The role of shocks and the velocity gradient in the relative orientation of the magnetic field and dense gas clouds

Abstract

We investigate the alignment mechanism between the magnetic field and interstellar clouds formed by the collision of warm atomic gas. We find that the magnetic field, initially oriented parallel to the flow, is perturbed by a fast MHD shock, which amplifies magnetic field fluctuations parallel to the shock front. Behind the shock, the compressive downstream velocity field further amplifies the magnetic field component parallel to the shock front. This mechanism causes the magnetic field to become increasingly parallel to the dense layer, and the development of a shear flow around the latter. Furthermore, the bending-mode perturbations on the dense layer are amplified by the non-linear thin-shell instability (NTSI), stretching the density structures formed by the thermal instability, and rendering them parallel to the bent field lines. By extension, we suggest that a tidal stretching velocity gradient such as that produced in gas infalling into a self-gravitating structure must straighten the field lines along the accretion flow, orienting them perpendicular to the density structures. We also find that the upstream superalfvénic regime transitions to a transalfvénic regime between the shock and the condensation front, and then to a subalfvénic regime inside the condensations.

11 pages, 11 figures, 3 animations. Accepted in Astronomy & Astrophysics

References in corpus (13)

• Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions
• Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
• The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows
• A robust numerical scheme for highly compressible magnetohydrodynamics: Nonlinear stability, implementation and tests
• Rapid Molecular Cloud and Star Formation: Mechanisms and Movies
• Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
• Physical Processes in Star Formation
• From parallel to perpendicular -- On the orientation of magnetic fields in molecular clouds
• Formation of HI Clouds in Shock-compressed Interstellar Medium: Physical Origin of Angular Correlation Between Filamentary Structure and Magnetic Field
• The role of magnetic fields in the structure and interaction of supershells
• The relative orientation between the magnetic field and gas density structures in non-gravitating turbulent media
• HI-H${}_2$ transition: exploring the role of the magnetic field
• The physical and the geometrical properties of simulated cold HI structures

Cited by in corpus (1)

• Magnetic fields in the intracluster medium with TNG-Cluster: properties, morphology, and tangential anisotropy