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The present-day metallicity structure of the Galactic disk is the product of billions of years of chemodynamical evolution. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to measure 8-10 GHz radio continuum and hydrogen radio recombination line (RRL) emission toward 82 Galactic HII regions. Since collisionally excited lines from metals (e.g., oxygen, nitrogen) are the primary cooling mechanism in ionized gas, the HII region electron temperature is empirically correlated to the nebular metallicity. We use the RRL-to-continuum ratio to derive electron temperatures and infer metallicities of these Galactic HII regions. Including previous single dish studies, there are now 167 nebulae with radio-determined electron temperatures and either parallax or kinematic distance determinations. The HII region oxygen abundance gradient across the Milky Way disk has a slope of -0.052 ± 0.004 dex/kpc. We find significant azimuthal structure in the metallicity distribution. The slope of the oxygen abundance gradient varies by a factor of ~2 between Galactocentric azimuths of 30 degrees and 100 degrees. Such azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms.
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Metallicity Structure in the Milky Way Disk
Metallicity structure provides a critical constraint on the formation history and subsequent chemical evolution of the Milky Way. In thermal equilibrium the abundance of the coolants (O, N, and other heavy elements) in the ionized gas regulates the electron temperature, with high abundances producing low temperatures. Here we attempt to better calibrate this relationship between the plasma electron temperature, Te, and O/H by observing [OIII] (52 and 88 μm), [NIII] (57 μm), and [NII] (122 μm) toward 9 HII regions with the Herschel telescope. We derive Te in HII regions with radio recombination lines (RRLs) and use them as proxies for the nebular O/H abundances. We derive ionic abundance ratios in the well studied HII region W3A to test our calibration and analysis procedures. We find that the O/H abundance ratio varies by a factor of 5 across W3A with uncertainties that are as large as 50%, inconsistent with previous results. We suspect that the standard calibration procedures employed by Herschel, which assume the source is uniform, explains the large O/H variations in W3A.
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- Award ID(s):
- 2202340
- PAR ID:
- 10515848
- Publisher / Repository:
- AAS243 Abstracts
- Date Published:
- Journal Name:
- Bulletin American Astronomical Society
- Volume:
- 56
- Issue:
- 2
- ISSN:
- 0002-7537
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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