The Chemical Composition and Gas-to-Dust Mass Ratio of Nearby Interstellar Matter

Abstract

Recent results on nearby interstellar gas and interstellar byproducts within the solar system are used to select among the equilibrium radiative transfer models of the nearest interstellar material of Slavin and Frisch (2002). Assuming O/H~400 ppm, Models 2 and 8 are found to yield good fits to available data on local interstellar material, and pickup ions and anomalous cosmic rays inside of the heliosphere, with the exception of Ne. For these models, the density of interstellar gas at the entry point to the heliosphere is n(HI)~0.20 /cc and n(e-)~0.1 /cc. These models suggest the chemical composition of the nearby ISM is ~60--70% subsolar if S is undepleted (where HI and HII must be included for abundance calculations). Gas-to-dust mass ratios of 178-183 for solar abundances, or 611-657 for 70% solar abundances are found. The percentage of the dust mass that is carried by iron is directly correlated with the gas-to-dust mass ratio implying an Fe-rich grain core remains after grain destruction.