7. Discussing the presence of a spiral arm
As already pointed out in the previous sections, the CMDs of fields F3 and F4 (see Fig. 3) show the presence of a few stars brighter than V mag for F4 and F3 respectively and bluer than the average main sequence. Stars in this part of the CMD can be interpreted as tracers of a very young population located in a spiral arm. The idea of a spiral arm is strongly supported by the fact that we find this young population in the fields F3 and F4, which are in the direction of the Sgr-Car arm (see Fig. 1 and Fig. 12), whereas the fields F1 and F2, which are not expected to cross the spiral arm, do not show any evidence for such a population.
The spiral arm has been parameterized following Vallee (1995)where however the parameters of the spiral pattern are set to reproduce our data. We adopt an arm/inter-arm density ratio of 2 as found in external galaxies (Rix & Zaritsky 1995) and suggested for our own Galaxy by Efremov (1997). The spiral arm is supposed to have a Gaussian distribution with a of 300 pc as suggested by Taylor & Cordes (1993), and Vallee (1995). We impose that the age of the spiral arm population is younger than 1 y. From CMD simulations, we find out that the faintest magnitude of the blue population gives hints about the distance of the spiral arm from us (see Fig. 4). We find out that the distance of the spiral arm in the direction of F3 is about 1.3 kpc, with the maximum of the distribution at 2 kpc. In the direction of F4, we find that the spiral arm is at a distance of 1.5 kpc. These results are of course dependent on the adopted parameterization of the spiral arm. They are consistent with the spiral arm pattern defined by the pulsar distribution (Taylor & Cordes (1993)) or derived from optical observations for the local solar environment (Humphreys (1976)).
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000