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Register a new userEffects of Strong Magnetic Fields on Photoionised Clouds
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Simulations are presented of the photoionisation of three dense gas clouds threaded by magnetic fields, showing the dynamical effects of different initial magnetic field orientations and strengths. For moderate magnetic field strengths the initial radiation-driven implosion phase is not strongly affected by the field geometry, and the photoevaporation flows are also similar. Over longer timescales, the simulation with an initial field parallel to the radiation propagation direction (parallel field) remains basically axisymmetric, whereas in the simulation with a perpendicular initial field the pillar of neutral gas fragments in a direction aligned with the magnetic field. For stronger initial magnetic fields, the dynamics in all gas phases are affected at all evolutionary times. In a simulation with a strong initially perpendicular field, photoevaporated gas forms filaments of dense ionised gas as it flows away from the ionisation front along field lines. These filaments are potentially a useful diagnostic of magnetic field strengths in H II regions because they are very bright in recombination line emission. In the strong parallel field simulation the ionised gas is constrained to flow back towards the radiation source, shielding the dense clouds and weakening the ionisation front, eventually transforming it to a recombination front.
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The effects of initially uniform magnetic fields on the formation and evolution of dense pillars and cometary globules at the boundaries of H II regions are investigated using 3D radiation-magnetohydrodynamics simulations. It is shown, in agreement with previous work, that a strong initial magnetic field is required to significantly alter the non-magnetised dynamics because the energy input from photoionisation is so large that it remains the dominant driver of the dynamics in most situations. Additionally it is found that for weak and medium field strengths an initially perpendicular field is swept into alignment with the pillar during its dynamical evolution, matching magnetic field observations of the `Pillars of Creation in M16 and also some cometary globules. A strong perpendicular magnetic field remains in its initial configuration and also confines the photoevaporation flow into a bar-shaped dense ionised ribbon which partially shields the ionisation front and would be readily observable in recombination lines. A simple analytic model is presented to explain the properties of this bright linear structure. These results show that magnetic field strengths in star-forming regions can in principle be significantly constrained by the morphology of structures which form at the borders of H II regions.
We expand our study on the dispersion of polarization angles in molecular clouds. We show how the effect of signal integration through the thickness of the cloud as well as across the area subtended by the telescope beam inherent to dust continuum measurements can be incorporated in our analysis to correctly account for its effect on the measured angular dispersion and inferred turbulent to large-scale magnetic field strength ratio. We further show how to evaluate the turbulent magnetic field correlation scale from polarization data of sufficient spatial resolution and high enough spatial sampling rate. We apply our results to the molecular cloud OMC-1, where we find a turbulent correlation length of approximately 16 mpc, a turbulent to large-scale magnetic field strength ratio of approximately 0.5, and a plane-of-the-sky large-scale magnetic field strength of approximately 0.76 mG.
High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.
The Zeeman effect and dust grain alignment are two major methods for probing magnetic fields (B-fields) in molecular clouds, largely motivated by the study of star formation, as the B-field may regulate gravitational contraction and channel turbulence velocity. This review summarizes our observations of B-fields over the past decade, along with our interpretation. Galactic B-fields anchor molecular clouds down to cloud cores with scales around 0.1 pc and densities of 10^4-5 H_2/cc. Within the cores, turbulence can be slightly super-Alfvenic, while the bulk volumes of pa-rental clouds are sub-Alfvenic. The consequences of these largely ordered cloud B-fields on fragmentation and star formation are observed. The above paradigm is very different from the generally accepted theory during the first decade of the century, when cloud turbulence was assumed to be highly super-Alfvenic. Thus, turbulence anisotropy and turbulence-induced ambipolar diffusion are also revisited.
Magnetic fields are dynamically important in the diffuse interstellar medium. Understanding how gravitationally bound, star-forming clouds form requires modeling of the fields in a self-consistent, supernova-driven, turbulent, magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to follow the formation and early evolution of clouds with final masses of 3-8 $times 10^3 M_{odot}$ within such a simulation. We use the codes adaptive mesh refinement capabilities to concentrate numerical resolution in zoom-in regions covering single clouds, allowing us to investigate the detailed dynamics and field structure of individual self-gravitating clouds in a consistent background medium. Our goal is to test the hypothesis that dense clouds are dynamically evolving objects far from magnetohydrostatic equilibrium. We find that the cloud envelopes are magnetically supported with field lines parallel to density gradients and flow velocity, as indicated by the histogram of relative orientations and other statistical measures. In contrast, the dense cores of the clouds are gravitationally dominated, with gravitational energy exceeding internal, kinetic, or magnetic energy and accelerations due to gravity exceeding those due to magnetic or thermal pressure gradients. In these regions field directions vary strongly, with a slight preference towards being perpendicular to density gradients, as shown by three-dimensional histograms of relative orientation.
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