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J.L. Linsky, S. Redfield (JILA/U. Colorado)
We have constructed a three-dimensional model of the LIC based on GHRS, EUVE, and ground-based Ca~II spectra. Our model is based on (1) hydrogen column densities derived from GHRS deuterium column densities for 16 lines of sight to nearby stars, (2) EUVE spectra of three hot white dwarfs, and (3) Ca~II spectra for 13 lines of sight with absorption at the projected LIC velocity. The model is a sum of nine spherical harmonics that best fit the data for these 32 lines of sight. The model can be viewed at the Colorado Model of the Local Interstellar Medium website. The input data, prescription for computing the model, and a tool for calculating the hydrogen column density through the LIC along any line of sight are also available at this website. As new data appear the website will include new versions of the Colorado LIC model and models for nearby warm clouds in the LISM.
The LIC model is neither a long thin filamentary structure like optical images of some interstellar clouds, nor is it spherical in shape. As seen from the North Galactic Pole, the LIC is egg-shaped with an axis of symmetry that points in the direction l\approx 315\circ. Since the direction of the center of the Scorpius-Centaurus Association is l=320\circ, the shape of the LIC could be determined by the flow of hot gas from Sco-Cen. The model shows that the Sun is located just inside the LIC in the direction of the Galactic Center and toward the North Galactic Pole. The absence of Mg~II absorption at the LIC velocity toward \alpha~Cen indicates that the distance to the edge of the LIC in this direction is \leq 0.05 pc and the Sun should cross the boundary between the LIC and the G cloud in less than 3,000 yr. We estimate that the volume of the LIC is about 115 pc3 and its mass is about 0.79 M\odot.
The physical parameters and hydrogen column density of the LIC are roughly consistent with theoretical models of the warm ISM that assume pressure and ionization equilibrium. However, the empirical hydrogen ionization of the LIC is much higher (\approx 50%) and the gas temperature lower (7000~K) than the theoretical models predict. The high ionization can be explained by the recombination of the LIC gas following shock ionization from a nearby supernova. The high ionization increases the gas cooling which can explain why the gas is about 2400~K cooler than the ionization equilibrium models predict. Computed and observed temperatures are in agreement for a theoretical model with the observed LIC electron density.
This work is supported by NASA grants to the University of Colorado.
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