Velocity Anisotropy in Self-Gravitating Molecular Clouds. I: Simulation [GA]

The complex interplay between turbulence, magnetic fields, and self-gravity leads to the formation of molecular clouds out of the diffuse interstellar medium (ISM). One avenue of studying this interplay is by analyzing statistical features derived from observations, where the interpretation of these features is greatly facilitated by comparisons with numerical simulations. Here we focus on the statistical anisotropy present in synthetic maps of velocity centroid data, which we derive from three-dimensional magnetohydrodynamic simulations of a turbulent, magnetized, self-gravitating patch of ISM. We study how the orientation and magnitude of the velocity anisotropy correlate with the magnetic field and with the structures generated by gravitational collapse.
Motivated by recent observational constraints, our simulations focus on the supersonic (sonic Mach number $\mathcal{M} \approx 2 – 17$) but sub- to trans-alfvenic (alfvenic Mach number $\mathcal{M}_A \approx 0.2 – 1.2$) turbulence regime, and we consider clouds which are barely to mildly magnetically supercritical (mass-to-flux ratio equal to once or twice the critical value). Additionally we explore the impact of the turbulence driving mechanism (solenoidal or compressive) on the velocity anisotropy.
While we confirm previous findings that the velocity anisotropy generally aligns well with the plane-of-sky magnetic field, our inclusion of the effects of self-gravity reveals that in regions of higher column density, the velocity anisotropy may be destroyed or even reoriented to align with the gravitationally formed structures. We provide evidence that this effect is not necessarily due to the increase of $\mathcal{M}_A$ inside the high-density regions.

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F. Otto, W. Ji and H. Li
Tue, 10 Jan 17

Comments: 18 pages, 15 figures. Accepted for publication in ApJ