The Electron Temperature in Ionized Nebulae

We discuss the role of the electron temperature in abundance determinations in ionized nebulae (planetary nebulae and giant H II regions). We show that, even when observations provide a direct estimate of T e, abundance determinations may sometimes be significantly in error for reasons other than hypothetical temperature fluctuations or uncertainties in ionization correction factors, and we show striking examples both in the high and the low metallicity regimes. On the other hand, even without a direct measure of Te and under certain conditions, it is possible to give rough estimates (or limits) on the abundances, both in GHRs and in PNe. Although the main mechanisms determining the electron temperature in ionized nebulae have been known for half a century, we emphasize that there are still important problems to be solved. Finally, we advocate for a different description of temperature inhomogeneities than the scheme generally used.