(4) Dust Grain Carriers and the Diffuse Band Profiles
The optical properties of dust grains containing resonant impurities were
described in an invited review chapter by
Shapiro (1995) in a
book on the diffuse interstellar bands ("DIBs"). The optical properties
of grains with resonant impurities have long been of astronomical interest
in connection with the hypothesis that such grains are the source of
the mysterious DIBs. In the seven decades since their first discovery, no
convincing explanation for these DIBs has yet been found. The "embedded
cavity model" of Purcell and Shapiro (1977, ApJ, 214, 92), generalized and
applied to explain the observed absorption profiles of the DIBs by Shapiro
and Holcomb (1986, ApJ, 305, 433; 1986, ApJ, 310, 872) is the standard
theory of the optical properties of grains hosting impurity absorbers and
the standard model for the dust grain origin of the DIB profiles. This
model was used to derive scattering and absorption cross sections, extinction
profiles, and anomalous linear and circular polarization for such grains,
as well as the effective bulk dielectric constant for the composite material.
The results were summarized and conclusions were drawn based upon a comparison
of these theoretical extinction profiles with the observed profiles of DIBs,
including the recent results of an observational search for profile variations
amongst the stars in the Rho Oph interstellar dark cloud correlated with
variations in the wavelength of maximum interstellar polarization of starlight
by the dust in this direction. The current conclusion is that the dust
grain explanation for the observed profiles is still viable. It was further
pointed out that such a dust grain model is relevant even if there is someday a
convincing identification of the molecular species responsible for the bands
in the gas-phase, since there is the possibility of a dust-grain-residence
time or dust-grain-assisted formation for the molecule, and molecules which
are large enough may actually acquire optical properties similar to those
of small grains.