Antonio Peimbert (Instituto de Astronomia–UNAM, Mexico)
The two most common ways of determining chemical abundances on ionized regions are: 1.- the one that comes from assuming homogeneous temperature and using the observed collisionally excited line ratios (the direct method) and 2.- the one that comes from using recombination lines. There exists a systematic difference between the abundances derived from these two methods; temperature fluctuations are capable of producing these discrepancies. The presence of significant thermal inhomogeneities in an ionized region is capable of distorting the emitted line ratios of collisionally exited lines; equivalently, determining chemical abundances while assuming temperature homogeneity when there is none will produce an underdetermination of the abundances when using collisionally excited lines while the abundances determined using recombination lines will not be affected. Observational evidence supporting the presence of thermal inhomogeneities will be discussed; I will also discuss possible explanations for their presence. This problem is present in H II regions and Planetary Nebulae. The use of collsionally exited lines requires a correction, due to the presence of thermal inhomogeneities, to obtain accurate abundances. I recommend an average correction due to thermal inhomogeneities for statistical studies.