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Magnetic Fields via Polarimetry Progress of Grain Alignment Theory
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Magnetic Fields via Polarimetry: Progress of
Grain Alignment Theory
A. Lazarian a,
aUniversity of Wisconsin-Madison, Astronomy Department, 475 N. Charter St.,
Madison, WI 53706, e-mail: lazarian@astro.wisc.edu
Abstract
Most astrophysical systems, e.g. stellar winds, the diffuse interstellar medium, molec-
ular clouds, are magnetized with magnetic fields that influence almost all of their
properties. One of the most informative techniques of magnetic field studies is based
on the use of starlight polarization and polarized emission arising from aligned dust.
How reliable the interpretation of the polarization maps in terms of magnetic fields
is the issue that the grain alignment theory addresses. Although grain alignment is
a problem of half a century standing, recent progress achieved in the field makes us
believe that we are approaching the solution of this mystery. I review basic physical
processes involved in grain alignment and discuss the niches for different alignment
mechanisms. I show why mechanisms that were favored for decades do not look so
promising right now, while the radiative torque mechanism ignored for more than
20 years looks so attractive. I define the observational tests and outline the circum-
stances when grain alignment theory predicts that new yet untapped information
of magnetic field structure is available through polarimetry. In particular, I touch
upon mapping magnetic fields in circumstellar regions, interplanetary space and in
comet comae.
1 Introduction
Magnetic fields are of utmost importance most astrophysical systems. Con-
ducting matter is entrained on magnetic field lines and magnetic pressure
and tension are very important for its dynamics. For instance, galactic mag-
netic fields play key role in many processes, including star formation, medi-
ating shocks, influencing heat and mass transport, modifying turbulence etc.
Aligned dust grains trace the magnetic
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