Fantastic Striations and Where to Find Them: The 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.

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

“…This suppression may provide some clues as to the origin of these filamentary S features: by suppressing the contributions from the highest density voxels the most, the power-law heterogeneous alignment prescription has revealed that the highest S filamentary features could arise from emission contributions from the intermediate to higher density regions within the line-of-sight, at least from the z line-of-sight perspective, wherein the ordering influence of the mean magnetic field is apparent to the observer. On the other hand, since there is not a strong correlation between S and column density (see Section 4.2 below), these regions probably do not coincide with the self-gravitating regions; instead, they could arise at the intermediate-high density regions found within every lineof-sight, whose existence can be accounted for by referring to the dense secondary shock common to all oblique magnetized shocks described in Chen et al (2017). However, we emphasise that separating these features from the onedimensional PDFs (as presented in Figure 5) is not a simple matter, as simple thresholding is not sufficient to separate these features, so we must limit our conclusions.…”

“…The density PDFs are calculated using kernel density estimation using 50% of the voxels of each simulation. The quantity n p(n) is a rough measure of how much of the total emission would be expected from voxels of that density, and also highlights features of the shock not visible in only the PDF, such as the secondary subshock in Model D discussed inChen et al (2017). The middle panel makes clear that even considering the outsize contributions of the densest voxels, their paucity reduces their contribution to the total emission.…”

Dense Gas Kinematics and a Narrow Filament in the Orion A OMC1 Region Using NH3

“…In fact, the synthetic polarization vectors of the edge-on view of the King et al (2018) super-Alfvenic colliding flow model looks very similar to our F2 observations. Chen et al (2017) examined the impact of velocity perturbations in colliding flow simulations on the resulting field morphology. They found that perturbations varying spatially along the field can produce striations, in which the field is "rolled up", thereby producing a field aligned with the filament rather than perpendicular to it.…”

Probing the cold magnetised Universe with SPICA-POL (B-BOP)

“…These waves are excited as a result of the passage of Alfvén waves, which couple to other MHD modes through phase mixing (see Figure 3, right panel). In contrast, Chen et al (2017) proposed that striations do not represent real density fluctuations, but are rather a line-of-sight column-density effect in a corrugated layer forming in the dense post-shock region of an oblique MHD shock. High-resolution polarimetric imaging data would be of great interest to set direct observational constraints and discriminate between these possible models.…”

“…Low-density striations are remarkably ordered structures in an otherwise chaotic-looking turbulent medium. While the exact physical origin of both low-density striations (Heyer et al 2016;Tritsis & Tassis 2016Chen et al 2017) and high-density fibres (e.g., Clarke et al 2017; Zamora-Avilés, Ballesteros-Paredes, & Hartmann 2017) is not well understood and remains highly debated in the literature, there is little doubt that magnetic fields are involved. For instance, Tritsis and Tassis (2016) modelled striations as density fluctuations associated with magnetosonic waves in the linear regime (the column-density contrast of observed striations does not exceed 25%).…”