Compact H II regions - Hydrogen recombination and OH maser lines

Abstract

Hydrogen recombination lines at 15 and 22 GHz and hydroxyl (OH) maser emission at 1.6 GHz have been mapped toward nine compact H II regions using the VLA. The authors find that all OH masers have radial velocities which are redshifted from the velocities of the H II regions as deduced from their hydrogen recombination lines. The majority of the maser spots appear projected onto the most compact H II region in a complex. These observations are inconsistent with models in which the masers form in an expanding, shocked shell, and they are explained most simply if the OH maser sources are part of a remnant envelope which is still collapsing toward the newly formed star.