scholarly journals Observational Aspects of Magnetic Fields in Molecular Clouds

1990 ◽  
Vol 140 ◽  
pp. 293-300
Author(s):  
T. H. Troland
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Zeeman Effect ◽  
Growing Body ◽  
Field Strengths

A small but growing body of observational information now exists regarding magnetic field strengths in molecular regions. Most of these data come from study of the Zeeman effect in 18 cm OH lines. The field is strong enough in many such regions to be dynamically important.

scholarly journals Magnetic fields in the non-masing ISM

2007 ◽  
Vol 3 (S242) ◽  
pp. 47-54
Author(s):  
Richard M. Crutcher
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Linear Polarization ◽  
Formation Process ◽  
Molecular Clouds ◽  
Zeeman Effect ◽  
Flux Ratio

AbstractObservations of the Zeeman effect in OH and H2O masers provide valuable information about magnetic field strength and direction, but only for the very high density gas in which such masers are found. In order to understand the role of magnetic fields in the evolution of the interstellar medium and in the star formation process, it is essential to consider the maser results in the broader context of magnetic fields in lower density gas. This contribution will (very briefly) summarize the state of observational knowledge of magnetic fields in the non-masing gas. Magnetic fields in H I and molecular clouds may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Existing observations will be reviewed. Results are that the strength of interstellar magnetic fields remains roughly invariant at 5-10 microgauss between densities of 0.1 cm−3 < n(H) < 1,000 cm−3 but increases proportional to approximately the square root of density at higher densities. Moreover, the mass-to-flux ratio is significantly subcritical (strong magnetic support with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that MCs and GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend crucially on the mass-to-flux ratio in each clump. Present data suggest that magnetic fields play a very significant role in the evolution of molecular clouds and in the star formation process.

scholarly journals Observability of the Magnetic Field in Molecular Clouds

1990 ◽  
Vol 140 ◽  
pp. 304-304
Author(s):  
N. Bel ◽  
B. Leroy
Keyword(s):  
Magnetic Field ◽  
Molecular Clouds ◽  
Diatomic Molecules ◽  
Zeeman Effect ◽  
Interstellar Clouds ◽  
The Magnetic Field

We have done detailed calculations of the Zeeman effect in the dozen diatomic molecules identified in interstellar clouds.

scholarly journals 19. Astrophysical applications of selective magnetic rotation

1958 ◽  
Vol 6 ◽  
pp. 166-168
Author(s):  
Y. öhman
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Zeeman Effect ◽  
Emission Lines ◽  
Absorption Lines ◽  
Magnetic Rotation ◽  
Double Refraction ◽  
Line Absorption ◽  
Complicated Process ◽  
The Magnetic Field

When measuring the magnetic fields of sunspots the astronomer assumes that the magnetic field revealed by the inverse Zeeman effect is the same as if the splitting were produced by emission lines instead of absorption lines. No doubt this is in general a very fair approximation, but we have reason to remember sometimes that line absorption in the presence of magnetic fields is a very complicated process. In the immediate neighbourhood of absorption lines effects of magnetic rotation of the plane of polarization and magnetic double refraction may appear in the spectrum.

scholarly journals Could bow-shaped magnetic morphologies surround filamentary molecular clouds?

2019 ◽  
Vol 632 ◽  
pp. A68 ◽  
Author(s):  
M. Tahani ◽  
R. Plume ◽  
J. C. Brown ◽  
J. D. Soler ◽  
J. Kainulainen
Keyword(s):  
Magnetic Field ◽  
Monte Carlo ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Molecular Clouds ◽  
Monte Carlo Analysis ◽  
Line Of Sight ◽  
Field Reversal ◽  
Filamentary Structure ◽  
Western Side

Context. A new method based on Faraday rotation measurements recently found the line-of-sight component of magnetic fields in Orion-A and showed that their direction changes from the eastern side of this filamentary structure to its western side. Three possible magnetic field morphologies that can explain this reversal across the Orion-A region are toroidal, helical, and bow-shaped morphologies. Aims. In this paper, we constructed simple models to represent these three morphologies and compared them with the available observational data to find the most probable morphology(ies). Methods. We compared the observations with the models and used probability values and a Monte Carlo analysis to determine the most likely magnetic field morphology among these three morphologies. Results. We found that the bow morphology had the highest probability values, and that our Monte-Carlo analysis suggested that the bow morphology was more likely. Conclusions. We suggest that the bow morphology is the most likely and the most natural of the three morphologies that could explain a magnetic field reversal across the Orion-A filamentary structure (i.e., bow, helical and toroidal morphologies).

