Carbon monoxide (CO) is a compound with molecules each consisting of one oxygen and one carbon atom. In astronomy, CO emission lines are used to detect molecular clouds. Though the clouds are primarily molecular hydrogen, the hydrogen lines are weak and only detectable in nearby clouds. A series of very useful CO lines is due to the steps in CO's possible rotational energy (CO transitions), i.e.,
J(J+1)ℏ² Erot = ———————— 2I
where Erot is the energy level, I is the moment of inertia, ℏ is the reduced Planck constant, and J is 0, 1, 2, etc. The frequencies of lines produced by stepping up or down these energy levels are multiples of an increment near 115 GHz. Lines of interest include:
|CO(7-6)||806.7 GHz||0.37 mm|
|CO(6-5)||691.473 GHz||0.43 mm|
|CO(5-4)||576.3 GHz||0.52 mm|
|CO(4-3)||461.0 GHz||0.65 mm|
|CO(3-2)||345.8 GHz||0.87 mm|
|CO(2-1)||230.5 GHz||1.3 mm|
|CO(1-0)||115.3 GHz||2.6 mm|
|13CO(1-0)||110.2 GHz||2.72 mm|
These can be detected in distant galaxies (e.g., z=1) in the millimeter/submillimeter range (e.g., submillimeter galaxy designator). and suggest gas clouds, i.e., indicating starburst galaxies and possibly mergers. CO lines, such CO(1-0) are useful for determining redshifts. CO(6-5) is used for observing protoplanetary disks and very distant (z>6) molecular clouds. This series is known as the CO ladder (CO rotational ladder or CO J ladder) and when observed together (yielding their spectral line energy distribution (SLED)), can yield qualities of the gas, in particular, its temperature.
From CO line widths, using the virial theorem, if virial stability is assumed, the mass of the cloud can be calculated. Assuming a particular CO to H2 factor, the mass of a distant molecular cloud can be calculated.
CO incorporating rarer isotopes, such as C17O, C18O, or 13CO, are sometimes preserved better or otherwise easier to observe, thus are sometimes measured, using normal ratios of isotopes to extrapolate the mass of CO, and from that, the mass of H2 (CO to H2 factor). Large clouds have sufficient CO to saturate the lines of the most abundant isotope combination, 12C16O, so the observer may determine from it no more than a minimum amount of CO present. Inclusion of another isotope affects the wavelength of the CO lines sufficiently that they can be independently measured, thus the value in detecting isotopes in analyzing molecular clouds above a certain size.