SWAS Spacecraft
The Submillimeter Wave Astronomy Satellite
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Observing the Chemical Evolution of Molecular Clouds

After hydrogen and helium, carbon and oxygen are the most abundant elements in the Universe and are the building blocks of life as we know it. Molecules formed through the interactions of these two atoms, along with the more abundant hydrogen atom, dominate the chemistry in cloud interiors and set the energy balance of interstellar clouds. Ultimately, the chemical interactions of carbon and oxygen not only govern the formation of various molecular species but also play an important and uncertain role in the collapse of cloud material to form stars and planets.

The chemistry of molecular clouds is quite different than the chemistry we see in the laboratory on the Earth. For example, the gas in molecular clouds which can be considered dense by interstellar standards is still more rare than the best vacuum we can achieve in the laboratory. Models of time-dependent chemistry in interstellar clouds (Figure 4) predict that almost all of the gas phase carbon will be in the form of carbon monoxide (CO), with the oxygen predicted to be mostly in CO, molecular oxygen (O_2), and water (H_2O).

(Figure 4)

As an example of the importance of these species to the chemical evolution of molecular clouds, most of the nearly 100 molecular species that have been identified in the interstellar medium have abundances relative to the dominant hydrogen molecule (H_2) that are less than one part in 100 million (1 x 10-8), but the relative abundance of O_2 is predicted to be 9 x 10_2 and that of water
is 3 x 10_2.

While the CO molecule, and numerous lesser abundant molecules, are readily observable from ground-based observatories, both O_2 and H_2O cannot be observed in the cold environment typical of molecular clouds because of blockage from the Earth's atmosphere. Using special techniques, such as observations of isotopic water, H_2^18O, from NASA's Kuiper Airborne Observatory, some limits have been set on the abundances of water and molecular oxygen at single positions in a handful of cloud cores. However, the abundances of these key components of the interstellar medium remain poorly constrained by current observations and these observations have provided no information on their spatial distribution.

With the space-based platform of SWAS, astronomers will be able for the first time to directly observe
O_2 and H_2O over many positions in numerous molecular cloud cores. A primary goal will be to detect or set an upper limit on the water and molecular oxygen abundance (relative to H_2) For molecular oxygen the detection limit is below most of the predictions from current chemical models and detection of the emission from these species will have important consequences on the theories of the evolution of molecular clouds and the process of star formation.