BENDING INSTABILITIES IN STELLAR-SYSTEMS

Citation
D. Merritt et Ja. Sellwood, BENDING INSTABILITIES IN STELLAR-SYSTEMS, The Astrophysical journal, 425(2), 1994, pp. 551-567
Citations number
22
Categorie Soggetti
Astronomy & Astrophysics
Journal title
ISSN journal
0004637X
Volume
425
Issue
2
Year of publication
1994
Part
1
Pages
551 - 567
Database
ISI
SICI code
0004-637X(1994)425:2<551:BIIS>2.0.ZU;2-R
Abstract
We discuss the physics of bending instabilities in inhomogeneous stell ar systems and propose a simple criterion for stability. We derive the bending modes of a family of thin disk models and show that long-wave length modes in thin systems with realistic density profiles are not a lways stabilized by gravity, in contrast to an infinite sheet. We also present the results of N-body experiments with finite-thickness disks that confirm instability to bending even when the velocity anisotropy is much less extreme than the critical value for instability in an in finite slab. We suggest that the most important mechanism that stabili zes bending in realistic models is the out-of-phase response of stars that encounter the bend with a frequency greater than their free verti cal oscillation frequency kappa(z). This mechanism successfully accoun ts for our N-body results, and for the behavior of the infinite slab a t short wavelengths, where gravity can be neglected. It further predic ts stability at all wavelengths to longitudinal bending modes for pres sure-supported systems in which the ratio of orbital oscillation frequ encies is less extreme than about 2:1, that is, for which the isodensi ty contours are rounder than about 1:3. (Oblate systems can be flatter if the velocities are azimuthally biased.) The latter number is in ag reement with the behavior of bending modes in N-body models and in uni form spheroids, as well as with the absence of elliptical galaxies fla tter than about E7.