Parker Instability in a Self-Gravitating Magnetized Gas Disk: I. Linear Stability Analysis

To be a formation mechanism of such large-scale structures as giant molecular clouds (GMCs) and HI superclouds, the classical Parker instability driven by external gravity has to overcome three major obstacles: The convective motion accompanying the instability generates thin sheets than large condensations. The degree of density enhancement achieved by the instability is too low to make dense interstellar clouds. The time and the length scales of the instability are significantly longer and larger than the estimated formation time and the observed mean separation of the GMCs, respectively. This paper examines whether a replacement of the driving agent from the external to the self gravity might remove these obstacles by activating the gravitational instability in the Galactic ISM disk. Self-gravitating, magnetized, gas disk bound by a hot halo medium is subject to a Parker-type instability driven by the self gravity, the usual Jeans instability and the convection. If the disk is thicker than a certain critical value, the Parker-type instability due to perturbations of undular mode assists the Jeans gravitational instability to form large-scale structures. Under the external gravity, the convection triggered by interchange mode perturbations is to grow most rapidly; under the self gravity, however, the Jeans gravitational instability can have a growth rate higher than the limiting growth rate of the convective instability. The self gravity can thus suppress the convective motions, and a cooperative action of the Jeans and the Parker instabilities can remove all the obstacles confronting the classical version of the Parker instability. The mass and mean separation of Supercomputing Center, KISTI, Daejeon, 305-600, Korea Astronomy Program, SEES, Seoul National University, Seoul 151-742, Korea Department of Infrared Astrophysics, ISAS/JAXA, Sagamihara, Kanagawa, Japan e-mail: [email protected] e-mail: [email protected]