Astrochemistry: an unfinished symphony

Book chapter, 2009

Alex Dalgarno has introduced many of the themes that define current research in astrochemistry. Variations on these themes will continue to be played out well into the future. Clear and unambiguous tests of theories of formation of interstellar molecules remain elusive; however, the understanding of fluorine chemistry appears to offer one success story. The predicted chemical activity in X-ray dominated regions and in shock waves is being studied with increasingly sensitive observations. Molecules whose chemistry and internal excitation are strongly coupled are especially valuable. In some cases, observations of these reactive species can provide probes of specific chemical rates in interstellar gas. Superthermal OH observed in a shock front appears to offer special diagnostic value. Even cosmologists have recognized that chemistry plays an important role in the early evolution of matter in the Universe around the epoch of recombination. The hydrogen molecule is being used to test cosmological theories of dark matter.