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Traditionally, the main problem considered in connection with
circular interstellar polarization is the meaning of the wavelength
where changes the sign
. Its proximity to
was treated as the indicator of the dielectric nature
of interstellar grains. This feature was established by Martin ([1972])
in the first
serious theoretical paper which accompanied the discovery of interstellar circular
polarization. Martin ([1972], [1974]) found that the condition
could be satisfied if the
imaginary part of the refractive index is low at visible wavelengths
(
). However, Shapiro ([1975]) using DDA
calculations for parallelepipeds of magnetite showed
that a strongly absorbing conductor could reproduce
this condition as well.
Given conclusion is, possibly, not unquestionable
because of doubt about the application of DDA to highly
absorbing particles.
Chlewicki and Greenberg ([1990]) reexamined the relationship
between the interstellar linear and circular polarization on the basis of
Kramers-Kronig relations. They concluded that the
observed connection between
and
was reached independently of specific characteristics of grains,
so long as they provide the ``correct''5 linear
polarization.
Note also that the shape effects for circular polarization were not
analyzed: Martin ([1972], [1974]) considered
infinite cylinders with PF orientation and
,
Chlewicki and Greenberg ([1990]) performed
calculations for infinite cylinders with PDG and
IDG orientations and
.
Calculations made for spheroidal particles
show that the position where the circular
polarization changes the sign shifts with variations of the particle
inclination in the same manner as the position of
.
This is clearly seen from Fig. 12 (right panels) where
the efficiency factors are plotted for prolate spheroids with
:
reduction of
leads to increase of the
values
where the circular polarization factors intersect the zero level.
This means that the positions of both
and
should move to shorter wavelengths if the direction of alignment
approaches the line of sight.
Such a behaviour is seen in Fig. 19 (middle panel) for prolate spheroids
while for oblate particles the displacement takes place in the opposite
direction. Figure 19 (upper panel) also shows changes of
circular polarization
with wavelength for particles of astrosil and
AC1. Comparison with the corresponding picture for linear polarization
(Fig. 16, upper left panel) allows one to see an
approximate coincidence of
and
for both dielectric and absorbing particles.
Finally, we can conclude that
the wavelength where the interstellar circular polarization changes
the sign and its coincidence with
tell us almost nothing about the absorptive properties of interstellar
grains but the dependence
and the positions
of their maxima/minima can serve for clearing up the
dust properties.
It is important
that from the point of view of circular
polarization there exists a large difference between prolate
and oblate grains. The latter
particles always produce much larger polarization (Fig. 19).
Note also that the circular polarization observed for stars
indicates the complex structure of interstellar
magnetic fields in their directions and gives information on the
birefringence in the dust
cloud nearest to the observer.