3. Color - magnitude diagram of the CG 30/CG 31/CG 38 region
In Fig. 2 we present the (V-I) - V diagram for the 6168 stars. We notice that the diagram has a very well defined blue confinement. Stars along the blue demarcation are probably among the most distant and less reddened in the sample. We were immediately struck by the almost linear shape of the blue confinement, which is somewhat surprising. In a study of the reddening close to the plane in a nearby direction (l, b) = (262o, +4o), Jonch-Sorensen & Knude (1994) noticed a blue protrusion from the confinement representing the most distant less reddened stars, see Fig. 10 of Jonch-Sorensen & Knude (1994). The protrusion was suggested to be a metal weak 8 Gyr old population. This protrusion seems absent in Fig. 2. The absence is possibly caused by the interplay of the limiting V and I magnitudes of 205 and 185-190 respectively, for V = 20 the I limit may exclude the bluest stars; another explanation is that (V-I) is less sensitive to blanketing than (b-y) which was used in the study mentioned. This could in fact influence the results deduced from the (V-I) distribution in the cloud and reference fields respectively, in order to have included remote relatively blue stars the I limit should have been fainter.
The small photometric errors cause the well defined appearance of Fig. 2. The widening of the main sequence band is a real effect and not due to errors increasing with magnitude. The stars are almost exclusively spread to the red for two physical reasons: partly because they may be reddened and partly because the less luminous stars are intrinsically redder. Another noteworthy trait is the sparsely populated but linear confinement of the stellar distribution to the bright and red side in the diagram, an interpretation is offered in Sect. 6.
© European Southern Observatory (ESO) 1999
Online publication: October 14, 1999