Carbon Monoxide

Molecule (CO) used to map the distribution of matter, especially molecular hydrogen, H2, in interstellar space. Molecular hydrogen is by far the dominant molecule in molecular clouds, but is very difficult to detect. One reason is that the strength of spectral lines from molecules is related to how asymmetric the molecule is. Since the hydrogen molecule is perfectly symmetric (containing two H atoms), its spectral lines are extremely weak. In addition, the energy required to change the rotational state of a molecule is dependent on its mass. Since the hydrogen molecule is the lightest of all molecules, a significant amount of energy (~500 Kelvin) must be absorbed to change its rotational state. In a cloud with an average temperature of ~10 Kelvin, this is an unlikely event and most of the hydrogen molecules remain in their ground state. In contrast, CO is asymmetrical and relatively heavy (28 vs. 2 amu). It has been shown that for every CO molecule there are about 10,000 H2 molecules meaning that we can trace molecular hydrogen through the emission from the CO molecule. This is the primary method use to locate molecular clouds.

In the Milky Way, the CO luminosity is closely correlated with the virial masses of the clouds (below), justifying the use of CO as a tracer of the mass of H2. The best fit to these data for clouds with masses between 105 and 2 x 106 Msun yields a constant of proportionality of 3.0 x 1020 H2 cm-2 (K km s-1)-1. The similarity of the clouds in other galaxies (M31, M33, and IC 10) to the Milky Way justifies the use of the same CO-H2 proportionality in external galaxies.

Some or all content above used with permission from J. H. Wittke.

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