SIMBAD references

Based on high-quality observations of multiplet V1 of O II and the NLTE atomic computations of O II, we study the density and temperature of a sample of H II 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 H II regions. The temperatures derived from (1) recombination lines (RLs) of O II, and (2) RLs of H I together with Balmer continua are lower than those derived from forbidden lines, while the densities derived from RLs of O II 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 RLs. These results imply that the proper abundances are those derived from RLs 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 abundance discrepancy factor and t ² values observed in H II regions.