Dynamics of the Parker-Jeans instability in a galactic gaseous disk

Linear analysis and nonlinear three-dimensional simulation of magnetohydrod ynamics of a gas layer are carried out in local corotating coordinates, tak ing into account the effect of self-gravitational force. The gas is subject to the Jeans instability when there is no magnetic held, or to the Parker- Jeans instability when a magnetic field is present. We study the evolution of these instabilities in several cases which vary in factors such as the r otational speed, strength of magnetic fields, external pressure, external g ravitational force, and directions of perturbations. We find that the growt h rate of the instabilities and the shape of the dense blobs that aggregate in the nonlinear stage depend sensitively on these factors. Our result sho ws that the Jeans instability is stabilized by fast rotation while the Park er-Jeans mode may still be unstable. When the gas rotation is negligible, o ur nonlinear simulations show that the gas may form dense blobs or filament s that are perpendicular or parallel to the magnetic held, depending on the strength of external pressure and the direction of initial perturbations. When rotation is included, the gas forms coherent long filaments with their major axes perpendicular to the magnetic held. When we adopt typical param eters of nearby molecular clouds, the separation of these filaments is abou t 5 pc, consistent with the observation. This suggests that molecular cloud s with a coherent filamentary structure may be attributed to the Parker-Jea ns instability of a gaseous disk under the influence of rotation. This stud y sheds light on how interstellar/intergalactic gas aggregates to form mole cular clouds and seeds of stars, and thus on the star-forming process at it s very early stage.