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The Galactic disk: study of four low latitude Galactic Fields

We present deep V and I photometry of stellar field 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. Two of them are suspected to cross the inner spiral arm. More than $10^5$ stars are detected in total, down to a magnitude of V $\sim 23.5$. 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$\pm 60$ pc seems to be favored. The data are very sensitive to the star formation rate of the thin disk. A decreasing star formation rate (SFR) is necessary to reproduce the distribution of the stars in the colour-magnitude diagrams (CMDs) as well as the luminosity functions. A constant or a strongly increasing SFR as derived using Hipparcos data for the solar neighborhood (Bertelli et al 1999) 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 SFR cannot be considered as representative for the whole thin disk. From the star-counts the local neighborhood mass density in stars more massive than 0.1 M$_\odot$ is found to be 0.036--0.02 M$_\odot$ pc$^{-3}$. Finally, the location of inner spiral arm is discussed. We find evidence of a population younger than 10$^8$ yr distributed in a spiral arm at a distance of 1.3$\pm 0.2$ and 1.5 $\pm 0.2$\,Kpc in the directions l$\sim$\,292 and l$\sim$\,305 respectively. This result is consistent with the spiral arm pattern defined on the basis of pulsars and young associations.