Magnetic Fields in Molecular Clouds: Observations Confront Theory

Crutcher, R. M.

doi:10.1086/307483

Cited by 843 publications

(1,057 citation statements)

References 43 publications

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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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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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“…Crutcher 1999, with a = 1.2 and b = 0.3 following Mckee et al 1993). The physical parameters of the clouds are derived from NH 3 (Fiebig 1990) and C 3 H 2 studies (Cox et al 1989;Cox, priv.…”

Section: Ccs Resultsmentioning

confidence: 99%

Zeeman line splitting measurements sampling dense gas in dark cloud and star-forming cores

Uchida

Fiebig

Güsten

2001

A&A

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Abstract. We present measurements of Zeeman splitting in SO JN = 12 − 11 line emission/absorption arising from dense thermal gas (>10 6 cm −3 ) associated with ten compact H II and star-forming regions. No line-of-sight magnetic field, above an average 1σ limit of 0.57 mG, is detected toward any of these sources. The W3(OH) region, found in a previous study to have a line-of-sight field of 3 mG in its dense gas, is thus unique in this respect. The implication of this study is that the magnetic fields in dense clouds are supercritical in the sense that they are not capable of supporting their host clouds against gravitational collapse. The relatively low average field limit is consistent with those theories predicting the quick onset of and short timescales for ambipolar diffusion. Also presented are (1) Zeeman splitting measurements of the CCS JN = 10 − 01 line probing the 10 4−5 cm −3 gas towards three positions in the Taurus star forming region, giving a 1σ average upper limit of 66 µG, and (2) measurements of Zeeman splitting in the OH 2 Π 3/2 J = 7/2 F = 3 + − 3 − hyperfine line toward the compact H II regions DR-21 and G10.62.

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“…Although it is difficult to precisely measure the mass-to-Ñux ratio in cores with observations, the most massive cores seem to be in magnetically supercritical state, in which the magnetic force is not strong enough to support the core (Bertoldi & McKee 1992 ; see also Crutcher 1999). Scott & Black (1980) made detailed numerical calculations of the collapse of a magnetically supercritical core and found that the collapse occurs preferentially along the Ðeld direction to form a disklike subregion with higher density.…”

Section: Relationship Between Core Evolution Andmentioning

confidence: 99%

Submillimeter Imaging Polarimetry of the NGC 7538 Region

Momose

Tamura

Kameya

et al. 2001

ApJ

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Imaging polarimetry of the 850 km continuum emission in the NGC 7538 region, obtained with the SCUBA Polarimeter, is presented. The polarization map is interpreted in terms of thermal radiation by magnetically aligned dust grains. Two prominent cores associated with IRS 1 and IRS 11, IRS 1(SMM), and IRS 11(SMM), are found in the surface brightness map. Although these cores look similar in surface brightness, their polarization shows striking di †erences. In IRS 11(SMM), the polarization vectors are extremely well-ordered, and the degrees of polarization are quite high with an average of D3.9%. In IRS 1(SMM), on the other hand, the directions of polarization vectors are locally disturbed, and the degrees of polarization are much lower than those of IRS 11(SMM). These di †erences suggest that small scale Ñuctuations of the magnetic Ðeld are more prominent in IRS 1(SMM). This can be interpreted in terms of the di †erence in evolutionary stage of the cores. Inside IRS 1(SMM), which seems to be at a later evolutionary stage than IRS 11(SMM), substructures such as subclumps or a cluster of infrared sources have already formed. Small scale Ñuctuations in the magnetic Ðeld could have developed during the formation of these substructures. The distribution of magnetic Ðeld directions derived from our polarization map agrees well with those of molecular outÑows associated with IRS 1(SMM) and IRS 11(SMM). Comparisons of energy densities between the magnetic Ðeld and the outÑows show that the magnetic Ðeld probably plays an important role in guiding the directions of the outÑows.

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Magnetic Fields in Molecular Clouds: Observations Confront Theory

Cited by 843 publications

References 43 publications

The dynamics of charged dust in magnetized molecular clouds

The dynamics of charged dust in magnetized molecular clouds

Zeeman line splitting measurements sampling dense gas in dark cloud and star-forming cores

Submillimeter Imaging Polarimetry of the NGC 7538 Region

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