Molecular clouds infrared spectroscopy

Dense molecular clouds, often also called dark clouds, block entirely the light of stars which lie behind them, and can therefore be studied observationally only by radio astronomy or infrared techniques. These clouds have a visual extinction in excess of A 10 which corresponds to a gas density of n lO cm" and a kinetic temperature usually well below T 100 K, typically between 10 and 25 K. Within the last ten years, the investigation of these dark molecular clouds has become almost entirely the domain of radio astronomy although now the first very promising results by infrared astronomy reveal the power of this new branch of spectroscopy. 

Infrared spectroscopy enables us to obtain information on the chemical composition and structure of icy grains in interstellar molecular clouds [3], Table 9.3 summarizes the abundance of molecules identified [4]. Among these species, the predominance of H2O ice is clear, its abundance being one order of magnitude greater than that of all odier molecules. The molecules CO and CO2 are those next most abundant, following H2O. Small amounts of reduced molecules, hydrocarbons and NH3 are also observed. 

A large tenuous cloud surrounds the object and is seen as a reflection nebula illuminated by the starlight that escapes above and below the ring of dust. The discovery of this object has provided dramatic evidence supporting earlier hypotheses that circum-stellar envelopes of infrared stars must be flattened. The large optical depth of the toroid produces a featureless, mid-infrared spectrum (Forrest eit 1976) but the chemical nature of the cloud has been deduced from optical spectroscopy of the reflection nebula (Crampton, Cowley and Humphreys, 1975) and by detection of a molecular cloud association with the source (Lo and Bechis, 1976 and Zuckermann ad 1976). These observations show that the 0.1 M cloud is carbon-rich, and, in fact has led to the suggestion that the source may be the progenitor of a planetary nebula. 

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