of the B1 cloud in Perseus, along with - . ^ ) the central embedded star. Two contours are shown for each line, an inner one near the peak intensity and an outer one at half that value.

collisional deexcitation of the line is also low, and OH serves as a tracer only of rather diffuse material. Figure 6.6 provides one example, the well-studied B1 cloud in Perseus. Here, the OH map covers a region almost as extended as that seen in the more abundant 13C16O. Both species saturate in intensity outside the densest central region. The (2,2) transition of NH3, on the other hand, is seen only near the embedded star.

6.3.1 Zeeman Splitting

The main utility of OH is not as a tracer of cloud gas but as a probe of the local magnetic field strength. The physical basis for these measurements is the Zeeman effect.2 The energy of an OH molecule situated in a magnetic field B depends on the relative orientation of the field and the molecule's magnetic moment (:

Since the magnetic moment of OH arises principally from the presence of an unpaired electron, proper evaluation of Emag requires a perturbative analysis of the electronic wave function. Such analysis shows that ( has contributions from both the electron's orbital and spin angular momenta:

The first term is e

2mec where —e and me are, respectively, the electronic charge and mass. The reader may verify that equation (6.29) would also result classically if the electron's magnetic moment arose from the

Magnetic fields in the atomic envelopes of molecular clouds are observed through the Zeeman effect in HI.

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