The galactic abundance gradient

Abstract

Chemical abundances in a large and representative sample of galactic H II regions covering a wide range in galactocentric radius RG were measured using radio and optical spectroscopy. Accurate electron temperatures in 67 H II regions spanning the range RG = 3.5-13.7 kpc were determined using radio recombination lines and these temperatures were applied to optical spectra of 33 of the same H II regions in order to determine the abundances of O, N, S, Ne, Ar, and He(+). Among other results, it is found that some H II regions have electron temperatures below 5000 K and that the radio-determined electron temperatures agree well with those obtained from the optical line ratios, in the light of standard models of H II regions. A gradient of H II region electron temperature with distance from the galactic center is found which equals +433 + or - 40 K/kpc, while the oxygen abundance gradient is -0.07 + or - 0.015 dex/kpc. The nitrogen abundance gradient is similar to that of oxygen, -0.09 + or 0.015 dex/kpc, while the sulfur abundance gradient (-0.01 + or - 0.02 dex/kpc) is significantly flatter than that of oxygen. No significant gradient in He(+)/H(+) is detected. In addition, evidence indicates that the abundance gradients may be steeper over the inner regions of the galactic disk.