Observational Constraints to the Evolution of Massive Stars

We consider some aspects of the evolution of massive stars which can only be elucidated by means of "indirect" observations, i.e. measurements of the effects of massive stars on their environments. We discuss in detail the early evolution of massive stars formed in high metallicity regions as inferred from studies of HII regions in external galaxies.Comment: 6 pages, 1 figure; Invited Paper presented at the Roma-Trieste Workshop 1999 "The Chemical Evolution of the Milky Way: Stars versus Clusters", Vulcano Island (ME, Italy), 20-24 September, 1999, eds. F. Giovannelli & F. Matteucci, Kluwer-Holland (in press

Physical conditions and molecular chemistry of the Central Molecular Zone

We discuss the physical conditions of the different kineniatical components of the Central Molecular Zone. In particular we compare the properties of the clouds moving with in elongated orbits along the Galactic bar with those of the well-known Galactic center ring (GCR) clouds (Sgr A, Sgr B2,...). We show that all the components contain dense clouds that can withstand the tidal shear. The SiO abundance in the clouds with non-circular velocities is high (~ 10−8), in perfect agreement with that of the GCR clouds. We discuss the role of the UV radiation and C-shocks in the heating of the neutral gas and the high abundances of some molecules like SiO. The SiO emission in the clouds moving in elongated trajectories is probably due to the cloud collisions expected in the inner regions of a bar