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D. E. Welty (U. Chicago)
For cool, neutral, predominantly atomic gas at T = 80~K, the thermal widths (FWHM) of absorption lines range from 0.26 km~s-1 (Fe) to 0.73 km~s-1 (Li), and the internal turbulent motions may be comparable to the sound speed (~ 0.7 km~s-1). High-resolution (FWHM = 0.3--1.8 km~s-1) optical spectra of Na~I, K~I, Ca~I, Ca~II, and Ti~II absorption reveal complex structure along many lines of sight, with a median separation between adjacent components of order 1 km~s-1. Furthermore, those adjacent components may have very different properties (e.g., line width, Na~I/Ca~II ratio). Such high-resolution spectra are therefore required for discerning and determining the properties of individual interstellar clouds. Detailed comparisons of the high-resolution line profiles for different species can yield estimates for cloud temperatures and turbulent velocities, relative depletions, and information on cloud structure. Component structures derived from the high-resolution optical spectra may be used to model the absorption-line profiles of many other neutral and singly ionized species seen in lower resolution UV spectra --- yielding additional diagnostics for the physical conditions (density, temperature, ionization, radiation field) characterizing the individual clouds. The resulting individual cloud abundances and physical parameters provide more relevant and stringent tests for models of cloud structure and chemistry.
This work has been supported by NASA LTSA grant NAG5-3228.
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The author(s) of this abstract have provided an email address for comments about the abstract: welty@oddjob.uchicago.edu