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

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9. Shells in spiral and dwarf galaxies

In this paper we have addressed the basic question of the conditions for triggered star formation in superbubble shells. Although physically reasonable and therefore expected to occur in general, only a few examples are observed where star formation sites are located at the rim of bubbles and just embedded into shells. As the closest case to us our local bubble could be envoked where Lindblad's ring seems to house the Orion, Perseus, and Sco-Cen OB associations on its periphery (Elmegreen, 1992). Comeron et al. (1993) and Comeron & Torra (1994) show that Cygnus OB1 - OB9 may be due to gravitationally unstable perturbations of the Cygnus Superbubble, which is powered by the Cygnus OB2 association. As examples of more distant star-forming loops which are connected with shells resulting from supernovae or accumulated stellar winds, Schwarz (1987) has presented e.g. the Mon OB 1 loop, Cep OB 3 loop, and a few others. In addition, G54.4-0.3 (Junkes et al., 1992a, 1992b) reveals the striking features of triggered star formation connected with a supernova shell.

Because observations of star-forming supernova shells within our Milky Way are difficult to perform in complementary spectral ranges since they are affected e.g. in X-rays by interstellar hydrogen absorption and in the UV and optical by dust extiction, extragalactic candidates could serve for better studies, e.g. in the LMC. The LMC-2 bubble (Wang & Helfand, 1991), LMC-4 bubble (Vallenari et al., 1993), Constellation III (Dopita et al., 1985) and also 30 Dor reveal impressive structures of present star-forming episodes triggered and located in giant shells. (We refer the interested reader to the comprehensive paper on triggered star formation by Elmegreen, 1992).

The holes in the HI distribution of spiral galaxies like M 31 (Brinks & Bajaja, 1986) or in dwarf galaxies like Ho II (Puche et al., 1992) are in most cases due to the energy released from young stars and we may ask the question if the expanding shells can fragment and possibly trigger the next generation of the star formation: The observations may be compared with computer simulations of the shell expansion, so that we can discuss whether the conditions for fragmentation and molecularization can be fulfilled.

The evolution of an expanding shell in a dwarf galaxy with the thick HI disk is shown in Fig. 5. It fragments almost everywhere. The fragments become molecular if the density of the ambient medium is high enough according to Fig. 3. Thus the star formation in dwarf galaxies propagates in all directions, with the speed of propagation of [FORMULA] turning all the galaxy into a starburst within a few [FORMULA] yr.

The evolution of a shell in a spiral galaxy similar to Milky Way composed of multi-component disk where the galactic differential rotation and the force perpendicular to the galactic plane are included is given in Fig. 7. The fragmentation happens close to the galactic plane in an unstable region near the tips of the elongated bubble. In spiral galaxies, the star formation can propagate only in some directions in the galaxy plane with a lower speed than in dwarf galaxies of [FORMULA]. The propagating star formation may result in spiral structures (Jungwiert & Palou, 1994), however it can never turn a spiral galaxy into a starburst.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998