Energy balance in two phase models for temperature fluctuations in HII regions

Abstract

Aims. The main objective of this article is to provide a simple physical framework with permits a quantitative comparison of measurements of the temperature fluctuations in the ionized interstellar medium with possible mechanisms which can produce them. Methods. We assume a generalized two phase ISM and derive expressions relating the mean amplitude of the temperature fluctuations to the temperatures of the phases and to the energy input, in excess of the basic component due to photoionization, required to maintain them. We apply these expressions to a set of limiting cases for the temperature and density differences between the phases. Finally we compare the most plausible case with the most complete data set available: the temperature fluctuations observed in the Orion Nebula. Results. We first list the cases considered and our general inferences: (a) Very hot tenuous substrate and warm moderately dense clouds; discarded as requiring too much excess energy input. (b) Two phases with equal densities but different temperatures, both warm; feasible but not general. (c) Two warm phases at moderately different temperatures and densities; the most probable case. This case is then used to quantify a specific hypothesis, reconnection of turbulent magnetic fields, as the source of the fluctuations observed in the Orion Nebula. Field strengths of a few hundred microG are required, not out of line with the limited observations available. Time variability on scales of months is a testable prediction of the scenario.