Measurement of Zeeman pattern with VLBI

Magnetic field is believed to play an important role in the evolution of compact HII regions where massive stars are formed. In fact, it exists very few methods to measure it. One of the best method used, is based on the Zeeman effect: under the effect of the magnetic field, a line can be split into two separated lines of opposite circular polarization. The separation in frequency between the lines is directly proportional to the intensity of the magnetic field. One of the difficulty of the method, is to made a correct identification of the Zeeman pairs. The very high resolution achievable by VLBI allows us to demonstrate whether the pairs are spatially coincident (as expected in true Zeeman pairs) or not. The OH emission traces the densest pockets of the neutral gas environment. In addition, because of its non zero electronic angular momentum, the OH radical is a natural magnetometer allowing us to determine both the intensity and the direction of the magnetic field in the outer layers of the compact HII regions. In this poster I reported on the possibilities of VLBI observations of OH masers in the first excited state (J = 5/2) lying at 6 GHz.