Dust in the local interstellar wind

The gas-to-dust mass ratios found for interstellar dust within the solar sy stem, versus values determined astronomically for the cloud around the sola r system, suggest that large send small interstellar grains have separate h istories and that large interstellar grains preferentially detected by spac ecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectr um and Bur rate of interstellar dust within the heliosphere are combined. w ith information about the density, composition, and relative how speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, R-g/d = 94(-38)(+46 ). This ratio is dominated by the larger near-micron-sized grains. Includin g an estimate for the mass of smaller grains, which do not penetrate the he liosphere owing to charged grain interactions with heliosheath and solar wi nd plasmas, and including estimates for the mass of the larger population o f interstellar micrometeorites, the total gas-to-dust mass ratio in the clo ud surrounding: the solar system is half this value. Based on in situ data, interstellar dust grains in the 10(-14) to 10(-13) g mass range are undera bundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such sma ll grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abun dances in the nearest interstellar clouds. Measurements of interstellar abs orption lines formed in the cloud around the solar system, as seen in the d irection of is an element of CMa, give R-g/d = 427(-207)(+72) for assumed s olar reference abundances and R-g/d = 551(-251)(+61) for assumed B star ref erence abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that s weptup stardust is present. Such high values for diffuse interstellar cloud s are strongly supported by diffuse cloud data seen toward lambda Sco and 2 3 Ori, provided B star reference abundances apply. If solar reference abund ances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. T he cloud surrounding the solar system exhibits enhanced gas-phase abundance s of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H-0 indic ates either variable gas ionization or the decoupling of neutral gas and du st over parsec scale lengths. These abundances, combined with grain destruc tion models, indicate that the nearest interstellar material has been shock ed with shocks of velocity similar to 150 km s(-1). If solar reference abun dances are correct, the low R-g/d value toward lambda Sco may indicate that at least one cloud component in this direction contains dust grains that h ave retained their silicate mantles and are responsible for the polarizatio n of the light from nearby stars seen in this general region. Weak friction al coupling between gas and dust in nearby low density gas permit inhomogen eities to be present.