Abundances in Diffuse Nebulae

A scheme for determining nebular abundances is presented that allows for temperature variations in a nebula along the line of sight. It is demonstrated that neglect of temperature fluctuations, for certain conditions of excitation, can cause significant underestimates of the abundances of oxygen, nitrogen, and neon with respect to that of hydrogen. For a wide range of excitation conditions, reliable abundance ratios of neon and nitrogen relative to oxygen, and neon and oxygen relative to helium may be obtained even when these abundances cannot be obtained relative to hydrogen. The determination of abundances of neon and nitrogen with respect to that of oxygen is also not very sensitive to temperature variations in the nebula. Recent observations are used in conjunction with improved atomic parameters to obtain abundances for some of the brightest H ii regions. These abundances have been corrected, where the observations permit, for changes in the electron temperature along the path of observation through the nebula