Zeeman splitting of 18 centimeter OH lines toward Cassiopeia A and other sources

Heiles, C.; Stevens, M.

doi:10.1086/163903

Cited by 29 publications

(8 citation statements)

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“…In addition to the molecular cloud envelope observations, we briefly observed the polarized OH maser sources W49 and W3(OH) in order to verify the observing setup and data processing procedures. In addition, we observed the previously well-studied OH Zeeman absorption line source NGC2024 (Orion B, W12) in order to verify that the procedures produced the same B LOS , including the sign or direction of the magnetic field, as had been obtained previously (Crutcher & Kazès 1983;Heiles & Stevens 1986;Crutcher et al 1999;Bourke et al 2001). (Note that the field direction reported in the Crutcher & Kazès (1983) paper was wrong; the correct direction is given in the latter three papers.)…”

Section: Green Bank Telescope Observationsmentioning

confidence: 79%

Testing Magnetic Star Formation Theory

Crutcher

Hakobian

Troland

2009

ApJ

143

228

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Zeeman observations of molecular clouds yield the line-of-sight component B LOS of the magnetic vector B, which makes it possible to test the two major extreme-case theories of what drives star formation -ambipolar diffusion or turbulence. However, only one of the three components of B is measurable, so tests have been statistical rather than direct, and they have not been definitive. We report here observations of the Zeeman effect in the 18-cm lines of OH in the envelope regions surrounding four molecular cloud cores toward which detections of B LOS have been achieved in the same lines, and evaluate the ratio of mass to magnetic flux, M/Φ, between the cloud core and envelope. This relative M/Φ measurement reduces uncertainties in previous studies, such as the angle between B and the line of sight and the value of [OH/H]. Our result is that for all four clouds, the ratios R of the core to the envelope values of M/Φ are less than 1. Stated another way, the ratios R ′ of the core to the total cloud M/Φ are less than 1. The extreme case or idealized (no turbulence) ambipolar diffusion theory of core formation requires the ratio of the central to total M/Φ to be approximately equal to the inverse of the original subcritical M/Φ, or R ′ > 1. The probability that all four of our clouds have R ′ > 1 is 3 × 10 −7 ; our results are therefore significantly in contradiction with the hypothesis that these four cores were formed by ambipolar diffuson. Highly super-Alfvénic turbulent simulations yield a wide range of relative M/Φ, but favor a ratio R < 1, as we observe. Our experiment is limited to four clouds, and we can only directly test the predictions of the extreme-case "idealized" models of ambipolar-diffusion driven star formation

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Section: Green Bank Telescope Observationsmentioning

confidence: 79%

Testing Magnetic Star Formation Theory

Crutcher

Hakobian

Troland

2009

ApJ

143

228

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“…In determining the mean field, we exclude the known HII region S88B since it is not representative of the envelopes of molecular clouds. However, we include Cassiopeia A (Cas A), for which field strengths and OH column densities were determined by Heiles & Stevens (1986). The line-of-sight toward this source intersects molecular clouds in the Perseus arm of the Milky Way.…”

Section: Analysis and Resultsmentioning

confidence: 99%

A Survey of Magnetic Field Strengths in the Envelopes of Molecular Clouds via the 18 cm OH Zeeman Effect

Thompson

Troland

Heiles

2019

ApJ

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We present the results of an extensive Arecibo observational survey of magnetic field strengths in the inter-core regions of molecular clouds to determine their role in the evolution and collapse of molecular clouds as a whole. Sensitive 18 cm OH Zeeman observations of absorption lines from Galactic molecular gas in the direction of extragalactic continuum sources yielded 38 independent measurements of magnetic field strengths. Zeeman detections were achieved at the three sigma level toward 9 clouds, while the others revealed sensitive upper limits to the magnetic field strength. Our results suggest that total field strengths in the inter-core regions of GMCs are about 15 µG.

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“…CN Zeeman-splitting measurements have also provided rather low upper limits to the Ðeld strength compared with estimated equipartition values (Crutcher et al 1996) and also magnetic Ðeld detections well below equipartition (Crutcher et al 1999). A rather low average Ðeld of ]9 kG was also found in Cas A by Heiles & Stevens (1986).…”

Section: Cloud and Flow T Opologymentioning

confidence: 92%

A Super‐Alfvenic Model of Dark Clouds

Padoan

Nordlund²

1999

ApJ

349

376

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Supersonic random motions are observed in dark clouds and are traditionally interpreted as Alfve n waves, but the possibility that these motions are has not been ruled out. In this work we super-Alfve nic report the results of numerical experiments in two opposite regimes : and whereMach numberÈthe ratio of the rms velocity to the speed. Our results show Alfve nicAlfve n that models with are consistent with the observed properties of molecular clouds that we have M A ? 1 tested (statistics of extinction measurements, distribution of integrated antenna temperature, Zeemansplitting measurements of magnetic Ðeld strength, line width versus integrated antenna temperature of molecular emission-line spectra, statistical B-n relation, and scatter in that relation), while models with have properties that are in conÑict with the observations. We Ðnd that both the density and the M A D 1 magnetic Ðeld in molecular clouds may be very intermittent. The statistical distributions of the magnetic Ðeld and gas density are related by a power law, with an index that decreases with time in experiments with decaying turbulence. After about one dynamical time it stabilizes at B P n0.4. Magnetically dominated cores form early in the evolution, while later on the intermittency in the density Ðeld wins out, and also cores with a weak Ðeld can be generated by mass accretion along magnetic Ðeld lines.

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Zeeman splitting of 18 centimeter OH lines toward Cassiopeia A and other sources

Cited by 29 publications

References 0 publications

Testing Magnetic Star Formation Theory

Testing Magnetic Star Formation Theory

A Survey of Magnetic Field Strengths in the Envelopes of Molecular Clouds via the 18 cm OH Zeeman Effect

A Super‐Alfvenic Model of Dark Clouds

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