Change of Magnetic Field-Gas Alignment at the Gravity-Driven Alfvénic Transition in Molecular Clouds: Implications for Dust Polarization Observations

Abstract

Diffuse striations in molecular clouds are preferentially aligned with local magnetic fields whereas dense filaments tend to be perpendicular to them. When and why this transition occurs remain uncertain. To explore the physics behind this transition, we compute the histogram of relative orientation (HRO) between the density gradient and the magnetic field in 3D MHD simulations of prestellar core formation in shock-compressed regions within GMCs. We find that, in the magnetically-dominated (sub-Alfvénic) post-shock region, the gas structure is preferentially aligned with the local magnetic field. For overdense sub-regions with super-Alfvénic gas, their elongation becomes preferentially perpendicular to the local magnetic field instead. The transition occurs when self-gravitating gas gains enough kinetic energy from the gravitational acceleration to overcome the magnetic support against the cross-field contraction, which results in a power-law increase of the field strength with density. Similar results can be drawn from HROs in projected 2D maps with integrated column densities and synthetic polarized dust emission. We quantitatively analyze our simulated polarization properties, and interpret the reduced polarization fraction at high column densities as the result of increased distortion of magnetic field directions in trans-or super-Alfvénic gas. Furthermore, we introduce measures of the inclination and tangledness of the magnetic field along the line of sight as the controlling factors of the polarization fraction. Observations of the polarization fraction and angle dispersion can therefore be utilized in studying local magnetic field morphology in star-forming regions.

Effects of grain alignment efficiency on synthetic dust polarization observations of molecular clouds

“…The calculation of the synthetic Stokes parameters from simulation data is outlined in Paper I, which follows existing convention (e.g., Lee & Draine 1985;Fiege & Pudritz 2000;Kataoka et al 2012;Planck Collaboration Int. XX 2015;Chen et al 2016). Under the assumption that polarized emission is in optically thin conditions, the gas is isothermal, and that the dust grains are in thermal equilibrium with the gas, the synthetic Stokes parameters in a Cartesian coordinate system are:…”

The structure and statistics of interstellar turbulence

“…If molecular clouds form in the turbulent ISM via large-scale compression of the warm diffuse trans-Alfvénic HI primarily along the field lines, then turbulence in the clouds will naturally be super-Alfvènic [78]. A closely related subject is the relative orientation of the magnetic field direction with respect to filamentary density structures in simulated turbulent molecular clouds [79,80]. Observations of striations and filaments in local molecular clouds and their close environments [81][82][83][84][85][86] suggest that the filaments are preferentially oriented perpendicular to the magnetic field lines, while the low-density subcritical striations are parallel.…”

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

“…Thus, one can think of f also as being the relative angle between the magnetic field and the filamentary axis of structures seen in mass surface density maps. Note that the convention we adopt for f follows PlanckXXXV but is shifted p 2 from that defined in Soler et al (2013) and Chen et al (2016). where γ is the angle between the local magnetic field relative to the plane of the sky, while χ is the angle of the magnetic field on the plane of the sky relative to the "north" axis (see Figure 6).…”

“…The HRO is a statistical tool that quantifies the magnetic field orientation relative to the gradient of the column density. It can be performed on polarization observations (e.g., PlanckXXXV) as well as numerical simulations (e.g., Planck Collaboration et al 2015; Chen et al 2016) to study the mutual dependence of magnetic fields on density structures.…”

“…where γ is the angle between the local magnetic field relative to the plane of the sky, while χ is the angle of the magnetic field on the plane of the sky relative to the "north" axis (see Figure 6). For a coordinate orientation where the y-axis can be defined as "north" with the line of sight directed along the zaxis, the relative Stokes parameters can be written as (see Chen et al 2016):…”