High-Velocity H₂O Masers Associated Massive Star FormationRegions

We report on the results of ¹²CO (1-0) emission associated with H₂O masers and massive star formation regions to identify high-velocity H₂O masers. Several masers have a large blue-shift, even up to 120km.s^-1, with respect to the CO peak, but no large redshifted maser appears. This result suggests that high-velocity H₂O masers can most probably occur in high mass star-forming regions and quite a number of masers stem from the amplifications of a background source, which may enable those undetectable weak masers to become to an observable level.