Spectrophotometry of four H II regions in the Perseus arm and a reassessment of galactic abundance gradients

Abstract

The results from deep spectrophotometry of four Perseus arm H ii regions (NGC 1624, NGC 2359, NGC 7538, and NOC 7635) obtained with the KPNO lIDS are presented. The observed line strengths are used to derive electron temperatures, densities, and the relative concentrations of H, He, N, 0, S, Cl, and Ar in the nebulae. These abundances are combined with similarly derived abundances for seven other well-observed galactic H ii regions from previously published spectrophotometry and are used to estimate the magnitude of galactic abundance gradients over the 8-14 kpc region of the solar neighborhood. Significant radial gradients for oxygen and nitrogen are found: d log (O/H)/dR = -0.059 + 0.017 kpc-1 and d log (N/H)/dR = -0.083 + 0.018 kpc , while only marginal evidence for a helium gradient is found: d log (He/H)/dR = -0.008 + 0.008 kpc 1 Little evidence for a gradient in S/H is found. From the red [N ii] and [S ii] lines we found d log (N+/S+)/dR = - 0.068 + 0.024 kpc 1, which suggests that a radial gradient in SlO exists in the Galaxy. The 0/H gradient found in this study is similar to that found from planetary nebulae, while the N/H gradient is significantly smaller. These results suggest that most of the nitrogen in the interstellar medium of the Galaxy was produced by primary nucleosynthesis processes. We also find evidence that the radial 0/H and N/H gradients for H ii regions in a given spiral arm are 2 to 3 times steeper than the global gradients over the 8-14 kpc interval in galactocentric distance. For four H ii regions in the Sagittarius arm we found d log (0/H)/dR = - 0.24 kpc-1 and d log (N/H)/dR = -0.16 kpc-1. For five H ii regions in the Perseus arm we found d log (0/H)/dR = -0.18 kpc-1 and d log (N/H)/dR = -0.15 kpc-1. The larger magnitudes of these gradients are greater than those of the global gradients by amounts which exceed reasonable errors in abundances and/or the relative distances for the individual H ii regions. Subject headings: galaxies: Milky Way - galaxies: structure - nebulae: abundances