Grazyna Stasinska (LUTH, Observatoire de Paris, Meudon, France)
The discrepancy between abundances derived from collisionally excited lines (CEL) and recombination lines (RL) in H II regions and planetary nebulae is still not fully understood.
As I understand, the effective recombination rates for the lines have been computed for electron temperatures “typical” of photoionized nebulae, i.e. Te < 20 000 K. What if inside the nebulae there are zones of much higher temperatures? Such zones can be produced inside dusty filamentary nebulae, as shown by Stasinska & Szczerba (2001), or in zones of shocked gas inside the nebulae. Could there not be a process occurring only at temperatures higher that 20 000 K, that would enhance the production of recombination lines (something similar to dielectronic recombination at high temperatures) ? Of course, CELs would also be affected by such high temperature zones, but differently and this could perhaps resolve the RL/CEL discrepancy.
I know that, presently, temperature diagnostics from recombination lines always indicate low temperatures. But those could be flawed as well, if the high temperature zones give a noticeable contribution.