Galactic H II regions: Evolution and galactic structure.

摘要

Nebulae ionized by OB stars, H II regions, are the formation sites of massive stars in our Galaxy. Since OB stars are short-lived, H II regions trace the locations of recent star formation. H II region classifications are based mainly on angular size, which is not an adequate indicator of an H I1 region's evolutionary state. The study of Galactic structure with H II regions has been hampered by the kinematic distance ambiguity (KDA) and by incomplete H II region samples. A large sample of H II regions is necessary to establish an H II region evolutionary sequence and to ascertain Galactic structure. Our sample contains 301 previously known ultra-compact (UC), compact, and diffuse inner Galaxy H II regions and 149 H II regions which were discovered by an ongoing survey using the Green Bank Telescope (GBT). The combined sample is sensitive enough to locate all H II regions ionized by single O-stars within the Solar orbit.;H II regions are commonly thought to evolve from UC to compact, before ending their lives as diffuse nebulae. The infrared, radio continuum, and molecular H II region properties derived here show no evidence that UC and compact nebulae are in different evolutionary stages. The properties of diffuse H II regions, however, suggest that the diffuse class does represent a later evolutionary state. We propose replacing the current classification scheme with one based on the physical size and radio continuum luminosity.;We resolve the KDA for 392 H II regions. The resulting nebular distances give a face-on map of the most recent sites of star formation in the first Galactic quadrant. This map shows two spiral arc features at Galactocentric radii of 4.5 and 6kpc, and a paucity of H II regions within 3.5 kpc of the Galactic center. The location of these features is independent of the choice of rotation curve, streaming motions, or a change in the Galactic rotation parameters. The GBT survey found 16 first-quadrant sources with negative recombination line velocities, placing them beyond the Solar orbit. These sources show good agreement with the Outer Arm of the Galaxy and appear to follow the Galactic warp.