Densities, temperatures, pressures, and abundances derived from OII recombination lines in HII regions and their implications

Abstract: Based on high quality observations of multiplet V1 of OII and the NLTE atomic computations of OII we study the density and temperature of a sample of HII regions. We find that the signature for oxygen rich clumps of high density and low temperature is absent in all objects of our sample: one extragalactic and eight Galactic HII regions. The temperatures derived from: a) recombination lines of OII, and b) recombination lines of HI together with Balmer continua are lower than those derived from forbidden lines, while the densities derived from recombination lines of OII are similar or smaller than densities derived from forbidden lines. Electron pressures derived from collisionally excited lines are about two times larger than those derived from recombination lines. These results imply that the proper abundances are those derived from recombination lines and suggest that other processes in addition to direct photoionization, such as dissipation of turbulent energy in shocks, magnetic reconnection, and shadowed regions, might be responsible for the large ADF and t2 values observed in HII regions.