ALMA OBSERVATION OF THE 658 GHz VIBRATIONALLY EXCITED H2O MASER IN ORION KL SOURCE I

We present an observational study of the vibrationally excited H2O line at 658 GHz (v(2) = 1, 1(1,0)-1(0,1)) toward Orion KL using the Atacama Large Millimeter/Submillimeter Array (ALMA). This line is clearly detected at the position of the massive protostar candidate, Source I. The spatial structure is compact, with a size of about 100 AU, and is elongated along the northeast-southwest low-velocity (18 km(-1)) bipolar outflow traced by 22 GHz H2O masers, SiO masers, and thermal SiO lines. A velocity gradient can be seen perpendicular to the bipolar outflow. The overall spatial and velocity structure seems to be analogous to that of the 321 GHz H2O maser line previously detected with ALMA and vibrationally excited SiO maser emission. The brightness temperature of the 658 GHz H2O line is estimated to be higher than 2 x 10(4) K, implying that it is emitted via maser action. Our results suggest that the 658 GHz H2O maser line is emitted from the base of the outflow from a rotating and expanding accretion disk as observed for the SiO masers and the 321 GHz H2O maser. We also search for two other H2O lines at 646 GHz (9(7,3)-8(8,0) and 9(7,2)-8(8,1)), but they are not detected in Orion KL.