Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21713203b&link_type=abstract
American Astronomical Society, AAS Meeting #217, #132.03; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
Galactic chemical evolution models are important for understanding how galaxies form and evolve. A key observational constraint for these models is the spatial distribution of abundances in the Galaxy. We use hydrogen radio recombination line and continuum free-free emission observed with the Green Bank Telescope to calculate electron temperatures in 81 Galactic HII regions. Since metals, such as oxygen, are the main coolants in the photo-ionized nebular gas, the electron temperature is directly related to the distribution of heavy elements in the Milky Way. We convert the electron temperatures to metallicity (12 + Log(O/H), based on a sub-sample of objects with both electron temperature and O/H determinations. We find a radial metallicity gradient in the Galactic disk of about -0.03 to -0.06 dex/kpc with no significant discontinuity from 4-18 kpc. This is in contrast to previous studies that found a discontinuity in the gradient at 10 kpc. The radial metallicity gradient varies with azimuth, however, suggesting that the Galactic disk is not uniformly mixed as is often assumed in chemical evolution models.
The new GBT HRDS Galactic HII regions will sample a new zone in Galactic azimuth since they are systematically at the far kinematic distance. Electron temperature determinations for the HRDS nebulae may provide direct evidence for local enrichment. Moreover, the negative velocity HRDS sources are not only at the important Galactocentric radius zone of 9-12 kpc, but also lie at very large distances, > 12 kpc, from the Sun. Electron temperature determinations for these nebulae will probe the metallicity of an entirely new part of the Milky Way.
Anderson David L.
Balser Dana S.
Bania Thomas M.
Rood Robert T.
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