GALACTIC TEMPERATURE AND METALLICITY GRADIENTS FROM ULTRACOMPACT H-IIREGIONS

We report observations in the H42 alpha, H66 alpha, H76 alpha, and H93 alpha radio recombination lines (RRL) toward 28 ultracompact (UC) H I I regions distributed in Galactocentric radius from 0 to 17 kpc. It wa s possible to fit the observed line intensities with a single set of e lectron density (n(e)) and temperature (T-e)-values for 17 nebulae dis tributed in D-G from 4 to 11 kpc using a non-LTE model analysis. A Gal actocentric T-e gradient was found of the form T-e(K)=(5537 +/- 387) (320 +/- 64)D-G, where D-G 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. sa mple to the higher density of our sample. As density increases from 10 (3) cm(-3) to 10(5) cm(-3) metal coolants are quenched and T-e increas es. From the combination of photoionization-statistical equilibrium mo dels at the densities of UC H II. regions and the derived T-e gradient , we find a Galactocentric oxygen abundance gradient d(O/H)/d(D-G) = - 0.047 +/- 0.009 dex kpc(-1) under the assumption that the T-e 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 opt ical observations. The observed line widths are correlated with densit y in the sense that the denser regions are more likely to have large t urbulent motions and/or bulk motions such as outflows, rotation, shock s, etc. Helium RRLs were detected in a subset of six nebulae. The He+/ H+ abundances are typically in the range 0.07-0.12 by number, consiste nt with the range found by other investigators from lower resolution o bservations. However, one source in our sample, G32.80A, has a ratio o f 0.158 +/- 0.047, possibly indicating local enrichment of helium.