Clumpy shocks as the driver of velocity dispersion in molecular clouds: the effects of self-gravity and magnetic fields

Abstract

We revisit an alternate explanation for the turbulent nature of molecular cloudsnamely, that velocity dispersions matching classical predictions of driven turbulence can be generated by the passage of clumpy material through a shock. While previous work suggested this mechanism can reproduce the observed Larson relation between velocity dispersion and size scale (σ ∝ L Γ with Γ ≈ 0.5), the effects of self-gravity and magnetic fields were not considered. We run a series of smoothed particle magnetohydrodynamics experiments, passing clumpy gas through a shock in the presence of a combination of self-gravity and magnetic fields. We find powerlaw relations between σ and L throughout, with indices ranging from Γ = 0.3 − 1.2. These results are relatively insensitive to the strength and geometry of magnetic fields, provided that the shock is relatively strong. Γ is strongly sensitive to the angle between the gas' bulk velocity and the shock front, and the shock strength (compared to the gravitational boundness of the pre-shock gas). If the origin of the σ − L relation is in clumpy shocks, deviations from the standard Larson relation constrain the strength and behaviour of shocks in spiral galaxies.

The Early Stage of Molecular Cloud Formation by Compression of Two-phase Atomic Gases

“…One of the differences between two-phase and WNM colliding flows is that the two-phase colliding flows are highly inhomogeneous. Our results showed that the inhomogeneity of colliding flows enhances longitudinal velocity dispersion for θ ∼ 0 • (also see Inoue & Inutsuka 2012;Carroll-Nellenback et al 2014;Forgan & Bonnell 2018). We also found density enhancement for θ ∼ 0 • in the models where the field strength is relatively large and the Alfvén Mach number of the colliding flow is close to unity in Fig.…”