Spectroscopy Between the Stars

The emission and absorption spectra of interstellar molecules are reviewed with special consideration of recent observational and technical advances in the shorter submillimeter wave region of the electromagnetic spectrum. Single-dish observations have contributed in the past probably most of the information about the structure of interstellar molecular clouds. At present about 120 interstellar molecules have been identified in interstellar clouds and circumstellar envelopes, evidence of a rich and diversified chemistry. CO, the most abundant interstellar molecule and other diatomic molecules and radicals are found throughout molecular clouds, whereas the more complex molecules are found in high-density cores, which are often the sites of active star formation. These locations represent prime targets for the search for larger molecules, such as glycine. The ignition of young stars is accompanied by strong heating of the surrounding material by radiation and/or shocks, leading to photoevaporation of molecules depleted on dust grains driving a "hot core" chemistry, traceable by its rich organic chemistry and its prevailing high excitation conditions (up to about 2000 cm^-1). However, in the list of detected interstellar molecules many simple hydrides are still missing, e.g. SH, PH, PH₂, etc., which constitute the building blocks for larger molecules. With the technological opening of the terahertz region (ν ∼1 THz corresponds to λ ∼0.3 mm) to both laboratory and interstellar spectroscopy, great scientific advances are to be expected. Amongst these will be the direct detection of the lowest rotational transitions of the light hydrides, the low energy bending vibrations of larger (linear) molecules, and possibly the ring-puckering motion of larger ring molecules such as the polycyclic (multiring) aromatic hydrocarbons. This revised version was published online in June 2006 with corrections to the Cover Date.