Galactic Temperature and Metallicity Gradients from Ultracompact H II Regions

Abstract

We report observations in the H42α, H66α, H76α, and H93α radio recombination lines (RRL) toward 28 ultracompact (UC) H II regions distributed in Galactocentric radius from 0 to 17 kpc. It was possible to fit the observed line intensities with a single set of electron density (n<SUB>e</SUB>) and temperature (T<SUB>e</SUB>)-values for 17 nebulae distributed in D<SUB>G</SUB> from 4 to 11 kpc using a non-LTE model analysis. A Galactocentric T<SUB>e</SUB> gradient was found of the form T<SUB>e</SUB>(K) = (5537±387) + (320±64)D<SUB>G</SUB>, where D<SUB>G</SUB> is the Galactocentric distance in kpc. On average, our sample is hotter than that found by Shaver et al. (1983) by approximately 1000 K. We attribute the higher average temperatures of the sources in our sample relative to those in the Shaver et al. sample to the higher density of our sample. As density increases from t0 cm<SUP>-3</SUP> to 10<SUP>5</SUP> cm<SUP>-3,</SUP> metal coolants are quenched and T<SUB>e</SUB> increases. <P />From the combination of photoionization-statistical equilibrium models at the densities of UC H II regions and the derived Te gradient, we find a Galactocentric oxygen abundance gradient d(O/H)/d(D<SUB>G</SUB>) =-0.047±0.009 dex kpc<SUP>-1</SUP> under the assumption that the T<SUB>e</SUB> gradient is primarily determined by metal abundances. Within the uncertainties, the O/H gradient is the same as that found by Shaver et al. from optical observations. The observed line widths are correlated with density in the sense that the denser regions are more likely to have large turbulent motions and/or bulk motions such as outflows, rotation, shocks, etc. Helium RRLs were detected in a subset of six nebulae. The He<SUP>+</SUP>/H<SUP>+</SUP> abundances are typically in the range 0.07-0.12 by number, consistent with the range found by other investigators from lower resolution observations. However, one source in our sample, G32.80A, has a ratio of 0.158±0.047, possibly indicating local enrichment of helium.