Fantastic Striations and Where to Find Them: Origin of Magnetically Aligned Striations in Interstellar Clouds

Abstract

Thin, magnetically aligned striations of relatively moderate contrast with the background are commonly observed in both atomic and molecular clouds. They are also prominent in MHD simulations with turbulent converging shocks. The simulated striations develop within a dense, stagnated sheet in the mid plane of the post-shock region where magnetically-induced converging flows collide. We show analytically that the secondary flows are an inevitable consequence of the jump conditions of oblique MHD shocks. They produce the stagnated, sheet-like sub-layer through a secondary shock when, roughly speaking, the Alfvénic speed in the primary converging flows is supersonic, a condition that is relatively easy to satisfy in interstellar clouds. The dense sub-layer is naturally threaded by a strong magnetic field that lies close to the plane of the sub-layer. The substantial magnetic field makes the sheet highly anisotropic, which is the key to the striation formation. Specifically, perturbations of the primary inflow that vary spatially perpendicular to the magnetic field can easily roll up the sheet around the field lines without bending them, creating corrugations that appear as magnetically-aligned striations in column density maps. On the other hand, perturbations that vary spatially along the field lines curve the sub-layer and alter its orientation relative to the magnetic field locally, seeding special locations that become slanted overdense filaments and prestellar cores through enhanced mass accumulation along field lines. In our scenario, the dense sub-layer unique to magnetized oblique shocks is the birthplace for both magnetically-aligned diffuse striations and massive star-forming structures.

13 pages, 13 figures, submitted to ApJ

References in corpus (8)

• Athena: A New Code for Astrophysical MHD
• Magnetically Aligned Velocity Anisotropy in the Taurus Molecular Cloud
• Formation of Magnetized Prestellar Cores with Ambipolar Diffusion and Turbulence
• Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
• Striations in the Taurus molecular cloud: Kelvin-Helmholtz instability or MHD waves?
• Striations in molecular clouds: Streamers or MHD waves?
• Simulations of the non-linear thin shell instability
• The interplay of the collisionless nonlinear thin-shell instability with the ion acoustic instability

Cited by in corpus (8)

• From diffuse gas to dense molecular cloud cores
• Self-gravitating Filament Formation from Shocked Flows: Velocity Gradients across Filaments
• Magnetized interstellar molecular clouds: II. The Large-Scale Structure and Dynamics of Filamentary Molecular Clouds
• The JCMT BISTRO-2 Survey: Magnetic Fields of the Massive DR21 Filament
• The relative orientation between the magnetic field and gas density structures in non-gravitating turbulent media
• Gas kinematics around filamentary structures in the Orion B cloud
• CO enhancement by magnetohydrodynamic waves; Striations in the Polaris Flare
• Relative Alignment Between Magnetic Fields and Molecular Gas Structure in Molecular Clouds