High resolution H I observations of H II regions. II - H I and the magnetic field near W3

H I absorption observations of the W3 complex, conducted using the Westerbork Synthesis Radio Telescope (angular resolution 13 arcsec, velocity resolution 1 km/s), are presented. The observed velocity structure is explained by the influence of the shock front associated with the H II region W4 on the W3 molecular cloud. The model provides an explanation for the observed velocities of ionized, neutral, and partially ionized gas in the various components of the W3 complex. The line-of-sight magnetic field in the absorbing gas is determined from the H I Zeeman effect. Significant small-scale structure is found. The magnetic field exhibits a centrally peaked morphology of the line-of-sight magnetic field toward W3A in the -45.8 km/s H I component. It is suggested that the magnetic field is a vestige of recent formation of massive stars in that area. The estimated time scale for ambipolar diffusion is consistent with this hypothesis