The Three-dimensional Structure of the Warm Local Interstellar Medium. II. The Colorado Model of the Local Interstellar Cloud

In this second paper in a series on the structure of the local interstellar medium (LISM), we construct a three-dimensional model of the local interstellar cloud (LIC) based on Hubble Space Telescope (HST), Extreme Ultraviolet Explorer (EUVE), and ground-based Ca II spectra. Starting with hydrogen column densities derived from deuterium column densities measured with the Goddard High Resolution Spectrograph instrument on HST for 16 lines of sight to nearby stars, we derive a model consisting of the sum of nine spherical harmonics that best fit the data. We then rederive the model by including the lines of sight to three hot white dwarfs observed by EUVE and 13 lines of sight with Ca II column densities at the projected LIC velocity. The LIC model is clearly not a long thin filamentary structure like optical images of some interstellar clouds (e.g., reflection nebulae in the Pleiades), but neither 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 ≈ 315°. Since the direction of the center of the Scorpius-Centaurus association is l = 320°, 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 α Cen indicates that the distance to the edge of the LIC in this direction is ≤0.05 pc and the Sun should cross the boundary between the LIC and the Galactic (G) cloud in less than 3000 yr. We estimate that the volume of the LIC is about 93 pc3 and its mass is about 0.32 M☉. The physical parameters and hydrogen column density of the LIC are roughly consistent with theoretical models of the warm interstellar medium that assume pressure and ionization equilibrium. However, the empirical hydrogen ionization of the LIC is much higher and the gas temperature lower than the theoretical models predict. Therefore, the ionization is likely due to shock activity from a nearby supernova that has not yet reached equilibrium. The higher ionization increases the gas cooling, which can explain why the gas is 2400 K cooler than the ionization equilibrium models predict. Computed and observed temperatures are in agreement for a model with the observed LIC electron density.

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