The Bar Mode Instability: Gravitational Torques and Bar Formation

Details The Bar Mode Instability: Gravitational Torques and Bar Formation The Bar Mode Instability: Gravitational Torques and Bar Formation

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999sf99.proc..203i&link_type=abstract

Star Formation 1999, Proceedings of Star Formation 1999, held in Nagoya, Japan, June 21 - 25, 1999, Editor: T. Nakamoto, Nobeyam

Astronomy and Astrophysics

Astrophysics

Scientific paper

Dynamic nonaxisymmetric instabilities in rapidly rotating fluids have several potential applications in astrophysics. Here, we investigate the effects of nonaxisymmetric instabilities on the evolution of rapidly rotating polytropes and then apply our results to the star formation process. The most unstable nonaxisymmetric mode is the lowest order mode, the bar mode. Contrary to its name, the bar mode produces barlike distortions only in the central regions of the polytropes; it trails into spiral arms as the equator is approached. The winding gives rise to self-gravitational torques which control the nonlinear evolution of the object. Here, we use the linear eigenfunctions of Toman et al. (1998) to calculate the self-gravitational torque Υ. We then use Υ to study the longterm evolution of the bar mode. We compare predictions of our ``quasi-linear'' theory to several fully nonlinear hydrodynamics simulations. We find that: (i) the saturation of the bar mode instability is due to the self-interaction gravitational torque. Saturation occurs before fission. If the growth of the bar mode is unchecked, fission likely ensues. (ii) The quasi-linear theory accurately predicts the extent of the bar and hence the mass and other global properties of the bars. (iii) The quasi-linear theory suggests that bars will show two stages of evolution; a dynamic growth phase where the mode saturates followed by a secular phase where the bar slowly sheds angular momentum to the spiral arms.

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