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Magnetic fields are believed to play an important role in the evolution of molecular clouds, from their large scale structure to dense cores, protostellar envelopes, and protoplanetary disks. How important is unclear, and whether magnetic fields are the dominant force driving star formation at any scale is also unclear. In this review we examine the observational data which address these questions, with particular emphasis on high angular resolution observations. Unfortunately the data do not clarify the situation. It is clear that the fields are important, but to what degree we dont yet know. Observations to date have been limited by the sensitivity of available telescopes and instrumentation. In the future ALMA and the SKA in particular should provide great advances in observational studies of magnetic fields, and we discuss which observations are most desirable when they become available.
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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.
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.
We expand on the dispersion analysis of polarimetry maps toward applications to interferometry data. We show how the filtering of low-spatial frequencies can be accounted for within the idealized Gaussian turbulence model, initially introduced for single-dish data analysis, to recover reliable estimates for correlation lengths of magnetized turbulence, as well as magnetic field strengths (plane-of-the-sky component) using the Davis-Chandrasekhar-Fermi method. We apply our updated technique to TADPOL/CARMA data obtained on W3(OH), W3 Main, and DR21(OH). For W3(OH) our analysis yields a turbulence correlation length $deltasimeq19$ mpc, a ratio of turbulent-to-total magnetic energy $leftlangle B_{mathrm{t}}^{2}rightrangle /leftlangle B^{2}rightrangle simeq0.58$, and a magnetic field strength $B_{0}sim1.1:mathrm{mG}$; for W3 Main $deltasimeq22$ mpc, $leftlangle B_{mathrm{t}}^{2}rightrangle /leftlangle B^{2}rightrangle simeq0.74$, and $B_{0}sim0.7:mathrm{mG}$; while for DR21(OH) $deltasimeq12$ mpc, $leftlangle B_{mathrm{t}}^{2}rightrangle /leftlangle B^{2}rightrangle simeq0.70$, and $B_{0}sim1.2:mathrm{mG}$.
Of all the factors that influence star formation, magnetic fields are perhaps the least well understood. The goal of this paper is to characterize the 3D magnetic field properties of nearby molecular clouds through various methods of statistically analysing maps of polarized dust emission. Our study focuses on nine clouds, with data taken from the Planck Sky Survey as well as data from the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry observations of Vela C. We compare the distributions of polarization fraction ($p$), dispersion in polarization angles ($mathcal{S}$), and hydrogen column density ($N_{rm H}$) for each of our targeted clouds. To broaden the scope of our analysis, we compare the distributions of our clouds polarization observables with measurements from synthetic polarization maps generated from numerical simulations. We also use the distribution of polarization fraction measurements to estimate the inclination angle of each clouds cloud-scale magnetic field. We obtain a range of inclination angles associated with our clouds, varying from 16{deg} to 69{deg}. We establish inverse correlations between $p$ and both $mathcal{S}$ and $N_{rm H}$ in almost every cloud, but we are unable to establish a statistically robust $mathcal{S}$ versus $N_{rm H}$ trend. By comparing the results of these different statistical analysis techniques, we are able to propose a more comprehensive view of each clouds 3D magnetic field properties. These detailed cloud analyses will be useful in the continued studies of cloud-scale magnetic fields and the ways in which they affect star formation within these molecular clouds.
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