Abstract
Strong circular polarization of OH masers at 1665 and 1667 MHz lines has been observed towards the molecular cloud cores associated with HII regions. Magnetic field strengths of a few mG are derived from the Zeeman splitting of OH lines. For instance, a magnetic field of about 4 mG in the masing region of W3(OH) has been estimated by OH-line Zeeman splitting (Davies, 1974). VLBI observations show that the OH maser spots project onto or very close to the surface of associated compact HII regions (Reid et al., 1986). The observational evidence demonstrates that the scales of OH maser components surrounding a compact HII region (R ~ 1016 cm) are about 1014 cm in diameter with an amplification pathlength of ~1015 cm. Hence the magnetic fields determined by the Zeeman splitting of OH maser lines appear partly very close to the associated HII region. Elitzur (1979) has theoretically obtained similar results as above.
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