Coagulation as Unifying Element for Interstellar Polarization

Abstract

We calculate the extinction and polarization for aligned dust aggregates consisting of 4, 8, 16, 32, and 64 spherical particles (monomers). We consider a power-law size distribution of monomers and optical constants related to interstellar values. Our calculations match the polarization and extinction by the general diffuse interstellar medium. They also match the linear correlation between the observed shift (increase) of the wavelength of maximum polarization with increasing total to selective extinction (RV). The model naturally incorporates the low values of RV at small wavelength of maximum polarization observed in molecular clouds at high Galactic latitude. A further result of the calculations is a systematic and correlated decrease in polarization efficiency with increasing aggregate size, ruling out very large aggregates if polarization is observed. Our results also support the idea that the observed change of features in molecular clouds such as the Taurus dark clouds is rather the result of a change in alignment than a change in particle size, although the small degree of coagulation that can still be inferred is in agreement with a simple coagulation model. Therefore, small aggregates in a limited size range serve as a good and most natural model structure for interstellar dust particles, with the number of monomers being the only parameter to be adjusted.