Composition, Structure, and Size Distribution of Dust in the Local Interstellar Cloud

The average properties of interstellar dust have previously been inferred from remote astronomical observations of interstellar extinction and from gas depletion measurements. In addition to recent high-resolution observations of gas absorption spectra, in situ measurements of dust in the solar system provide an alternative approach to deducing the properties of interstellar dust, in particular, those of dust in the Local Interstellar Cloud (LIC), in which the Sun resides. We constrain the composition and structure of dust in the LIC by the dust-phase elemental abundances derived from gas absorption measurements and by the dynamical behavior inferred from dust impact measurements. The elemental abundances of the LIC dust are consistent with core-mantle grains consisting of Mg-rich pyroxene and Mg-rich olivine with inclusions of troilite, Fe-rich kamacite, and corundum in the core and organic refractory compounds of C, N, and O in the mantle. The mass of the organic refractory mantle is comparable to the mass of the silicate core that is abundant in pyroxene compared to olivine. Taking into account these results, the dynamical behavior of the LIC dust in the solar system indicates that bare silicates and bare carbonaceous materials may be present as grains smaller than 10-17 kg. The LIC grains with mass exceeding 10-17 kg are most likely aggregates of submicron-sized silicate core, organic mantle grains. The mass distribution of dust in the LIC can be well explained by coagulation growth of core-mantle grains but is scarcely explained by severe destruction of grains in interstellar shocks.