The Magnetohydrodynamics of Shock‐Cloud Interaction in Three Dimensions

Abstract

The magnetohydrodynamic evolution of a dense spherical cloud as it interacts with a strong planar shock is studied, as a model for shock interactions with density inhomogeneities in the interstellar medium. The cloud is assumed to be small enough that radiative cooling, thermal conduction, and self-gravity can be ignored. A variety of initial orientations (including parallel, perpendicular, and oblique to the incident shock normal) and strengths for the magnetic field are investigated. During the early stages of the interaction (less than twice the time taken for the transmitted shock to cross the interior of the cloud) the structure and dynamics of the shocked cloud is fairly insensitive to the magnetic field strength and orientation. However, at late times strong fields substantially alter the dynamics of the cloud, suppressing fragmentation and mixing by stabilizing the interface at the cloud surface. Even weak magnetic fields can drastically alter the evolution of the cloud compared to the hydrodynamic case. Weak fields of different geometries result in different distributions and amplifications of the magnetic energy density, which may affect the thermal and non-thermal x-ray emission expected from shocked clouds associated with, for example, supernovae remnants.

Radiative Interaction of Shocks With Small Interstellar Clouds as a Pre-Stage to Star Formation

“…Rather than using a discrete cloud boundary, we use the same type of smooth cloud density and temperature profiles as, among others, Nakamura et al (2006) and Shin et al (2008). This means assuming pressure equilibrium and a density profile of the form…”

The interaction of a magnetohydrodynamical shock with a filament

“…However, because a large grid was required for their cloud they were unable to run a 'high' resolution simulation to test this. One of the few 3D MHD resolution tests in the literature was performed by Shin et al (2008) for a spherical cloud using a non-AMR code at resolutions of R 120 and R 60 and concluded that most aspects of the MHD shock-cloud interaction were well converged at both resolutions. To our knowledge, the only resolution tests for a 3D purely hydrodynamic shock-filament interaction were performed by Pittard & Goldsmith (2016), who demonstrated that convergence was possible at a resolution of R 32 .…”

“…Other studies have reported on the effects of additional processes on the interaction, such as magnetic fields (e.g. Mac Low et al 1994;Shin-S., Stone & Snyder 2008), radiative cooling (e.g. Mellema, Kurk & Röttgering 2002;Fragile et al 2004;Yirak, Frank & Cunningham 2010) and thermal conduction (e.g.…”

“…Furthermore, if the field is parallel to the shock normal a 'flux rope' is formed behind the cloud since the field is preferentially amplified at that point due to shock-focusing. 3D simulations show that when the magnetic field is strong and aligned either perpendicularly or obliquely to the shock normal the cloud takes on a sheet-like appearance at late times and becomes orientated parallel to the post-shock field (Shin et al 2008). A perpendicular field can better deflect the flow around the cloud and reduce mixing, whereas a parallel field allows the cloud to be permeated by the flow and this enhances mixing (Li, Frank & Blackman 2013).…”

3D simulations of microquasar jets in clumpy stellar winds

“…Otherwise, the lateral magnetic pressure can effectively prevent clump penetration into the jet. Moreover, assuming that clumps managed to enter the jet, magnetic pressure could suppress the development of shocks, induce magnetic dissipation and non-thermal activity, and strongly change the clump disruption process, softening or potentiating it (e.g., Jones et al 1996;Shin et al 2008). In any case, the flow evolution under a dynamically dominant magnetic field is out of the scope of this work.…”