The Metallicity-Electron Temperature Relationship in HII Regions

Abstract

HII region 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) and free-free continuum emission are accurate extinction-free tracers of the HII region electron temperature. Since metals act as coolants in HII regions 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 HII region 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 HII regions with different ionizing radiation fields, densities, and metallicities. We find that electron temperatures derived under the assumption of LTE are about 20% systematically higher due to non-LTE effects, but overall LTE is a good assumption for cm-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 range 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.