Magnetic Fields in Molecular Clouds

Crutcher, R. M.

doi:10.1146/annurev-astro-081811-125514

Cited by 834 publications

(810 citation statements)

References 113 publications

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“…On the observational side, only a handful of magnetic field strength estimates have been reported at densities 10 5 cm −3 . While low-density HI structures (such as Giant Molecular Clouds) are found to be globally magnetically subcritical (Heiles & Crutcher 2005), most observations towards dense molecular cloud cores on sub-pc scales obtained so far suggest that they are approximately critical or supercritical (Crutcher 2009). Typically, magnetic fields detected in these dense molecular clouds have strengths 0.1-1.5 mG (Crutcher et al 1999(Crutcher et al , 2004Falgarone et al 2008;Heyer & Brunt 2012).…”

Section: Background: the Role Of Magnetic Fields In Star Formationmentioning

confidence: 98%

CN Zeeman observations of the NGC 2264-C protocluster

Maury¹,

Wiesemeyer

Thum

2012

A&A

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From an observational point of view, the role of magnetic fields in star formation remains unclear, and two main theoretical scenarios have been proposed so far to regulate the star-formation processes. The first model assumes that turbulence in star-forming clumps plays a crucial role, and especially that protostellar outflow-driven turbulence is crucial to support cluster-forming clumps; while the second scenario is based on the consideration of a magnetically-supported clump. Previous studies of the NGC 2264-C protocluster indicate that, in addition to thermal pressure, some extra support might effectively act against the gravitational collapse of this clusterforming clump. We previously showed that this extra support is not due to the numerous protostellar outflows, nor the enhanced turbulence in this protocluster. Here we present the results of the first polarimetric campaign dedicated to quantifying the magnetic support at work in the NGC 2264-C clump. Our Zeeman observations of the CN(1-0) hyperfine lines provide an upper limit to the magnetic field strength B los 0.6 mG in the protocluster (projected along the line of sight). While these results do not provide sufficiently tight constraints to fully quantify the magnetic support at work in NGC 2264-C, they suggest that, within the uncertainties, the core could be either magnetically super or sub-critical, with the former being more likely.

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Section: Background: the Role Of Magnetic Fields In Star Formationmentioning

confidence: 98%

CN Zeeman observations of the NGC 2264-C protocluster

Maury¹,

Wiesemeyer

Thum

2012

A&A

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“…For example, Gabici et al (2010) argued that to explain the TeV emission of the molecular clouds near the SNR W 28, an average diffusion coefficient around 10% of the Galactic standard diffusion coefficient is required, albeit not only in the molecular clouds but everywhere around the SNR. Crutcher (2012) found that the maximum strength of the interstellar magnetic field stays constant at ∼10 µG up to densities n H ∼ 300 cm −3 , and above 300 cm −3 it increases following a power law with exponent ≈2/3. If one assumes that the magnetic turbulent strength in MC clumps is of order ∼10 µG as well, with a Kolmogorov-type power law for the magnetic turbulent power spectrum and the size of the MC clumps ∼1 pc as the maximum wavelength, one can obtain a much lower diffusion coefficient in dense clump regions with < ∼ 1% Galactic standard for 1−1000 TeV CRs (e.g., Fatuzzo et al 2010).…”

Section: Hess J1731-347 Still Residing Inside the Main-sequence Bubblementioning

confidence: 98%

A young supernova remnant illuminating nearby molecular clouds with cosmic rays

Cui

Pühlhofer

Santangelo

2016

A&A

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The supernova remnant (SNR) HESS J1731-347 displays strong nonthermal TeV γ-ray and X-ray emission, thus the object is presently accelerating particles to very high energies. A distinctive feature of this young SNR is the nearby (∼30 pc in projection) extended source HESS J1729-345, which is currently unidentified but is in spatial projection coinciding with known molecular clouds (MC). We model the SNR evolution to explore whether the TeV emission from HESS J1729-345 can be explained as emission from runaway hadronic cosmic rays (CRs) that are illuminating these MCs. The observational data of HESS J1729-345 and HESS J1731-347 can be reproduced using core-collapse SN models for HESS J1731-347. Starting with different progenitor stars and their presupernova environment, we model potential SNR evolution histories along with the CR acceleration in the SNR and the diffusion of the CRs. A simplified three-dimensional structure of the MCs is introduced based on 12 CO data of that region, adopting a distance of 3.2 kpc to the source. A Monte Carlo based diffusion model for the escaping CRs is developed to deal with the inhomogeneous environment. The fast SNR forward shock speed, as implied from the X-ray data, can easily be explained when employing scenarios with progenitor star masses between 20 M and 25 M , where the SNR shock is still expanding inside the main-sequence (MS) bubble at present time. The TeV spectrum of HESS J1729-345 is satisfactorily fitted by the emission from the highest energy CRs that have escaped the SNR, using a standard Galactic CR diffusion coefficient in the interclump medium. The TeV image of HESS J1729-345 can be explained with a reasonable three-dimensional structure of MCs. The TeV emission from the SNR itself is dominated by leptonic emission in this model. We also explore scenarios where the shock is starting to encounter the dense MS progenitor wind bubble shell. The escaping hadronic CR hypothesis for the γ-ray emission of HESS J1729-345 can still hold, but even in this case our model cannot easily account for the TeV emission from HESS J1731-347 in a hadronic scenario.

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“…Magnetic fields (e.g., Mouschovias 2001;Crutcher 2012;Li et al 2014, p. 101) and turbulence (e.g., Krumholz & McKee 2005;Padoan & Nordlund 2011;Federrath & Klessen 2012;Padoan et al 2014, p. 77) are both likely to be more important in influencing the gravitational stability of molecular gas and thus the regulation of star formation.…”

Section: Sfr Gasmentioning

confidence: 99%

“…The fiducial direction is q =  60 , though various orientations are explored. The fiducial magnetic field strength is set to be m 10 G, following Zeeman measurements of GMC field strengths (Crutcher 2012). Additionally, we test non-magnetized as well as more strongly magnetized ( m 30 G) cases to explore the effects of magnetic field strength.…”

Section: Initial Conditionsmentioning

confidence: 99%

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 Molecular Clouds

Cited by 834 publications

References 113 publications

CN Zeeman observations of the NGC 2264-C protocluster

CN Zeeman observations of the NGC 2264-C protocluster

A young supernova remnant illuminating nearby molecular clouds with cosmic rays

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

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