AAS 196th Meeting, June 2000
Session 26. The Galactic ISM: Observations and Modelling
Display, Tuesday, June 6, 2000, 10:00am-6:30pm, Empire Hall South

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[26.05] Ionization of the Local Cloud: Reconciling Line of Sight and In Situ Data

J. D. Slavin (Eureka Scientific, Inc.), P. C. Frisch (University of Chicago)

We present results of photoionization calculations that model the physical characteristics of interstellar material within three parsecs of the Sun (a.k.a. the Local Fluff). Determining the ionization of the interstellar material flowing past the Solar System is of great interest for studies of both the Local Interstellar Medium (LISM) and the heliosphere. Combining measurements of ion column densities (\ion{H}{1}, \ion{C}{2}, \ion{C}{2}*, \ion{N}{1}, \ion{O}{1}, \ion{Mg}{1}, \ion{Mg}{2}) in nearby stars with observations of pickup ions within the solar system (He, N, O, Ne) places tight constraints both on photoionization models of the Local Fluff and gas-phase elemental abundances. Line of sight column densities constrain the overall properties of the Local Fluff, while the pickup ion data constrain interstellar physical properties at the entry point to the heliosphere.

Our photoionization models include EUV radiation from stellar sources (WDs, B stars), the hot gas from the Local Bubble and (for some models) radiation from a conductive interface at the boundary between the Local Fluff and the hot gas. Because our models include the variation in electron density and ionization with cloud depth we are able to reconcile seemingly contradictory evidence regarding the electron density in the Local Fluff. By determining the total gas phase elemental abundances for several elements (e.g. Mg, Fe) we are able to make estimates of the mass tied up in grains in the LISM for comparison with the directly observed interstellar dust inside the Solar System.

This research has been supported by the NASA Sun-Earth Connection Heliospheric Physics/Cosmic Particle Theory and Analysis Program.

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