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Astron. Astrophys. 347, 442-454 (1999)

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8. Conclusions

In this paper we modeled the generation of spiral structure in a multi-phase, star-forming disk. The gaseous and stellar phases interact as well by their common gravity as by mutual phase transitions due to star formation and stellar death. The main results of our paper can be summarized as follows:

  1. The multi-phase disks which undergo a gas-star phase transformations are unstable with respect to nonaxial perturbations. The global spiral modes grow exponentially and saturate in a way, similar to that found in the one-component case.

  2. The spiral pattern grows faster and saturates on a higher level compared to a one-component stellar disk with the same mass and rotation. The spiral mode remains well developed, and keeps its properties unaltered if most of the gas is transformed into the "stellar" component with a stiffer equation of state. The cold gas phase remains a good tracer of the spiral structure even if the system contains about a few percent of the total mass of the disk.

These results demonstrate that the destabilizing role of the cold gas component previously known for two-component systems with fixed background properties can be extended to multi-component star-forming disks which are allowed for rapidly changing background properties.

Further research, however, should be performed with respect to self-gravitating multi-component disks. One obvious generalization is necessitated by the simplified model of the interaction chain used in our paper. Interactions between the different phases in a star-forming region are regulated mainly by the time dependent balance between the three components: gas, molecular clouds and stars (e.g. Theis et al. 1992, Shore & Ferrini 1995). Therefore, in a next step the gaseous phase should be split into clouds and an inter-cloud medium matching the chemo-dynamical approach of e.g. Samland et al. (1997). Another aspect is the stochasticity provided by the star-formation in individual clouds. Thus, in extension of Gerola & Seiden's (1978) analysis based on cellular automata, a study of the interplay between large-scale structure formation (including self-gravity) and stochastic processes would be interesting.

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999