CHANGE OF MAGNETIC FIELD-GAS ALIGNMENT AT THE GRAVITY-DRIVEN ALFVÉNIC TRANSITION IN MOLECULAR CLOUDS: IMPLICATIONS FOR DUST POLARIZATION OBSERVATIONS

• Published in: The Astrophysical journal Vol. 829; no. 2; pp. 84 - 103
• Main Authors: Chen, Che-Yu, King, Patrick K., Li, Zhi-Yun
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.10.2016; IOP Publishing
• Subjects: ACCELERATION; ALFVEN WAVES; Alignment; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CLOUDS; Computational fluid dynamics; Computer simulation; COSMIC DUST; COSMIC GASES; DENSITY; Density gradients; DISPERSIONS; Dust; Dust emission; Elongation; EMISSION; Emission analysis; Field strength; Filaments; Fluid flow; GRAVITATION; Histograms; Inclination; ISM: clouds; ISM: magnetic fields; Kinetic energy; MAGNETIC FIELDS; Magnetism; MAGNETOHYDRODYNAMICS; magnetohydrodynamics (MHD); Molecular clouds; MOLECULES; Morphology; ORIENTATION; POLARIZATION; Power law; SIMULATION; Star formation; STARS; stars: formation; Striations; THREE-DIMENSIONAL CALCULATIONS; TURBULENCE; TWO-DIMENSIONAL CALCULATIONS
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/0004-637X/829/2/84

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 three-dimensional magnetohydrodynamic (MHD) simulations of prestellar core formation in shock-compressed regions within giant molecular clouds. 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. 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 two-dimensional 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.