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The Three-Dimensional Physical Structure of the Local Interstellar Medium

TitleThe Three-Dimensional Physical Structure of the Local Interstellar Medium
Publication TypeThesis
Year of Publication2003
AuthorsRedfield, S

The Local Interstellar Medium (LISM) is a complex environment consisting of collections of warm, partially ionized gas. Ultraviolet (UV) observations of nearby stars have demonstrated that the LISM is organized into individual clouds of gas moving at various discrete velocities. The primary goal of this thesis is to develop a three- dimensional model of the physical structure of the LISM.

This thesis includes a complete collection of high-resolution UV observations of the LISM from spectra of stars within 100 parsecs. The measurements of numerous ions include HI, DI, CII, CIII, NI, NII, OI, MgII, Al II, Si II, Si III, Ca II, and Fe II. Although most of these ions lie in the UV waveband observed by the Hubble Space Telescope (HST), observations in the far-UV with the Far Ultraviolet Spectroscopic Ex- plorer (FUSE) and in the optical with ultra-high resolution ground-based spectrographs are also included to complement the inventory of LISM observations.

The use of multiple ionic species allows for accurate measurements of such fun- damental physical quantities as: projected velocity, depletion and abundances, temper- ature, turbulent velocity, and ionization structure. We find evidence for the interaction of warm clouds in the LISM from decelerated velocities at the leading edge of the Local Interstellar Cloud (LIC). We search for small-scale structure in the LISM by observing a cluster of Hyades stars. No signficant variations in column density are observed for angular distances of < 8°. By estimating the length scale of the absorbing material by assuming a constant D/H ratio and a homogeneous hydrogen number density, we model the morphology of the LIC using a series of spherical harmonics. We present measurements of temperature and turbulent velocity by comparing the line widths of various observed ions with different atomic masses.

We observe spatial variations in all measured quantities, which indicates that coherent, discrete clouds of warm partially ionized material populate the LISM. Corre- lating many lines of sight allows us to determine the basic morphology of warm clouds of gas in the LISM, and to probe the three-dimensional physical structure of gas in our local environment.