2007A&A...464..631Z

We investigate temperature fluctuations in HII regions in terms of a two-phase model, which assumes that the nebular gas consists of a hot and a cold phase. We derive general formulae for T([OIII]), the [OIII] forbidden line temperature, and T(HI), the hydrogen Balmer jump temperature, in terms of the temperatures of the hot and cold phases, T_h and T_c. For large temperature differences, the values of t² required to account for the observed difference between T([OIII]) and T(HI) are much lower than those deduced using the classical formulae that assume random and small amplitude temperature fluctuations. One should therefore be cautious when using a two-phase model to account for empirically derived t² values. We present a correction of a recent work by Giammanco & Beckman (2005A&A...437L..11G), who use a two-phase model to estimate the ionization rate of HII regions by cosmic rays. We show that a very small amount of cold gas is sufficient to account for t² values typically inferred for H II regions.