Filamentary flow and magnetic geometry in evolving cluster-forming molecular cloud clumps

Abstract

We present an analysis of the relationship between the orientation of magnetic fields and filaments that form in 3D magnetohydrodynamic simulations of cluster-forming, turbulent molecular cloud clumps. We examine simulated cloud clumps with size scales of L ∼ 2-4 pc and densities of n ∼ 400-1000 cm −3 with Alfvén Mach numbers near unity. We simulated two cloud clumps of different masses, one in virial equilibrium, the other strongly gravitationally bound, but with the same initial turbulent velocity field and similar mass-to-flux ratio. We apply various techniques to analyze the filamentary and magnetic structure of the resulting cloud, including the DisPerSE filamentfinding algorithm in 3D. The largest structure that forms is a 1-2 parsec-long filament, with smaller connecting sub-filaments. We find that our simulated clouds, wherein magnetic forces and turbulence are comparable, coherent orientation of the magnetic field depends on the virial parameter. Subvirial clumps undergo strong gravitational collapse and magnetic field lines are dragged with the accretion flow. We see evidence of filament-aligned flow and accretion flow onto the filament in the subvirial cloud. Magnetic fields oriented more parallel in the subvirial cloud and more perpendicular in the denser, marginally bound cloud. Radiative feedback from a 16 M star forming in a cluster in one of our simulations ultimately results in the destruction of the main filament, the formation of an H ii region, and the sweeping up of magnetic fields within an expanding shell at the edges of the H ii region.

Compressed Magnetic Field in the Magnetically Regulated Global Collapsing Clump of G9.62+0.19

“…A clear decreasing trend in the ∆θ vs. F 8µm relation is found as: with the correlation coefficient R=0.63; further indicating that the magnetic field surrounding the expanding Hii regions becomes to follow the outlines of the expanding shells and is approximately parallel to the ionization (or shock) front. In numerical simulations of expanding Hii regions, a shell of material is swept up as the Hii region grows and the magnetic field inside the shell is approximately parallel to the ionization front (Arthur et al 2011;Klassen, Pudritz & Kirk 2017), which is consistent with our findings here.…”

“…In numerical simulations, the magnetic field strength is enhanced by a factor of about 5 to 6 in the compressed shell when comparing the magnetic field strength inside the expanding Hii regions (Klassen, Pudritz & Kirk 2017). If we adopt the same enhancement factor, the magnetic field strength inside the expanding Hii regions ("B" and "C") should be ∼200 µG.…”

“…Magneto-hydrodynamic (MHD) simulations demonstrated that the global magnetic field lines may roughly trace the outline of the expanding shell from a young massive star and are parallel to the long axis of the adjacent compressed filament (e.g., Klassen, Pudritz & Kirk 2017), a strong evidence for stellar feedback. Recent near-infrared polarization observations have found that the magnetic field in the shells near Hii regions or infrared bubbles is curved and following the shells, and the magnetic field strength in the shells is significantly enhanced compared to the ambient field strength (e.g., Chen et al 2017).…”

Magnetic Fields in the Infrared Dark Cloud G34.43+0.24

“…Filamentary structures exist in molecular clouds, with sizes ranging from a few to tens of parsecs (André et al, 2014;Wang et al, 2016). Recent magnetohydrodynamic (MHD) simulations (Klassen et al, 2017;Li et al, 2018;Gómez et al, 2018) probing the formation of large-scale filamentary clouds suggest a complex evolutionary process involving the interaction and fragmentation of dense, velocity-coherent, fibers into chains of cores, resembling observations in nearby clouds (e.g., L1495/B213 and Musca cloud; Hacar et al, 2013Hacar et al, , 2016. The simulations show that global magnetic fields are expected to be roughly perpendicular to the longer axes of dense filamentary clouds.…”

“…Several velocity coherent fibers are identified inside the clouds and appear to be supportable along the main filament. In 3D MHD simulations of cluster-forming turbulent molecular cloud clumps, Klassen et al (2017) found that B-fields are oriented parallel to sub-virial clouds and perpendicular to denser gravitationally-bound clouds.…”