Annu. Rev. Astron. Astrophys. 1990. 28:
37-70 Copyright © 1990 by Annual Reviews. All rights reserved |
5.1 Continuum Polarization
5.1.1
1. The averaged value of max is 0.55 µm
(151a),
with extremes from about 0.34 µm to about 1
µm. The values of max,
determined by a least-squares
fitting to Serkowski's law rather than a direct search for the maximum,
depend mainly upon the observations at extreme wavelengths.
2. The polarization typically rises with wavelength
through the ground-based UV to a maximum in the optical and then
falls slowly through the NIR. Such behavior bears little resemblance
to the extinction law, which keeps rising monotonically, except for the
bump, towards shorter wavelengths throughout the observable UV. The
grains responsible for the extinction in the ground-based UV do not
participate in polarization because they are not elongated and/or not aligned.
3. The value of max is almost proportional to RV
(25,
167,
168),
although there is more scatter in the relationship than for the
extinction laws. To a large extent, optical polarization measurements
can substitute for NIR extinction in obtaining RV.
4. The polarization law p() varies as p() -1.8 for both diffuse
dust and outer-cloud dust in the range 0.9 µm < < 5 µm
(108).
The polarization law exponent is less well determined than for extinction,
varying between -1.5 and -2.0 for various samples, but it is certainly
similar to the value for extinction (-1.7 - -1.8; see
Section 2.1.3).
Note that
this relation involves the absolute polarization, not relative to
p(max). The
optical p() does vary strongly with RV (see point 2
above), and the silicate feature has strong polarization which dominates
for > 5 µm. The
independence of p() from RV again
suggests that the size distribution of large grains is similar for
clouds and the diffuse ISM.
5. The maximum value of p(max) / A(max) is about 0.03 mag-1, far less
than from perfectly aligned spinning cylinders [0.22 mag-1
(111)].
This is interesting because the polarization direction closely follows the
contours of the edges of several molecular clouds, presumably in regions
where hydrogen changes its state from molecular to atomic relatively
abruptly in space and perhaps in time. If the alignment mechanism keeps
grains aligned under these adverse conditions, one would expect almost
complete alignment when conditions are favorable, and a larger value of
p(max) /
A(max) than is
observed, in directions where the line of sight is
perpendicular to the field. Perhaps there are two or more separate
types of grains, only some of which are aligned. Alternatively, all
grains might be well aligned but have shapes that are less efficient
than a spinning cylinder for producing polarization. A third
possibility is that there is always a randomly oriented component to the
field.
6. Polarization in the UV is unknown except for two stars
(58).
These limited data suggest that the bump is unpolarized.
Upcoming observations from space (the WUPPE experiment on ASTRO
missions) should provide many data. The polarization of the
2175 bump has been predicted
if the bump is produced by aligned graphite
(36).
An explanation for the form of the polarization law
(111)
assumes that grains can be aligned only if they contain one or more
``superparamagnetic'' particles (magnetite or other magnetic materials),
which dissipate rotational energy as heat. Large grains are
preferentially aligned because they are relatively likely to contain
inclusions. Polarization is not specific to any particular grain model;
if the large grains are aligned, and a model predicts the extinction
correctly, it will do well for the polarization also.
5.1.2
Grain alignment is even more difficult for dense clouds than for the
diffuse ISM
(71).
Alignment depends upon the grain
being far from equilibrium with its surroundings, in which case there is
no preferred axis by the equipartition of energy. Deep inside a cloud, a
grain should come to thermal equilibrium with the dense surrounding gas.
The fact that polarization is observed shows that the rotation of
aligned grains within clouds is not thermalized; they are presumably
kept spinning in a particular direction because of the ejection of
particles (H2 after formation, or electrons) from particular sites
(134),
so the
momentum of the ejected particles is not random. However, deep inside a
cloud one would expect the gas impinging upon the grain to already be
overwhelmingly H2. Probably there also needs to be an enhanced
dissipation of energy by superparamagnetic inclusions in the grains.