Temperature fluctuations in H II regions: t2 for the two-phase model
Department of Astronomy, Peking University, Beijing 100871, PR China e-mail: email@example.com
2 Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, PR China
3 Harvard-Smithsonian Centre for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
Accepted: 22 November 2006
Aims.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.
Methods.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, Th and Tc.
Results.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, 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.
Key words: ISM: general / ISM: HII regions
© ESO, 2007