Observations of polarized light from stars.

Hall, John S.; Mikesell, A. H.

doi:10.1086/106256

Cited by 59 publications

(38 citation statements)

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“…The notion of a magnetized interstellar medium (ISM) dates back more than half a century, to at least the detection of polarized starlight (Hall, 1949;Hiltner, 1949) and its interpretation as coming from the absorption of the unpolarized starlight by magnetically aligned grains in the foreground medium (Davis and Greenstein, 1951;see Andersson et al, 2015 for a recent review). With the advent of observational capabilities, the magnetic fields in the ISM in general, and star-forming molecular clouds in particular, are becoming increasingly better characterized.…”

Section: Observational Motivationsmentioning

confidence: 99%

The Role of Magnetic Fields in the Formation of Protostellar Discs

Wurster

2018

Front. Astron. Space Sci.

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The formation of a protostellar disc is a natural outcome during the star formation process. As gas in a molecular cloud core collapses under self-gravity, the angular momentum of the gas will slow its collapse on small scales and promote the formation of a protostellar disc. Although the angular momenta of dense star-forming cores remain to be fully characterized observationally, existing data indicates that typical cores have enough angular momenta to form relatively large, 100 au-scale, rotationally supported discs, as illustrated by hydrodynamic simulations. However, the molecular clouds are observed to be permeated by magnetic fields, which can in principle strongly affect the evolution of angular momentum during the core collapse through magnetic braking. Indeed, in the ideal magnetohydrodynamic (MHD) limit, magnetic braking has been shown to be so efficient as to remove essentially all of the angular momentum of the material close to the forming star such that disc formation is suppressed. This failure to produce discs in idealized cores is known as the magnetic braking catastrophe. The catastrophe must be averted in order for the all-important rotationally supported discs to appear, but when and how this happens remains debated. We review the resolutions proposed to date, with emphasis on misalignment, turbulence and especially non-ideal effects. Non-ideal MHD accounts for charged and neutral species, making it a natural extension to the ideal MHD approximation, since molecular clouds are only weakly ionized. The dissipative non-ideal effects diffuse the magnetic field to weaken it, and the dispersive term redirects the magnetic field to promote or hinder disc formation, dependent upon the magnetic geometry. When self-consistently applying non-ideal processes, rotationally supported discs of at least tens of au form, thus preventing the magnetic braking catastrophe. The non-ideal processes are sensitive to the magnetic field strength, cosmic ray ionization rate, and gas and dust grain properties, thus a complete understanding of the host molecular cloud is required. Therefore, the properties of the host molecular cloud -and especially its magnetic field -cannot be ignored when numerically modelling the formation and evolution of protostellar discs.

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Section: Observational Motivationsmentioning

confidence: 99%

The Role of Magnetic Fields in the Formation of Protostellar Discs

Wurster

2018

Front. Astron. Space Sci.

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“…Ever since the surprising discovery of the polarization of starlight more than half a century ago ( Hall 1949;Hall & Mikesell 1949;Hiltner 1949aHiltner , 1949b, a great deal of attention has focused on the physical and dynamical properties of interstellar dust grains as the primary source of the observed phenomenon. From the very earliest, it was recognized that the selective extinction properties of dust grains determined by their size, configuration, and orientation must be responsible for the polarization.…”

Section: Introductionmentioning

confidence: 99%

Laboratory Experiments on Rotation and Alignment of the Analogs of Interstellar Dust Grains by Radiation

Abbas

Craven

Spann

et al. 2004

ApJ

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The processes and mechanisms involved in the rotation and alignment of interstellar dust grains have been of great interest in astrophysics ever since the surprising discovery of the polarization of starlight more than half a century ago. Numerous theories, detailed mathematical models, and numerical studies of grain rotation and alignment with respect to the Galactic magnetic field have been presented in the literature. In particular, the subject of grain rotation and alignment by radiative torques has been shown to be of particular interest in recent years. However, despite many investigations, a satisfactory theoretical understanding of the processes involved in grain rotation and alignment has not been achieved. As there appear to be no experimental data available on this subject, we have carried out some unique experiments to illuminate the processes involved in the rotation of dust grains in the interstellar medium. In this paper we present the results of some preliminary laboratory experiments on the rotation of individual micron /submicron-sized, nonspherical dust grains levitated in an electrodynamic balance evacuated to pressures of $10 À3 to 10 À5 torr. The particles are illuminated by laser light at 5320 8, and the grain rotation rates are obtained by analyzing the low-frequency ($0-100 kHz) signal of the scattered light detected by a photodiode detector. The rotation rates are compared with simple theoretical models to retrieve some basic rotational parameters. The results are examined in light of the current theories of alignment.

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“…Dust observations have long been seen as a tool for determining the magnetic field structure in molecular clouds through polarimetric observations. Polarization of starlight has been discovered in 1949 [30,31] and was almost immediately recognized as arising from absorption and scattering of starlight by elongated dust grains that are partially aligned by the magnetic field [32]. Thus, dust observations have become a tool to determine the structure of the galactic magnetic field at various scales.…”

Section: Magnetic Fieldmentioning

confidence: 99%

Dust in Star Forming Regions - A Review

Wiebe¹

2017

Proceedings of Frontier Research in Astrophysics – II — PoS(FRAPWS2016)

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Space dust is a major astronomical factor, which affects most observations in UV, optical, infrared, submillimeter, etc. Most often it is an obstacle, preventing us from getting undistorted information on various astronomical objects. Dust in star-forming regions is different. It is not only an obstacle but also an important tracer of physical conditions in these regions, a catalyst for interstellar chemistry, and a progenitor for planetary systems. In order to clarify all these various roles, we need to know properties of space dust and how they may change on different stages of the star formation process.

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Observations of polarized light from stars.

Cited by 59 publications

References 0 publications

The Role of Magnetic Fields in the Formation of Protostellar Discs

The Role of Magnetic Fields in the Formation of Protostellar Discs

Laboratory Experiments on Rotation and Alignment of the Analogs of Interstellar Dust Grains by Radiation

Dust in Star Forming Regions - A Review

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