"Solid State Chemistry in Dense Clouds Along Quiescent Lines of Sight"
We will study the infrared spectra from 5.3 to 21.8 micrometers through dense interstellar clouds, with little or no star formation activity, to assess the early chemistry of molecular cloud dust. Dense clouds produce molecules and ices critical to star and planet formation. The formation of organic compounds in these ices is one of the first steps toward the complex molecular materials needed for life. Infrared spectroscopy provides a powerful tool for the study of the composition and evolution of interstellar ices. The most diagnostic features of solid-state materials occur in the mid-infrared. To date, mid-infrared absorption studies have primarily been toward embedded protostars where the ice may well have been processed either thermally or by far ultraviolet photons from the star. Such sightlines demonstrate a preponderance of simple molecules (water, methanol, carbon monoxide, carbon dioxide, and ammonia) and energetically processed species (nitriles and cyanates) in the surrounding ices, revealing that protostars strongly influence their circumstellar environments. Lines of sight to these objects are unlikely to be representative of dense cloud materials as a whole. A more complete understanding of the composition of dense clouds and their chemical dynamics requires that we also probe lines of sight through the general quiescent cloud medium. We have obtained low resolution spectra from the Spitzer Space Telescope's Infrared Spectrometer in the wavelength region that includes the absorption features of water, methanol, methane, and carbon dioxide, plus high resolution IRS spectra for selected sources, to study detailed band profiles. We will correlate band strengths with the amount of dust obscuration to determine the abundances and densities required for the ice components to appear, and study the chemical changes in molecular clouds as a function of temperature and density. These observations will provide a snapshot of the chemical state of a molecular cloud prior to the formation of stars, and a general baseline for studies of dust chemistry in regions of star formation.