Dust in the Local Interstellar Wind

Abstract

The gas-to-dust mass ratios found for interstellar dust within the Solar System, versus values determined astronomically for the cloud around the Solar System, suggest that large and small interstellar grains have separate histories, and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow 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 +46−38 . This ratio is dominated by the larger near-micron sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere due to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the Solar System is half this value. Based on in situ data, interstellar dust grains in the of 10 −14 to 10 −13 g mass range are underabundant in the Solar System, compared to an MRN mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of ǫ CMa, give−207 for assumed solar reference abundances, and R g/d =551 +61 −251 for assumed B-star reference abundances. These values exceed the in situ value, suggesting either grain mixing or grain histories are not correctly understood, -4or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen towards λ Sco and 23 Ori, provided B-star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the Solar System exhibits enhanced gas-phase abundances 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 • indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scalelengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ∼150 km s −1 . If solar reference abundances are correct, the low R g/d value towards λ Sco may indicat...

The Galactic Environment of the Sun: Interstellar Material Inside and Outside of the Heliosphere

“…Since early 2000, Ulysses has detected interstellar dust flux levels above 10 −4 m −2 s −1 again . Until early 2005 the upwind direction of the interstellar grains was within a few tens of degrees in agreement with the interstellar helium flow direction (Landgraf and Grün 1998;Frisch et al 1999;Krüger et al 2006). During 2000 and 2006 Ulysses was traveling through almost the same spatial regions and with almost identical detection geometries for interstellar grains; however the upwind direction of the grains was somewhat wider and shifted by about 30 degrees away from the helium flow direction towards southern ecliptic latitudes during the latter set of observations (Krüger et al 2007).…”

“…The LIC abundances of refractory elements found from these radiative transfer models are typical abundances for interstellar clouds where dust grains have been destroyed by processing through interstellar shock fronts, i.e. the Routly-Spitzer effect (Routly and Spitzer 1952;Jones et al 1994), and in the case of the LIC indicate grain processing by a shock with velocity ∼90-140 km s −1 (Frisch et al 1999). A problem with this interpretation is that models (e.g., Jones et al 1994) predict that silicate dust would be more destroyed than carbonaceous dust.…”

“…2.3, Fig. 6, Frisch et al 1999) with radii a 0.2 μm in comparison to the MRN power-law distribution of n(a) ∼ a −3.5 (Mathis et al 1977). The very smallest carbonaceous grains, either graphites or polycyclicaromatic-hydrocarbons (PAH) with radii <0.01 μm, appear to be missing from the CHISM entirely because all carbon is in the gas phase (Sect.…”

“…4.3, . The exclusion of small interstellar dust grains from the heliosphere has been discussed in many papers (Frisch et al 1999;Czechowski and Mann 2003a;Mann and Kimura 2000;Grün et al 2005).…”

“…The direction of peak polarization is shifted by ∼ 40 • from the He o upwind direction. The black arrow shows the upstream direction, and 1σ uncertainties, for interstellar dust flowing through the heliosphere based on Ulysses and Galileo data (Frisch et al 1999). The green arrow shows the upstream direction from observations of interstellar He o in the inner heliosphere.…”