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 molecularclouds (GMCs) and HI superclouds, the classical Parker instability driven byexternal gravity has to overcome three major obstacles: The convective motionaccompanying the instability generates thin sheets than large condensations.The degree of density enhancement achieved by the instability is too low tomake dense interstellar clouds. The time and the length scales of theinstability are significantly longer and larger than the estimated formationtime and the observed mean separation of the GMCs, respectively. This paperexamines whether a replacement of the driving agent from the external to theself gravity might remove these obstacles by activating the gravitationalinstability in the Galactic ISM disk. The self gravity can suppress theconvective motions, and a cooperative action of the Jeans and the Parkerinstabilities can remove all the obstacles confronting the classical version ofthe Parker instability. The mass and mean separation of the structuresresulting from the odd-parity undular mode solution are shown to agree betterwith the HI superclouds than with the GMCs. We briefly discuss how inclusionsof the external gravity and cosmic rays would modify behaviors of theodd-parity undular mode solution.Comment: 53 pages, 21 figure