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Figure 5.8 Near-infrared spectrum of the BN object in Orion, shown at three different observing times. The relative flux is plotted against the wave number k, defined here as 1/A.

For the CO molecule, J can change by either — 1 or +1 between different rovibrational states. The lines from J — 1 ^ J fall on the other side of the central gap, which lies to the right of the spectrum shown in Figure 5.9. The energies in this P-branch are where, again, J = 1, 2,3, etc. Because of the sign change from equation (5.15), the frequency falls below the gap for any J. The spacing between successive lines widens, and there is no convergence to a band head.

Populating the upper electronic levels of CO requires even more energetic environments. The first excited electronic level lies at an equivalent temperature of 9.3 x 104 K above ground. Downward rovibrational transitions give rise to bands in the ultraviolet regime. Fewer lines now separate the band head from the gap, since the difference Bv> - Bv•• in equation (5.15) is significantly greater. If enough energy is available, CO dissociates. As in H2, collisional dissociation is direct, while photodissociation occurs through a two-step process. That is, the molecule must first be excited by line absorption to a higher electronic level, from which it can either relax to the ground state or else fall apart into separate carbon and oxygen atoms.

We have seen how the lowest rotational levels of CO saturate in population as the cloud density climbs above ncrit for that species. The molecule then ceases to be a useful observational gauge of ntot. Additionally, we have noted that photons from a given CO molecule are absorbed and reemitted many times by other, identical molecules before leaving the cloud. In other words, the cloud becomes optically thick to this radiation, a condition that sets in first for the lowest transitions of the main isotope 12C16O. The detected radiation is then only sampling conditions in the sparse, outer regions of the cloud.

Since its discovery in 1969, interstellar ammonia has been one of the most widely used probes for higher-density molecular regions. Formed through a network of gas-phase reactions, the polyatomic NH3 has a much more complex set of transitions than CO and is therefore a more sensitive diagnostic of cloud conditions. Many useful transitions fall within a narrow frequency range, thus greatly reducing relative errors in instrumental calibration.

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