Gravitational Contraction Versus Supernova Driving and the Origin of the Velocity Dispersion–size Relation in Molecular Clouds

Abstract

Molecular cloud observations show that clouds have non-thermal velocity dispersions that scale with the cloud size as σ ∝ R 1/2 at constant surface density, and for varying surface density scale with both the cloud's size and surface density, σ 2 ∝ RΣ. The energy source driving these chaotic motions remains poorly understood. We describe the velocity dispersions observed in a cloud population formed in a numerical simulation of a magnetized, stratified, supernova-driven, interstellar medium, including diffuse heating and radiative cooling, before and after we include the effects of the self-gravity of the gas. We compare the relationships between velocity dispersion, size, and surface density measured in the simulated cloud population to those found in observations of Galactic molecular clouds. Our simulations prior to the onset of self-gravity suggest that external supernova explosions alone do not drive turbulent motions of the observed magnitudes within dense clouds. On the other hand, selfgravity induces non-thermal motions as gravitationally bound clouds begin to collapse in our model, approaching the observed relations between velocity dispersion, size, and surface density. Energy conservation suggests that the observed behavior is consistent with the kinetic energy being proportional to the gravitational energy. However, the clouds in our model show no sign of reaching a stable equilibrium state at any time, even for strongly magnetized clouds. We conclude that gravitationally bound molecular clouds are always in a state of gravitational contraction and their properties are a natural result of this chaotic collapse. In order to agree with observed star formation efficiencies, this process must be terminated by the early destruction of the clouds, presumably from internal stellar feedback.

Physical Properties of Molecular Clouds for the Entire Milky Way Disk

“…This increase can be explained by contraction of innerGalaxy GMCs (e.g., Ballesteros-Paredes et al 2011;Ibáñez-Mejía et al 2016); this is the solution we advocate here. However, contraction of the GMC cannot be the entire solution, since continued contraction would lead to very dense massive GMCs, which do not exist.…”

Galactic cold cores

“…Turbulence, for instance, is predicted to be highly linked to accretion and collapse (e.g., Burkert & Hartmann 2004), but these processes should still not be able to prevent accretion. Numerical models clearly indicate that molecular clouds are dynamically evolving and contracting gravitationally despite the presence of magnetic fields (Ibáñez-Mejía et al 2016). The same process of large scale global collapse drives the evolution of the internal substructure of the cloud (filament, clump), therefore gravity and mass inflow become the principal and dominant mechanisms determining changes in filamentary properties as a function of time.…”

The impact of stellar feedback on the density and velocity structure of the interstellar medium

“…Several candidates capable of driving the ISM turbulence exist, for example large-scale expanding outflows from high-pressure HII regions (Kessel-Deynet & Burkert 2003), stellar winds or supernovae (e.g. Kim et al 2001;de Avillez & Breitschwerdt 2004Joung & Low 2006), gravitational instabilities coupled with galactic rotation (Gammie et al 1991;Piontek & Ostriker 2004;Ibáñez-Mejía et al 2015;Krumholz & Burkhart 2016) and the Magneto-Rotational-Instability (MRI) (Balbus & Hawley 1991), to name a few.…”