Oxygen Recombination Line Abundances in Gaseous Nebulae
  The determination of the heavy element abundances from giant extragalactic H II regions has been generally based on collisionally excited lines. We will discuss the reasons to study the characteristics of recombination lines, and then use these lines to determine chemical abundances. Of these lines the oxygen (specifically the O II) lines are the most important; and, of them, the lines of multiplet 1 of O II are the most accessible. It has often been assumed that by measuring the intensity of a single line within a multiplet the intensities of all the lines in the multiplet can be determined; in recent studies we have found that the intensity ratios of lines within a multiplet can depend on density; we will present empirical density-intensity relationships for multiplet 1 based on recent observations of H II regions and planetary nebulae. From observations deof H II regions we find that the critical density for collisional redistribution of the multiplet 1 O II recombination lines amounts to 2800 +- 500 cm-3. We point out that the O/H recombination abundances of H II regions in the solar vicinity are in excellent agreement with the O/H solar value, while the abundances derived from collisionally excited lines are not. We present a calibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8 range based on O recombination lines. 
Antonio Peimbert and Manuel Peimbert