The presence of temperature fluctuations in photoionized regions has been a matter of debate since the pioneering work by Peimbert (1967). Although little doubt on their relevance in real nebulae seems possible on observational grounds (see the review by Peimbert 1995), their inclusion in a theoretical framework is still controversial, given both the incompleteness of the present theoretical scenario, and the technical difficulties implied by their inclusion in photoionization models. Plain photoionization theory predicts temperature fluctuations to be very small, mainly due to the steep dependence of the cooling rate on temperature, which implies that their lifetime would be quite short; yet, the existence of a mechanism steadily providing the energy required to feed them is not excluded by this line of reasoning. The importance of the question can be barely overrated given that, if temperature fluctuations turn out to be as big as observations suggest, the present determinations of chemical abundances should be rescaled by as much as +0.5 dex.
For a long time, the only way temperature fluctuations were accounted for in theoretical models was acknowledging the impossibility to reproduce the observed intensity of the most affected lines; see, e.g., Luridiana et al. (1999) for the case of NGC 2363, and Stasinska & Schaerer (1999) for the case of IZw18. Recently, Binette & Luridiana (2000) developed a model to quantify the effect of temperature fluctuations on the ionization and energy balance of nebulae. In the present work, we use their schema to investigate whether the kinetic energy provided by stellar winds could feed the temperature fluctuations observed in NGC 2363. Throughout the paper, by "stellar winds'' we generically refer to all those phenomena involving the ejection of mass from stars into the interstellar medium (ISM). Given the age range considered, the only contributors to stellar winds actually included in the calculations are hot stars and Wolf-Rayet stars (WRs).
The structure of the paper is as follows: in Sect. 2 we summarize the relevant properties of NGC 2363. Section 3 describes the model used to represent temperature fluctuations in nebulae. In Sect. 4 we estimate the kinetic luminosity of the stellar cluster in NGC 2363, and apply the method of Sect. 3 to determine whether stellar winds could be responsible for the temperature fluctuations observed in NGC 2363. Finally, in Sect. 5 we analyze several possible sources of uncertainty, and in Sect. 6 we summarize the main conclusions.
Copyright ESO 2001