Suppose we wish to observe some molecule through its emission in a particular spectral line. The detection of this line, which results from a transition between discrete energy levels, requires an ambient temperature high enough to excite the upper level of interest. Within quiescent clouds, this requirement generally singles out low-lying rotational transitions, with associated photon wavelengths in the millimeter regime. Even when it has such a transition, however, a highly complex molecule is intrinsically difficult to detect. In this case, a great many levels exist in any appreciable energy range. Many of these become populated in a sufficiently warm environment, so that the power in any one transition is relatively small.
Most of the molecules observed to date contain one or more carbon atoms. While no inorganic species found in space contain more atoms than NH3, organic molecules exist in complex rings and chains. Hence the carbon bond, which plays such a dominant role in terrestrial chemistry, is also important in the interstellar environment. Furthermore, since the cosmic abundance of oxygen exceeds that of carbon, it is no surprise that the relatively tightly bound CO is the most abundant species after H2 itself. The observations of CO, pursued since 1970, have yielded more information on star-forming regions than any other molecule.
Theorists employ time-dependent computer models to understand the pattern of chemical abundances in any region. These programs simulate large reaction networks, usually operating at fixed ambient density and temperature. With time, the reactions that create and destroy various species equilibrate, and the abundances approach steady-state values. Dense cores with no internal stars provide a particularly simple environment to test such schemes. Starting with reasonable initial conditions, the models have little difficulty matching the observed abundances of simple species like CO, CS, or HCO+. Such agreement represents a gratifying confirmation of the ion-molecule chemistry at the root of these networks. On the other hand, the models are not without problems. One finds that complex organics always build up in time initially and then disappear as carbon becomes locked up in CO. At the density and temperature of a typical dense core such as TMC-1, there would be nearly complete conversion of atomic carbon to CO by 1 x 106 yr.
Such a time is difficult to reconcile with our understanding of cloud history. Although no accurate ages for dense cores are available, the process of gravitational settling that creates them operates over a period of order 107 yr. The observed presence of organics therefore remains puzzling. In addition, the current chemical models have difficulty explaining the significant spatial variation in molecular abundances seen across TMC-1 and other starless cores. The two simplest possibilities- gradients in age or elemental composition- both seem rather contrived as
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