3.1. Color-Magnitude Diagram
The Color Magnitude Diagrams (CMD) of the observed field are presented in Fig. 3. The CMD of F1 is dominated by a wide RGB sequence, running from I ~ 20.6 down to the limiting magnitude of the photometry. The exact location of the TRGB (as derived in Sect. 3.2, below) is reported in the CMDs as a horizontal line to the red of the observed RGB. The sources brighter and redder than the RGB Tip are likely bright Asymptotic Giant Branch (AGB) stars, probably associated with an intermediate-age (and/or metal-rich) population (see Davidge, 2003, for an extensive discussion). The presence of a conspicuous plume of intermediate-young Main Sequence (MS) stars reaching I 19.4 is also evident on the blue side of the CMD, with a sharp blue edge at V - I 0.0. In the CMD of F2 only the RGB and AGB population are discernible, while the blue MS stars are completely absent. Star counts on the RGB indicate that the stellar density drops by a factor > 20 from F1 to F2.
Figure 3. Color Magnitude Diagrams of the two observed fields. The errobars show the average photometric uncertainty as a function of magnitude; the horizontal lines marks the position of the TRGB as determined in Sect. 3.2.
The contamination by foreground Galactic stars is negligible for the purposes of the present study. The Galactic model by Robin et al. (2003) predicts fewer than 200 Galactic stars in the observed fields within the color and magnitude ranges spanned by the CMDs of Fig. 3. By inspection of the synthetic CMD obtained with the Robin et al. (2003) model we conclude that even in the poorly populated F2 the only region of the CMD that may be significantly affected by Galactic contamination is that which hosts bright AGB stars, e.g. V - I 1.8 and V 20.6 (see Sect. 3.4, below).