Spiral structure and age distribution of the H II regions in the Milky Way.

ABSTRACT. Radioastronomical measurements giving thie ratio R = VHe+/VH+ were used to determine the age of galactic H II regions with ionization parameter U = Uorion = 65 pc cm. Models of evolution of R vs. age show that even when the well known gradient of metallicity is taken into account, a difference arises among the histogram of ages of the three analized arms. The Car-Sag structure is younger than inner Scutt-CruxNorma and the outer Per-Orion spiral arm. Keq wo'tJ : GALAXY-STRUCTURE - NEBULAE-H II REGIONS