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We have carried out wide-field BVRI CCD imaging of a selection of interstellar ``cirrus'' clouds using the Schmidt telescopes at KPNO and CTIO. In most cases, these images are sensitive enough to detect diffuse optical emission in all of the wavelength bands. The optical images have been compared with IRAS infrared maps to study the properties of the interstellar grains in these clouds.
We find that the broad morphologies of the optical and infrared emission are generally similar. Detailed analysis of the optical colors of these clouds shows that certain clouds have colors consistent with those expected for scattering of interstellar radiation field photons off of silicate dust grains. However, a subset of the clouds exhibit colors far too red to be accounted for by simple scattering. Earlier studies, based on photographic data and small-field CCD imaging, found similar results and it was suggested that the red colors could arise from luminescence in large molecules within the clouds. We use the observed optical colors of the clouds to test this model.
There is a broad range of optical colors from cloud to cloud and even within different areas of a given cloud. In the ``blue'' cirrus clouds, the ratio of B to 100$\mu m$ surface brightness over most regions is consistent with that expected if the same grains that emit the bulk of the infrared radiation are the primary scatterers. The observed blue flux per unit 100$\mu m$ flux is slightly below that predicted by isotropic scattering models, however, indicating that interstellar grains preferentially forward scatter. There are isolated areas within selected clouds that exhibit faint blue emission that was not detected by IRAS, suggesting the presence of a population of cold, efficient scatterers. The relationship between the optical and infrared emission in the ``red'' clouds is found to be much more complex suggesting that the different grain species in the clouds may be inhomogeneously mixed.
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