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Astron. Astrophys. 327, 1262-1270 (1997)

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1. Introduction

Thermal condensation of diffuse gas is a commonly invoked route for the formation of dense astrophysical structures. This thermal condensation may occur spontaneously or may be induced by variations of the external conditions. In the first situation (spontaneous condensation), the initially diffuse gas is in a state out of equilibrium in which the cooling dominates over the heating. Eventually, the gas reachs a thermo-chemical equilibrium in a dense and cool state. In the second situation (induced condensation), in absence of variation of external conditions the gas would reach (or be in) a state of thermo-chemical equilibrium in a diffuse and hot state, but in presence of appropriate variations of the external conditions, the gas evolves toward a cooler and denser state. Induced condensation of diffuse gas must play an important role in the large scale evolution of the medium; specially if the stimulating sources are able to induced condensation far away from them. The synchronization and the large scale patterns of star formation may be governed by this kind of stimulation. In fact, most of the models and numerical simulations of the star formation in disk galaxies include as a main process the so-called self-propagating star formation (Gerola & Seiden 1978; Seiden & Gerola 1982; Shore 1981, 1983; Palous et al. 1990; Commins & Shore 1990; Cammerer & Shchekinov 1994). This kind of systems are part of the wider class of reaction-diffusion systems (Kapral 1993, and references therein). In particular, the so-called exitable media are appropriate to capture the main features of the pattern formation in disk galaxies (Smolin 1996).

Among the external conditions whose variations would induce phase transitions are the external pressure and the ionizing flux. We focus our attention on the effect of the variation of the ionizing flux, keeping in mind that induced condensation by pressure variations may play a relevant role (Roberts 1969; Shapiro & Kang 1987). In particular, we focus our attention on flash-like variations; that is, during a "short" period of time, the ionizing flux is enhanced in comparison to the pre- and post-flash values. To illustrate this mechanism of induced condensation, two examples are given here: one on the context of pregalactic conditions (in a free metal cloud), and the other on the context of the actual interstellar medium (in a gas with solar abundances). In both cases the cause of the induced phase change is the same: the increase in the electron density due to the momentary increase of ionizing flux enhances the cooling rate. After the passing of the flash, the cooling rate remains enhanced due to the inertia of the ionization (i.e. the characteristic recombination time is much larger than the cooling one).
Many scenarios in which a flash of radiation affects the evolution of gas clouds can be imagined. For the metal free gas case, this kind of induced condensation might be relevant at the epoch of galaxy formation. In particular, the ionizing flash effect might be a key step in the sequence of events that have conduced to the formation of globular clusters. It has been stated by Cox (1985) that "such a rapid fragmentation of the halo almost certainly requires inducement by energy leaving the (primitive) disk". For the case of a gas with solar abundances, the evoked mechanisms of induction are diverse but generally associated to compression and convergent mass flows (see review by Elmegreen 1992). However, in addition to these mechanisms, the induced condensation by ionizing flashes appears as an effect to be considered.

In Sect. 2 the basic equations to follow the thermal and chemical evolution of a gas subject to variations of the gas pressure and the ionizing flux are given. In Sect. 3 the evolution of a free metal gas cloud subject to a flash of ionizing and dissociating radiation is analyzed, whereas, in Sect. 4, the evolution of a solar abundance gas subject to a flash of cosmic ray flux is considered. Finally, the conclusions are given in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998