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

Abstract

We use recent results on interstellar gas toward nearby stars and interstellar by-products within the solar system to select among the equilibrium radiative transfer models of the nearest interstellar material presented in Slavin & Frisch. For the assumption that O/H~400 parts per million, models 2 and 8 are found to yield good fits to available data on interstellar material inside and outside of the heliosphere, with the exception of the Ne abundance in the pickup ion and anomalous cosmic-ray populations. For these models, the interstellar medium (ISM) at the entry point to the heliosphere has n(H 0)=0.202-0.208 cm -3, n(He 0)=0.0137-0.0152 cm -3, and ionizations χ(H)=0.29-0.30, χ(He)=0.47-0.51. These best models suggest that the chemical composition of the nearby ISM is ~60%-70% subsolar if S is undepleted. Both H 0 and H + need to be included when evaluating abundances of ions found in warm diffuse clouds. Models 2 and 8 yield an H filtration factor of ~0.46. Gas-to-dust mass ratios for the ISM toward ɛ CMa are R gd =178-183 for solar abundances of Holweger or R gd =611-657 for an interstellar abundance standard 70% solar. Direct observations of dust grains in the solar system by Ulysses and Galileo yield R gd ~=115 for models 2 and 8, supporting earlier results (Frisch and coworkers). If the local ISM abundances are subsolar, then gas and dust are decoupled over small spatial scales. The inferred variation in R gd over parsec length scales is consistent with the fact that the ISM near the Sun is part of a dynamically active cluster of cloudlets flowing away from the Sco-Cen association. Observations toward stars within ~500 pc show that R gd correlates with the percentage of the dust mass that is carried by iron, suggesting that an Fe-rich grain core (by mass) remains after grain destruction. Evidently large dust grains (>10 -13 g) and small dust grains (<10<SUP>-13 g) are not well mixed over parsec length spatial scales in the ISM. It also appears that very small C-dominated dust grains have been destroyed in the ISM within several parsecs of the Sun, since C appears to be essentially undepleted. However, if gas-dust coupling breaks down over the cloud lifetime, the missing mass arguments applied here to determine R gd and dust grain mineralogy are not appropriate.