Ambipolar diffusion and star formation: Formation and contraction of axisymmetric cloud cores. I - Formulation of the problem and method of solution

Fiedler, Robert; Mouschovias, Telemachos Ch.

doi:10.1086/171336

Cited by 74 publications

(52 citation statements)

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“…A key quantity used to assess cloud stability and predict future outcomes is the ratio of mass, in the form of gas and dust, to the flux of the B-field threading that material. This "mass-to-flux" ratio (e.g., Mestel & Spitzer 1956;Mouschovias 1976aMouschovias , 1976bFiedler & Mouschovias 1992;Crutcher 2012) is normally indexed by its critical ratio, the value at which the gravitational and magnetic energy densities are equal, yielding a F M ratio of unity. If F M is less than unity, then B-fields dominate and the region is classified as subcritical.…”

Section: Introductionmentioning

confidence: 99%

Tracing the Magnetic Field of IRDC G028.23-00.19 Using NIR Polarimetry

Hoq

Clemens

Guzmán

et al. 2017

ApJ

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The magnetic field (B-field) of the starless dark cloud L1544 has been studied using near-infrared (NIR) background starlight polarimetry (BSP) and archival data in order to characterize the properties of the plane-of-sky B-field. NIR linear polarization measurements of over 1700 stars were obtained in the H band and 201 of these were also measured in the K band. The NIR BSP properties are correlated with reddening, as traced using the Rayleigh-Jeans color excess (H-M) method, and with thermal dust emission from the L1544 cloud and envelope seen in Herschel maps. The NIR polarization position angles change at the location of the cloud and exhibit their lowest dispersion there, offering strong evidence that NIR polarization traces the plane-of-sky B-field of L1544. In this paper, the uniformity of the plane-of-sky B-field in the envelope region of L1544 is quantitatively assessed. This allows evaluation of the approach of assuming uniform field geometry when measuring relative mass-to-flux ratios in the cloud envelope and core based on averaging of the radio Zeeman observations in the envelope, as done by Crutcher et al. In L1544, the NIR BSP shows the envelope B-field to be significantly non-uniform and likely not suitable for averaging Zeeman properties without treating intrinsic variations. Deeper analyses of the NIR BSP and related data sets, including estimates of the B-field strength and testing how it varies with position and gas density, are the subjects of later papers in this series.

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Section: Introductionmentioning

confidence: 99%

Tracing the Magnetic Field of IRDC G028.23-00.19 Using NIR Polarimetry

Hoq

Clemens

Guzmán

et al. 2017

ApJ

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“…Detailed numerical simulations by Fiedler & Mouschovias (1992, 1993) predicted a slope κ = 0.47 for contracting cores formed by gravitationallydriven ambipolar diffusion and evolving from initially magnetically subcritical to supercritical states. More recently, Li et al (2015) inferred κ = 0.41 from observations in the massive star forming region NGC 6334.…”

Section: Introductionmentioning

confidence: 99%

Magnetic field–gas density relation and observational implications revisited

Tritsis

Panopoulou

Mouschovias

et al. 2015

Mon. Not. R. Astron. Soc.

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We revisit the relation between magnetic-field strength (B) and gas density (ρ) for contracting interstellar clouds and fragments (or, cores), which is central in observationally determining the dynamical importance of magnetic fields in cloud evolution and star formation. Recently, it has been claimed that a relation B ∝ ρ 2/3 is statistically preferred over B ∝ ρ 1/2 in molecular clouds, when magnetic field detections and nondetections from Zeeman observations are combined. This finding has unique observational implications on cloud and core geometry: The relation B ∝ ρ 2/3 can only be realized under spherical contraction. However, no indication of spherical geometry can be found for the objects used in the original statistical analysis of the B −ρ relation. We trace the origin of the inconsistency to simplifying assumptions in the statistical model used to arrive at the B ∝ ρ 2/3 conclusion and to an underestimate of observational uncertainties in the determination of cloud and core densities. We show that, when these restrictive assumptions are relaxed, B ∝ ρ 1/2 is the preferred relation for the (self-gravitating) molecular-cloud data, as theoretically predicted four decades ago.

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“…Since then, much work has been done on AD-driven collapse of MCs (e.g. Spitzer 1968;Nakano & Tademaru 1972;Mouschovias 1976Mouschovias , 1977Mouschovias , 1979Nakano & Nakamura 1978;Shu 1983;Lizano & Shu 1989;Fiedler & Mouschovias 1992, 1993.…”

Section: Introductionmentioning

confidence: 99%

“…Twodimensional fully-implicit codes (e.g. Fiedler & Mouschovias 1992) were developed to avoid this problem, but complex code development would be required to extend this to three dimensions. In addition, implicit treatments can involve multiple iterations to converge, which may offset the advantage from the larger timestep.…”

Section: Introductionmentioning

confidence: 99%

Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics

McKee

Klein

et al. 2008

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Most numerical investigations on the role of magnetic fields in turbulent molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However, MCs are weakly ionized, so that the time scale required for the magnetic field to diffuse through the neutral component of the plasma by ambipolar diffusion (AD) can be comparable to the dynamical time scale. We have performed a series of 256 3 and 512 3 simulations on supersonic but sub-Alfvénic turbulent systems with AD using the Heavy-Ion Approximation developed in Li -2 -et al. (2006). Our calculations are based on the assumption that the number of ions is conserved, but we show that these results approximately apply to the case of time-dependent ionization in molecular clouds as well. Convergence studies allow us to determine the optimal value of the ionization mass fraction when using the heavy-ion approximation for low Mach number, sub-Alfvénic turbulent systems. We find that ambipolar diffusion steepens the velocity and magnetic power spectra compared to the ideal MHD case. Changes in the density PDF, total magnetic energy, and ionization fraction are determined as a function of the AD Reynolds number. The power spectra for the neutral gas properties of a strongly magnetized medium with a low AD Reynolds number are similar to those for a weakly magnetized medium; in particular, the power spectrum of the neutral velocity is close to that for Burgers turbulence.

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Ambipolar diffusion and star formation: Formation and contraction of axisymmetric cloud cores. I - Formulation of the problem and method of solution

Cited by 74 publications

References 0 publications

Tracing the Magnetic Field of IRDC G028.23-00.19 Using NIR Polarimetry

Tracing the Magnetic Field of IRDC G028.23-00.19 Using NIR Polarimetry

Magnetic field–gas density relation and observational implications revisited

Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics

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