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

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5. Summary and conclusions

The variation of the ionizing flux as a mechanism for stimulating the condensation of the diffuse gas was considered. To illustrate this effect, two situations were examined: one on the context of pregalactic conditions (a free metal cloud), and the other on the context of the actual interstellar medium (a gas with solar abundances). We have focused 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. In both cases the cause of the induced phase change is the same: the enhancement of the cooling rate by the increase of the electron density caused by the momentary increase of ionizing flux. After the passing of the flash, the cooling rate remains enhanced due to the inertia of the ionization. In the first case (metal free gas) we show that after the lapse of UV flux decrement, recombination continues, but at a lower rate than the cooling. The excess of electrons at these relatively low temperatures results in an enhancement of the [FORMULA] rate formation due to the enhanced abundance of the [FORMULA] intermediary. Even when [FORMULA], the abundance of [FORMULA] may reach a large enough value to produce considerable self-shielding. If the cloud reaches a critical value ([FORMULA]) for the optical depth at dissociating frequencies, the [FORMULA] abundance grows very fast, allowing the cooling of the cloud to temperatures of the order of [FORMULA]. The temperature drop occurs in a fraction ([FORMULA]) of the free fall time provoking a rapid decrease of the Jeans mass. However, if the post-flash dissociating level is large enough the [FORMULA] formation can be inhibit and the cloud remains warm.

In the second case (solar abundances gas) the dominant cooling mechanism of the warm neutral gas (the excitation of heavy ions by electron impacts) is proportional to the electron density, and therefore, the ionizing flash increases the cooling efficiency. We considers flash-like variations of the primary ionization rate by cosmic rays and calculate the marginal flash characteristics to induce warm gas condensation. We show that, for the expected states of the warm interstellar gas, ionizing flashes may easily induce the phase transition from the warm to the cool phase. The phase transition is completed in about [FORMULA] yr; however, the drop in the temperature from [FORMULA] to [FORMULA] occurs in about [FORMULA] yr.

The results indicate that the mechanism of induced condensation studied here might play a relevant role in the gas evolution of the diffuse gas in both, the pregalactic and the actual interstellar medium conditions.

The above results include only local processes. However, non-local processes like thermal conduction impose restrictions on the size of the condensing structures. Thermal conduction tends to attenuate temperature gradients, and therefore, it imposes a minimal value for the mass of the condensing structures (Corbelli & Ferrara 1995; Ibá nez & Rosenzweig 1995; Steele & Ibá nez 1997). Critical masses of the order of galaxy masses are obtained in the case of a metal-free gas (Ibá nez & Parravano 1983) and in the case of an actual interstellar gas, masses of the order of one solar mass are obtained (Parravano 1987; Parravano, Ibá nez & Mendoza 1993). However, a wide range of critical masses are obtained depending on the initial gas state, and ambient conditions. Therefore, in addition to the restrictions on [FORMULA] and [FORMULA] to induce the warm gas condensation, the cosmic ray flash should cover a region greater than the critical value imposed by thermal conduction and other diffusive processes.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998