Deep CCD photometry in the V and I pass-bands is presented for 4 low latitude Galactic fields. The studied fields are located between l=265 and l=305.
The scale height and the scale lenght of the various populations are derived. The data seem to favor a disk scale height around and a scale lenght 1100 pc. The star formation seems to have been decreasing in time, while a constant or increasing star formation rate as the one found using Hipparcos data in the solar neighborhood, cannot reproduce the features of the CMDs: too many blue stars are expected.
From the star-counts in the studied fields we derive the local mass density in stars more massive than 0.1 , found to be in the range 0.036-0.022 pc-3.
In two fields, namely F3 and F4 located at l=292 and l=305 evidence is found for the presence of a very young population. The data can be reproduced imposing the presence of a spiral arm. In these direction the Sgr-Car arm is crossed at a distance of 1.3 and 1.5 Kpc respectively. These results are consistent with previously defined spiral arm patterns (Taylor & Cordes (1993); Humphreys (1976)).
Due to the small field of view, two of the studied fields, F1 and F2 do not set strong constrains on the scale height and the scale lenght of the disk. A larger field of view (see e.g. the WFI at the 2.2m ESO telescope having 30´ 30´) would allow us to have good statistics down to faint magnitudes. At l=305, b=-4.8 more than 104 disk stars down to V=21 are expected in such a large field. In a forthcoming paper we will discuss the Galactic structure towards the center making use of the good statistics of the WFI data.
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000