HIGH-RESOLUTION H-I OBSERVATIONS OF H-II REGIONS .3. PHOTODISSOCIATION REGIONS AND THE MAGNETIC-FIELD NEAR ORION-B

H I observations of the H II region Orion B are presented with an angu lar resolution of 50'' (0.1 pc) and velocity resolution of 0.64 km s-1 . Three kinematically distinct H I layers are detected in absorption. It is argued that one of these H I components originates in a photodis sociation region (PDR) directly outside the ionization front in front of the H II region. In addition to the H I absorption, H I emission is detected, most likely originating in a PDR behind the H II region. Co mbining the H I emission and absorption data yields the conclusion tha t a range of temperatures exists in the PDRs, a conclusion corroborate d by existing [C II] 158 mum line measurements of the PDRs. While the H I absorption data show the presence of gas colder than approximately 20 K (which is too cold to contribute significantly to the [C II] emi ssion), the H I and [C II] data can be brought in agreement if the emi tting H I is a temperature of at least several hundred degrees. These results are interpreted in terms of a clumpy PDR model, where the cold er gas is located in dense clumps, which are embedded in a warmer, low -density interclump medium. An analysis of the Zeeman effect as determ ined from the observed circular polarization yields line-of-sight magn etic fields of 28 and 63 muG for two of the H I layers, at a reduced a ngular resolution of 100''. It is concluded that the magnetic field st rength and the total pressure in the dense PDR are higher than in the more tenuous cloud envelope. However, in the cloud envelope the pressu re is dominated by the magnetic field, while in the PDR turbulent pres sure dominates.