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

TL;DR

This study demonstrates that clumpy shocks, influenced by self-gravity and magnetic fields, can produce the observed velocity dispersion-size relations in molecular clouds, with implications for understanding turbulence in galactic environments.

Contribution

It extends previous models by including self-gravity and magnetic fields in simulations of clumpy shocks, showing their effects on velocity dispersion relations.

Findings

Power-law relations between velocity dispersion and size scale with indices 0.3-1.2.

Magnetic fields have limited impact on the velocity dispersion-size relation.

Shock strength and orientation significantly influence the velocity dispersion in molecular clouds.

Abstract

We revisit an alternate explanation for the turbulent nature of molecular clouds - namely, 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 ($\sigma \propto L^{\Gamma}$ with $\Gamma \approx 0.5$), 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 $\sigma$ and $L$ throughout, with indices ranging from $\Gamma=0.3-1.2$. These results are relatively insensitive to the strength and geometry of magnetic fields, provided that the shock is relatively strong. $\Gamma$ 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 $\sigma-L$ relation is in clumpy shocks, deviations from the standard Larson relation constrain the strength and behaviour of shocks in spiral galaxies.