Triggering the Collapse of Self-Gravitating Clouds by Torsional Alfvén Waves

Abstract Magnetohydrodynamic braking mechanisms of rotation are related to the production of copious fluxes of nonlinear torsional Alfvén waves under a variety of conditions in an interstellar medium and within molecular clouds. The present authors noted that these nonlinear torsional Alfvén waves may pinch ambient gas, sometimes leading to the gravitational collapse of gas clouds, and a spin up of the trapped ambient gas during propagation. They proposed this mechanism in order to interpret the observation that the infrared sources showing CO outflows in streamers tend to align along a spinning high-density helical streak-like structure in them, as seen in the case of L1641 in Orion. In the present paper, by adding a toroidal magnetic field we can simulate the magnetic pinching process of an isothermal cloud which is stabilized by a poroidal magnetic field, and then study the triggering of a gravitational instability of the cloud. Simulations are made by using a 3D smoothed particle magnetohydrodynamic code for the range of physical parameters and the mass of the clump appropriate for the situation of our present concern. It is shown that the magnetic effects of star formation in a large-scale field can actually trigger the gravitational collapse of another clump of mass on the same flux tube, which is well on the stable side of the criterion for the gravitational instability. It is suggested that this process, which takes place along a magnetic flux tube can form a long dense streak with a triggered collapse of mass relaying the nonlinear torsional Alfvén waves. This can explain the observation of winding dense streaks in the streamers along which infrared stars line up in some cases, as in 11641 in Orion.