Figure 6.9 upper panel: Subtraction of the right- and left-circularly polarized components of an OH line to yield the V-spectrum. lower panel: Observed I - and V-spectra of the 1665 MHz and 1667 MHz lines inB1.

derivative on the right side using the average of IR(v — vQ) and IL (v — vQ). Such an average is equal to one half of the I-spectrum, where the Stokes I-parameter is the sum of the two circularly polarized components. Finally, one determines B\\ through a fitting procedure. Note that a negative value of B\\ indicates a field pointing toward the observer.

The first successful application of this procedure in molecular clouds was toward NGC 2024 in the Orion A complex (Plate 1). In this case, the 1665 and 1667 MHz lines appear in absorption against the continuum radiation from a background HII region, and the derived B\\ is 38 ^G. Thus far, it has proved difficult to extend the technique to dense cores. The OH molecule is created through ion-molecule reactions, and the relative abundance of the species diminishes in very optically thick regions. Nevertheless, there are a handful of relevant observations. Figure 6.9 shows the Zeeman splitting in OH emission from the central region of the B1 cloud. The best-fit value for B\\ of —27 ^G indicates that the field is providing substantial support against the cloud's self-gravity. We will later study in depth this important aspect of cloud structure.

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