Observations of Far-Infrared Fine Structure Lines in the Interstellar Medium

Abstract

Fine structure lines in the far-infrared have many simple properties that make them useful tools for diagnosing astrophysical plasmas. The line ratios are sensitive and accurate probes of density, elemental abundances, and ionization structure; also, line shapes and Doppler shifts have proven handy in studies of gross dynamics of H II regions and galaxies. These lines have several analytical and practical advantages over spectral features in the optical and radio regions for measuring certain parameters of ionized regions.