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Hiiregion heavy-element abundances throughout the Galactic disk provide important constraints to theories of the formation and evolution of the Milky Way. In LTE, radio recombination line (RRL) emission and free–free continuum emission are accurate extinction-free tracers of the Hiiregion electron temperature. Since metals act as coolants in Hiiregions via the emission of collisionally excited lines, the electron temperature is a proxy for metallicity. Shaver et al. found a linear relationship between metallicity and electron temperature with little scatter. Here we use CLOUDY Hiiregion simulations to (1) investigate the accuracy of using RRLs to measure the electron temperature and (2) explore the metallicity–electron temperature relationship. We model 135 Hiiregions with different ionizing radiation fields, densities, and metallicities. We find that electron temperatures derived under the assumption of LTE are about 20% systematically higher owing to non-LTE effects, but overall LTE is a good assumption for centimeter-wavelength RRLs. Our CLOUDY simulations are consistent with the Shaver et al. metallicity–electron temperature relationship, but there is significant scatter since earlier spectral types or higher electron densities yield higher electron temperatures. Using RRLs to derive electron temperatures assuming LTE yields errors in the predicted metallicity as large as 10%. We derive correction factors for log(O/H) + 12 in each CLOUDY simulation. For lower metallicities the correction factor depends primarily on the spectral type of the ionizing star and ranges from 0.95 to 1.10, whereas for higher metallicities the correction factor depends on the density and is between 0.97 and 1.05.
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Azimuthal Metallicity Structure in the Milky Way Disk Revealed by Galactic HII Regions
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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- Award ID(s):
- 1714688
- PAR ID:
- 10342809
- Date Published:
- Journal Name:
- American Astronomical Society meeting #235
- Volume:
- 52
- Issue:
- 1
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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