Evolution of the spectrum and VLBI structure of W75N during the huge OH maser flare in 2003-2007

Abstract

We present results of single-dish and very long baseline interferometry (VLBI) observations of a strong 1000-Jy OH maser flare in the star-forming region W75N. The flare was first seen in 2003, and persisted for at least four years. Three major spectral features were present during the whole period of observations, with N1 being the strongest initially and N2 becoming stronger later. The large velocity range of 43 km s−1 observed during the flare is not kinematic in origin, but is caused by the Zeeman splitting in the strong magnetic field generated by the central star. The maps and proper motion of the maser spots were monitored over eight epochs, between 1998 and 2006, using the European VLBI Network (EVN) supplemented by published Very Large Baseline Array (VLBA) data. Mapping of the maser spots has shown that all the new spectral features of the flare were located close to the ultracompact H ii region VLA2, while the quiescent maser features were found near VLA1. We suggest that the flare was initiated by a disturbance, probably an magnetohydrodynamic (MHD) shock launched from the massive young star, also responsible for the excitation of the ultracompact H ii region. The proper motion of the majority of the maser spots was not detected at 1665 and 1667 MHz with the upper limit of several km s−1. The only positive detection of the proper motion was made for the two brightest flare features N1 and N2. The spot N2 is moving along the filamentary spot N3, with a tangential velocity of 14 km s−1. This motion is caused by the MHD shock propagating along the filament. The measured low upper limit for the velocity of OH maser spots is consistent with models of a slowly rotating Keplerian disc rather than with outflows, jets or expanding shells.