Dynamics of the Parker‐Jeans Instability in a Galactic Gaseous Disk

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Abstract
Linear analysis and nonlinear three-dimensional simulation of magnetohydrodynamics of a gas layer are carried out in local corotating coordinates, taking into account the effect of self-gravitational force. The gas is subject to the Jeans instability when there is no magnetic field, 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 rotational speed, strength of magnetic fields, external pressure, external gravitational force, and directions of perturbations. We find that the growth rate of the instabilities and the shape of the dense blobs that aggregate in the nonlinear stage depend sensitively on these factors. Our result shows that the Jeans instability is stabilized by fast rotation while the Parker-Jeans mode may still be unstable. When the gas rotation is negligible, our nonlinear simulations show that the gas may form dense blobs or filaments that are perpendicular or parallel to the magnetic field, 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 field. When we adopt typical parameters of nearby molecular clouds, the separation of these filaments is about 5 pc, consistent with the observation. This suggests that molecular clouds with a coherent filamentary structure may be attributed to the Parker-Jeans instability of a gaseous disk under the influence of rotation. This study sheds light on how interstellar/intergalactic gas aggregates to form molecular clouds and seeds of stars, and thus on the star-forming process at its very early stage.