Spiral structure and stellar populations in the solar neighbourhood (Prof. Jacques R. D. Lépine, IAG-USP - Brasil)

We present a new image of the spiral structure of the Galaxy, using as spiral arm tracer sources of CS molecular emission associated with star-formation regions. A spiral arm which passes close to the Sun, at a  smaller galactic radius, presents a strong break in angle, which is confirmed by means of other tracers, like Cepheids and Open Clusters. The break in angle of this arm is consistent with stellar orbits at the  4:1 resonance (4 epicycle rotations in one galactic revolution), with a maximum elongation close to the Sun. From the details of the spiral structure of our Galaxy, one can see that there is in the solar neighbourhood a mixture of stars belonging to distinct groups of orbits, and therefore, were born at different galactic radii, have different metallicities and occupy different regions in the U-V plane of galactic velocities. The analysis of the connexion between the spiral structure and the stellar populations  helps us to understand recent fine details of the metallicity gradients in the Galaxy, including the azimuthal gradient. This work was greatly beneficiated by the results of proper motion and parallax surveys like Hipparcos/Tycho2, and by the radio molecular survey made by Leonardo Bronfmann in Chile. The discovery of new and more distant Cepheids in the galactic plane, with their follow-up in the near infrared to determine their average magnitude during a period, will be made possible by the VISTA survey and will be of great importance for the galactic work.