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Astron. Astrophys. 328, 121-129 (1997)

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1. Introduction

Radiation, ionization and mechanical energy input into the interstellar medium due to the evolution of massive stars and their supernova explosions are processes accelerating the ambient interstellar medium, agglomerating the gas into expanding and decelerating shells. The shells may become unstable, transforming themselves into molecular clouds which seed further star formation.

The shells expand into the interstellar medium with velocities exceeding the sound speed. The gravitational instability of shock-compressed layers has been investigated by Elmegreen & Elmegreen (1978), and a new type of hydrodynamic instability of decelerating pressure-driven shocks has been discussed by Vishniac (1983). The evolution of decelerating shock waves in a uniform density medium, or in a medium with large-scale density gradients, has been investigated in numerical studies (Mac Low & McCray, 1988; Mac Low et al., 1989; Yoshida & Habe, 1992). In low-density regions, where the hot gas accelerates the swept-up shell, the Rayleigh-Taylor instability develops. High-density regions are more sensitive to cooling, forming a radiative thin shell. Vishniac (1994) and Elmegreen (1989, 1994) discuss the combination of the hydrodynamical instability of radiative, expanding and decelerating shocks with the effects of self-gravity.

The linear analysis of this 'Elmegreen-Vishniac' instability is described in the next paragraph. Later, a solution similar to Sedov's (1959) solution of an expanding blast wave is adopted and compared with a three-dimensional computer model. Then, a more realistic distribution of ambient medium and the galactic differential rotation are included. Finally, the conditions for the shell fragmentation and star formation in spiral and irregular galaxies are discussed. The differences in the shell fragmentation may be the reason for changing conditions for star formation, resulting in different gas consumption rates between spiral and irregural galaxies.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998