Abundance and excitation of H2,H+3 and CO in star-forming regions

Although most of the 123 interstellar molecules reported to date have been detected through millimeter wave emission line spectroscopy, this technique is inapplicable to non-polar molecules like H-2 and H-3(+) which are central to our understanding of interstellar chemistry. Thus, high resolution infrared absorption line spectroscopy plays an important role in interstellar studies: chemically important non-polar molecules can be observed, and their abundances and excitation conditions reference the same "pencil beam" absorbing column of gas. In this fashion, the abundance of cold H-2 in dark molecular clouds and star-forming regions can now be accurately measured and compared along the same line of sight with the abundance of its most commonly cited surrogate, CO. Also observed is the pivotal molecular ion H-3(+), whose abundance provides the most direct measurement of the cosmic ray ionization rate of H-2, a process that initiates the formation of many other molecules observed in dark clouds. For each of the distinct velocity components seen toward some embedded young stellar objects, we can determine the temperature, density, and abundance of CO, H-2, H-3(+), (CO)-C-13 and (CO)-C-12 thereby unraveling some of the internal structure of a star-forming cloud. These measurements represent significant tests of models of molecular clouds and star-forming regions.