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Astron. Astrophys. 328, 531-543 (1997)

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Collisional dynamics of the Milky Way

Evgeny Griv 1, 2, Michael Gedalin 1 and Chi Yuan 2

1 Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, Beersheva 84104, Israel
2 Institute of Astronomy & Astrophysics, Academia Sinica, P.O. Box 1-87, Taipei 11529, Taiwan

Received 28 October 1996 / Accepted 23 April 1997

Abstract

The effect of gravitational (elastic) encounters between stars and giant molecular clouds on the stability of small-amplitude perturbations of the Milky Way's self-gravitating disk is considered, using the exact Landau (Fokker-Planck type) collision integral, and compared with the results obtained by Griv & Peter (1996), who used the simple phenomenological Bhatnagar-Gross-Krook (Bhatnagar et al. 1954) collisional model. The present analysis is carried out for the case of a spatially inhomogeneous, highly flattened system, i.e., an inhomogeneous system in which the thickness is very small in comparison with the disk's radial extension. According to observations (Grivnev & Fridman 1990), the dynamics of a system with rare, [FORMULA] (and weak, [FORMULA]), interparticle encounters is considered, where [FORMULA] is the epicyclic frequency, [FORMULA] is the frequency of excited waves, and [FORMULA] yr-1 is the effective frequency of star-cloud encounters. The evolution of the stellar distribution is determined primarily by interactions with collective modes of oscillations - gravitational Jeans-type and gradient-dissipative modes - rather than by ordinary ("close") star-cloud encounters. On the basis of a local kinetic theory, it is shown that the Landau integral and the Bhatnagar et al. model give practically identical results in the case of perturbations with the wavelength [FORMULA] that is comparable to the mean epicyclic radius of stars [FORMULA], that is, in the case of the most dangerous, in the sense of the loss of stability, gravitational Jeans-type perturbations. The models, however, have essentially different qualitative and quantitative behaviors in the extreme limits of long-wavelength perturbations, [FORMULA], and of short-wavelength perturbations, [FORMULA]. Certain observational implications of the present theory are discussed.

Key words: galaxies: interactions – galaxies: kinematics and dynamics – ISM: clouds

Send offprint requests to: Dr. E. Griv at Beersheva (griv@chen.bgu.ac.il)

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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