The interstellar medium near stars with peculiar interstellar polarizations

Abstract

The properties of the interstellar medium along the line of sight to the small number of stars whose wavelength of maximum interstellar polarization is greater than or equal to 6500 Å are studied. The optical interstellar line measurements reported here confirm the hypothesis that the long λ_(max) phenomenon occurs in regions where the mean grain size is abnormally large, and that this takes place only in the densest parts of the interstellar medium. The grains grow by accretion from the gas, not by coalescence of previously existing grains. Furthermore, the material accreted in the dense clouds (i.e., the mantle) must have a different chemical composition from the grain core; it must consist of substances composed largely of hydrogen

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