5 The color-color diagram

Figure 14: Color-color diagram for the objects matched in U, V and $K_{\rm s}$ frames that had ALLFRAME photometry uncertainties <0.5 mag and magnitudes brighter than the 50% completeness limits in all bands. Field 1 objects are plotted with open squares and Field 2 with filled triangles. Overplotted is the Padova isochrone (Bertelli et al. 1994) for the solar metallicity and log(age)=7.0 yr that indicates the locus of the foreground stellar sequence. The reddening vector has dimensions corresponding to E(B-V)=0.1.

Figure 14 shows the color-color diagram for the objects detected in U-, V- and K-band in Field 1 (open squares) and Field 2 (filled triangles) that had ALLFRAME photometry uncertainties <0.5 mag and magnitudes brighter than the 50% completeness limits in all bands. The error-bars plotted are the ones given by ALLFRAME. A total of 99 objects in Field 1 and 90 in Field 2 satisfy these criteria. Most of them are located along the stellar sequence that crosses the color-color diagram in diagonal. Matching the UV and VK catalogues after the statistical subtraction of the foreground contamination, the stellar sequence disappears and only 54 and 44 objects with good photometry in all three filters are left in Field 1 and 2, respectively.

We use the color-color diagram presented in Fig. 14 to measure the reddening and the contamination by background galaxies.

5.1 Reddening

The objects crossing the color-color diagram in diagonal from top right to bottom left belong to the Milky Way. The quality of the fit of the stellar sequence with isochrones depends on the age of the isochrone with respect to stars. Because the foreground Milky Way stars span a range of ages, isochrones of different ages fit different parts of the stellar sequence. As an example, in order to guide the eye we overplotted the stellar loci of solar metallicity stars and log(age)=7.0 yr (Bertelli et al. 1994). The tight stellar sequence in the U-V vs. V-K color-color diagram shows that the foreground reddening in the two fields is the same. Its magnitude was measured by fitting the theoretical isochrones and it amounts to $E(B-V)=0.15\pm0.05$ mag. This is in excellent agreement with the Schlegel et al. (1998) extinction maps and the reddening determined by Fasset & Graham (2000), who used UBV color-color diagrams to measure a value of $E(B-V)=0.14\pm 0.02$ mag.

5.2 Star-galaxy separation

Using the color-color plots it is possible to distinguish the foreground dwarf stars from the most compact unresolved galaxies. Galaxies are located in the lower right part of the diagram in Fig. 14. The resolved galaxies are not plotted here since they got rejected by our selection of shape parameters in FIND and ALLFRAME. Note that the group of objects with -0.5<(V-K)<2 are the young stars detected in Field 1.