) and luminosity functions are analyzed using a revised version of the Padova software descr ibed in Bertelli et al. (1995). The interstellar extinction along the line of sight is derived and found to be in reasonable agreement with Mendez & v an Altena (1998) maps. Due to the low galactic latitude of the studied fiel ds, 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+/-60 pc seems to be favored. The data are very sensitive to t he star formation rate of the thin disk. A decreasing star formation rate i s necessary to reproduce the distribution of the stars in the colour-magnit ude diagrams as well as the luminosity functions. A constant or a strongly increasing star formation rate as derived using Hipparcos data for the sola r neighborhood (Bertelli et al. 1999) are marginally in agreement with the luminosity functions but they are at odds with the CMDs. The analysis of th ese 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 ma ss density in stars more massive than 0.1 M. is found to be 0.036-0.02 M. p c (-3). Finally, the location of inner spiral arm is discussed. We find evi dence of a population younger than 10(8) yr distributed in a spiral arm at a distance of 1.3+/-0.2 and 1.5+/-0.2 Kpc in the directions 1 similar to 29 2 and 1 similar to 305 respectively. This result is consistent with the: sp iral arm pattern defined on the basis of pulsars and young associations (Ta ylor & Cordes (1993); Humphreys (1976)). 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 E SO telescope having 30' x 30' would allow us to have good statistics down t o faint magnitudes.