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Molecular Tracers

Astronomers can learn much about the physical conditions in distinct regions of space from the emissions of minute amounts of certain gas molecules they contain. Temperatures, densities, and pressures; all of these may be deduced from the unique spectra exhibited by such tracers.

For instance, molecular gas, one comprised of molecules, traces cold temperatures. This is because molecules tend only to dissociate into individual atoms at warmer temperatures. Gas containing atoms, atomic gas, traces medium temperatures. However, because atoms lose their electrons -- become ionized -- at hotter temperatures, detection of ionized tracers indicates that very hot conditions prevail.

The presence of trace gases is revealed by their spectral lines. These same lines and their Doppler shifts may also provide information about the motions of the gases, their chemical interactions, the magnetic and gravitational fields present and the underlying physics associated with these processes.

Such information acts as a springboard from which astronomers can arrive at a coherent understanding of the physics, chemistry and dynamics of the present universe.

What Do Trace Molecules Tell Us?

Spectral emissions by molecules such as carbon monoxide, sulfur dioxide, ammonia and methane trace the conditions present in giant molecular clouds, or GMC's, in which stars are born. Most of the gases in GMC's are molecular, except in the hotter regions where starbirth is taking place.

By studying the spectral lines of the tracers, astronomers can pin point regions of high and low density. For instance, hydrogen cyanide (HCN) molecules are typically seen in regions of dense gas, indicating possible star forming regions. On the other hand, carbon monoxide (CO) molecules are found mostly in outlying, lower density regions. A picture emerges of a hot core where a protostar develops, with a cooler envelope surrounding it.

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Copyright © 1995: Board of Trustees, University of Illinois

Eleni Adrian, NCSA. Last modified by David Curtis, 6/24/95