Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud II: A Parameter Study

We use numerical simulations to study the effect of nonlinear MHD waves in a stratified, self-gravitating molecular cloud that is bounded by a hot and tenuous external medium. In a previous paper, we had shown the details of a standard model and studied the effect of varying the dimensionless amplitude ãd of sinusoidal driving. In this paper, we present the results of varying two other important free parameters: β0, the initial ratio of gas to magnetic pressure at the cloud midplane, and ν̃0, the dimensionless frequency of driving. Furthermore, we present the case of a temporally random driving force. Our results demonstrate that a very important consideration for the actual level of turbulent support against gravity is the ratio of driving wavelength λ0 to the the size of the initial non-turbulent cloud; maximum cloud expansion is achieved when this ratio is close to unity. All of our models yield the following basic results: (1) the cloud is lifted up by the pressure of nonlinear MHD waves and reaches a steady-state characterized by oscillations about a new time-averaged equilibrium state; (2) after turbulent driving is discontinued, the turbulent energy dissipates within a few sound crossing times of the expanded cloud; (3) the line-width–size relation is obtained by an ensemble of clouds with different free parameters and thereby differing time-averaged self-gravitational equilibrium states. The best consistency with the observational correlation of magnetic field strength, turbulent line width, Present address: National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan; [email protected]