How do velocity structure functions trace gas dynamics in simulated molecular clouds?
Abstract
Context. Supersonic disordered flows accompany the formation and evolution of molecular clouds (MCs). It has been argued that this is turbulence that can support against gravitational collapse and form hierarchical substructures.
Aims: We examine the time evolution of simulated MCs to investigate: What physical process dominates the driving of turbulent flows? How can these flows be characterised? Are they consistent with uniform turbulence or gravitational collapse? Do the simulated flows agree with observations?
Methods: We analysed three MCs that have formed selfconsistently within kiloparsecscale numerical simulations of the interstellar medium (ISM). The simulated ISM evolves under the influence of physical processes including selfgravity, stratification, magnetic fields, supernovadriven turbulence, and radiative heating and cooling. We characterise the flows using velocity structure functions (VSFs) with and without density weighting or a density cutoff, and computed in one or three dimensions. However, we do not include optical depth effects that can hide motions in the densest gas, limiting comparison of our results with observations.
Results: In regions with sufficient resolution, the densityweighted VSFs initially appear to follow the expectations for uniform turbulence, with a firstorder powerlaw exponent consistent with Larson's sizevelocity relationship. Supernova blast wave impacts on MCs produce shortlived coherent motions at large scales, increasing the scaling exponents for a crossing time. Gravitational contraction drives smallscale motions, producing scaling coefficients that drop or even turn negative as small scales become dominant. Removing the density weighting eliminates this effect as it emphasises the diffuse ISM.
Conclusions: We conclude that two different effects coincidentally reproduce Larson's size velocity relationship. Initially, uniform turbulence dominates, so the energy cascade produces VSFs that are consistent with Larson's relationship. Later, contraction dominates and the densityweighted VSFs become much shallower or even inverted, but the relationship of the global average velocity dispersion of the MCs to their radius follows Larson's relationship, reflecting virial equilibrium or freefall collapse. The injection of energy by shocks is visible in the VSFs, but decays within a crossing time.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 October 2019
 DOI:
 10.1051/00046361/201833970
 arXiv:
 arXiv:1908.03951
 Bibcode:
 2019A&A...630A..97C
 Keywords:

 turbulence;
 ISM: kinematics and dynamics;
 ISM: structure;
 ISM: clouds;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 21 pages, 12 figures, accepted for publication in A&