A unified model of interstellar dust.

We have simultaneously modeled both the interstellar extinction and polarization (both linear and circular) on the basis of a trimodal dust model: large silicate core-organic refractory mantle dust particles; very small carbonaceous particles responsible for the hump extinction; and PAH's responsible for the FUV extinction. The core-mantle particles which are the exclusive contributor to the interstellar polarization and the dominant contributor to the visual and NIR extinction are modeled as finite cylinders with a Gaussian size distribution in terms of perfect spinning alignment. Results for models using infinite cylinders are presented for comparison. Our model results are in good agreement with such observational constraints as the average interstellar extinction curve, the polarization law, the ratio of visual polarization to extinction (P/A)_v_, the scattering properties (albedos), the excess NIR polarization over the extrapolation of the Serkowski law. The (P/A)_v_ constraint imposed on other dust models (e.g., the silicate/graphite model, the composite dust model) leads to either a too low (P/A)_v_ value (the silicate/graphite model) or instability of particle structure (the composite dust model). The cosmic abundance constraints, in particular the evidence for lower oxygen abundance in the interstellar medium than in the solar system and the possible interstellar C/O ratios, are discussed extensively. Considering the uncertainties in the interstellar ``cosmic'' abundances, the C/O ratio and the interstellar non-dust elemental abundances, although our model requires a bit more carbon than the reference abundance, it is within the limit of an acceptable range and is a significant improvement over other models. All the other major reference abundance (cosmic - non-dust) constraints on O, N, Si, Mg, Fe are well satisfied.