scholarly journals Magnetic fields of rapidly oscillating Ap stars

1993 ◽  
Vol 139 ◽  
pp. 132-132
Author(s):  
G. Mathys
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Zeeman Effect ◽  
Rotation Frequency ◽  
Spectral Lines ◽  
Driving Mechanism ◽  
Line Splitting ◽  
Ap Stars ◽  
The Magnetic Field

Magnetic field appears to play a major role in the pulsations of rapidly oscillating Ap (roAp) stars. Understanding of the behaviour of these objects thus requires knowledge of their magnetic field. Such knowledge is in particular essential to interpret the modulation of the amplitude of the photometric variations (with a frequency very close to the rotation frequency of the star) and to understand the driving mechanism of the pulsation. Therefore, a systematic programme of study of the magnetic field of roAp stars has been started, of which preliminary (and still very partial) results are presented here.Magnetic fields of Ap stars can be diagnosed from the Zeeman effect that they induced in spectral lines either from the observation of line-splitting in high-resolution unpolarized spectra (which only occurs in favourable circumstances) or from the observation of circular polarization of the lines in medium- to high-resolution spectra.

scholarly journals Magnetic Fields & Ionized Gas in Elliptical Galaxy Halos

1990 ◽  
Vol 140 ◽  
pp. 459-462
Author(s):  
Richard G. Strom
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Elliptical Galaxy ◽  
Radio Sources ◽  
Early Type ◽  
Ionized Gas ◽  
X Ray ◽  
Galaxy Halos ◽  
Field Strengths

Faraday depolarization estimates of thermal densities within the components of double radio sources agree well with estimates from X-ray observations of hot halos around early-type galaxies, provided magnetic field strengths are close to their equipartition values. Internal Faraday dispersion is the main cause of the depolarization observed.

scholarly journals Magnetic Field Strengths of Spiral Galaxies and the Far-Infrared/Radio Correlation

1990 ◽  
Vol 140 ◽  
pp. 241-241
Author(s):  
A. J. Fitt ◽  
P. Alexander
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Field Distribution ◽  
Detection Rate ◽  
Spiral Galaxies ◽  
Far Infrared ◽  
Magnetic Field Distribution ◽  
Selection Effects ◽  
The Magnetic Field ◽  
Field Strengths

We have calculated equipartition magnetic fields for a complete, optically-selected sample of 165 spiral galaxies. The magnetic field distribution (fig. 1) is type independent, and shows remarkably little spread in values, around 1 decade in B. This is not due to selection effects because of the nature of the sample and the 95 percent detection rate.

scholarly journals Measurement of Magnetic-Field Strengths in Molecular Clouds: Detection of OH Line Zeeman Splitting

1987 ◽  
Vol 115 ◽  
pp. 55-57
Author(s):  
I. Kazès ◽  
R. M. Crutcher ◽  
T. H. Troland
Keyword(s):  
Magnetic Field ◽  
Radio Telescope ◽  
Molecular Clouds ◽  
Stokes Parameter ◽  
Zeeman Splitting ◽  
Spectral Lines ◽  
First Results ◽  
Extended Program ◽  
Interstellar Molecular Clouds ◽  
Field Strengths

We report here the first results of an extended program to measure magnetic-field strengths in interstellar molecular clouds. The very large radio telescope located near Nancay, France, has been used to measure the Stokes-parameter I and V spectra of the 1665 and 1667 MHz lines of OH in emission and in absorption from extended (non-masing) molecular clouds. Signals in the V spectra are produced by Zeeman splitting of the spectral lines; we derive magnetic-field strengths or limits from these data.

scholarly journals New OH Zeeman Measurements of Magnetic Field Strengths in Molecular Clouds

2001 ◽  
Vol 554 (2) ◽  
pp. 916-932 ◽  
Author(s):  
Tyler L. Bourke ◽  
Philip C. Myers ◽  
Garry Robinson ◽  
A. R. Hyland
Keyword(s):  
Magnetic Field ◽  
Molecular Clouds ◽  
Field Strengths
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