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

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Fragmentation of expanding shells in spiral and irregular galaxies

S. Ehlerova 1, J. Palou 1, Ch. Theis 2 and G. Hensler 2

1 Astronomical Institute, Academy of Sciences of the Czech Republic, Bon II 1401, 141 31 Prague 4, Czech Republic
2 Institut f"ur Astronomie und Astrophysik der Universit"at, D-24098 Kiel, Germany

Received 30 May 1997 / Accepted 28 July 1997


Conditions for the instability and the fragmentation of expanding and decelerating shells are discussed. A self-similar analytical solution is compared with the results of 3-dimensional computer simulations of expansions in homogeneous media. The amount of energy supply from the final number of young stars in an OB association, the value of the sound speed, the stratification and density of the ambient medium, the galactic differential rotation and the gravitational force perpendicular to the galactic plane influence formation of fragments. The typical size of unstable shells is 1 kpc for density [FORMULA]. In thick disk galaxies the fragmentation occurs in the nearly whole shell while in thin disks it is restricted to the galactic equator. Unstable fragments may become molecular and trigger the formation of molecular clouds where new stars are formed. We conclude that in dwarf galaxies the star formation may propagate in all directions turning the system into a starburst, contrary to spiral galaxies where the star formation propagates only in some directions in the thin strip near the symmetry plane.

Key words: stars: formation – ISM: bubbles; kinematics and dynamics – Galaxy: structure

Send offprint requests to: J. Palou 1

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998