Magnetic Fields in Interstellar Clouds From Zeeman Observations: Inference of Total Field Strengths by Bayesian Analysis

Crutcher, R. M.; Wandelt, B. D.; Heiles, Carl; Falgarone, É.; Troland, T. H.

doi:10.1088/0004-637x/725/1/466

Cited by 535 publications

(782 citation statements)

References 21 publications

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“…The total mass contained in the cube continuously increases with time. The magnetic field's initial direction is along that of the incoming flows, and its intensity is about 5 μG, consistent with observational values at these densities (Crutcher et al 2010). There is therefore a large-scale anisotropic component of the magnetic field throughout the simulation, as well as a turbulent component linked to the velocity perturbations imposed on the converging flows.…”

Section: Simulations Of Mhd Turbulencesupporting

confidence: 80%

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Ade

Aghanim

Alina

et al. 2015

A&A

126

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Polarized emission observed by Planck HFI at 353 GHz towards a sample of nearby fields is presented, focusing on the statistics of polarization fractions p and angles ψ. The polarization fractions and column densities in these nearby fields are representative of the range of values obtained over the whole sky. We find that: (i) the largest polarization fractions are reached in the most diffuse fields; (ii) the maximum polarization fraction p max decreases with column density N H in the more opaque fields with N H > 10 21 cm −2 ; and (iii) the polarization fraction along a given line of sight is correlated with the local spatial coherence of the polarization angle. These observations are compared to polarized emission maps computed in simulations of anisotropic magnetohydrodynamical turbulence in which we assume a uniform intrinsic polarization fraction of the dust grains. We find that an estimate of this parameter may be recovered from the maximum polarization fraction p max in diffuse regions where the magnetic field is ordered on large scales and perpendicular to the line of sight. This emphasizes the impact of anisotropies of the magnetic field on the emerging polarization signal. The decrease of the maximum polarization fraction with column density in nearby molecular clouds is well reproduced in the simulations, indicating that it is essentially due to the turbulent structure of the magnetic field: an accumulation of variously polarized structures along the line of sight leads to such an anti-correlation. In the simulations, polarization fractions are also found to anti-correlate with the angle dispersion function S. However, the dispersion of the polarization angle for a given polarization fraction is found to be larger in the simulations than in the observations, suggesting a shortcoming in the physical content of these numerical models. In summary, we find that the turbulent structure of the magnetic field is able to reproduce the main statistical properties of the dust polarization as observed in a variety of nearby clouds, dense cores excluded, and that the large-scale field orientation with respect to the line of sight plays a major role in the quantitative analysis of these statistical properties.

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Section: Simulations Of Mhd Turbulencesupporting

confidence: 80%

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Ade

Aghanim

Alina

et al. 2015

A&A

126

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“…This estimate is supported by observations (Crutcher 1999;Crutcher et al 2010) and numerical simulations (Burkhart et al 2009;Banerjee et al 2009;Molina et al 2012) in the regime of interest where the fields dynamically unimportant on supersonic scales (i.e., we again require MA > 1). 9 Note that the dust feels the total "large-scale" magnetic field (in contrast to the velocity field, where only v λ is important) so we do not need the magnetic field spectrum.…”

Section: Dust Magnetizationsupporting

confidence: 79%

The dynamics of charged dust in magnetized molecular clouds

Lee

Hopkins

Squire

2017

Monthly Notices of the Royal Astronomical Society

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We study the dynamics of large, charged dust grains in turbulent giant molecular clouds (GMCs). Massive dust grains behave as aerodynamic particles in primarily neutral dense gas, and thus are able to produce dramatic small-scale fluctuations in the dust-to-gas ratio. Hopkins & Lee (2016) directly simulated the dynamics of neutral dust grains in super-sonic MHD turbulence, typical of GMCs, and showed that the dust-to-gas fluctuations can exceed factor ∼ 1000 on small scales, with important implications for star formation, stellar abundances, and dust behavior and growth. However, even in primarily neutral gas in GMCs, dust grains are negatively charged and Lorentz forces are non-negligible. Therefore, we extend our previous study by including the effects of Lorentz forces on charged grains (in addition to drag). For small charged grains (sizes 0.1 µm), Lorentz forces suppress dust-to-gas ratio fluctuations, while for large grains (sizes 1 µm), Lorentz forces have essentially no effect, trends that are well explained with a simple theory of dust magnetization. In some special intermediate cases, Lorentz forces can enhance dust-gas segregation. Regardless, for the physically expected scaling of dust charge with grain size, we find the most important effects depend on grain size (via the drag equation) with Lorentz forces/charge as a second-order correction. We show that the dynamics we consider are determined by three dimensionless numbers in the limit of weak background magnetic fields: the turbulent Mach number, a dust drag parameter (proportional to grain size) and a dust Lorentz parameter (proportional to grain charge); these allow us to generalize our simulations to a wide range of conditions.

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“…In the non-colliding case, the gas mass mostly stays concentrated near the initialized levels, with especially the ambient, CNM gas evolving in a mostly quiescent manner. Although our simulations are initialized with relatively idealized conditions, both fiducial models develop a B versus n H behavior approximately consistent at least in general shape with the "Crutcher relation" (Crutcher et al 2010) 3 as the accumulation of gas to higher densities in turn compresses the magnetic fields along with it. The lower envelope of the phaseplot appears to exhibit a slight elbow near -10 cm 3 3 in both the colliding and non-colliding cases.…”

Section: Magnetic Field Strengths: B | | Versus N Hmentioning

confidence: 74%

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

Tan

Nakamura

et al. 2017

ApJ

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We investigate giant molecular cloud collisions and their ability to induce gravitational instability and thus star formation. This mechanism may be a major driver of star formation activity in galactic disks. We carry out a series of 3D, magnetohydrodynamics (MHD), adaptive mesh refinement simulations to study how cloud collisions trigger formation of dense filaments and clumps. Heating and cooling functions are implemented based on photodissociation region models that span the atomic-to-molecular transition and can return detailed diagnostic information. The clouds are initialized with supersonic turbulence and a range of magnetic field strengths and orientations. Collisions at various velocities and impact parameters are investigated. Comparing and contrasting colliding and non-colliding cases, we characterize morphologies of dense gas, magnetic field structure, cloud kinematic signatures, and cloud dynamics. We present key observational diagnostics of cloud collisions, especially: relative orientations between magnetic fields and density structures, like filaments; 13 CO( J=2-1), 13 CO( J=3-2), and 12 CO( J=8-7) integrated intensity maps and spectra; and cloud virial parameters. We compare these results to observed Galactic clouds.

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Magnetic Fields in Interstellar Clouds From Zeeman Observations: Inference of Total Field Strengths by Bayesian Analysis

Cited by 535 publications

References 21 publications

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Planckintermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

The dynamics of charged dust in magnetized molecular clouds

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

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