2016ApJ...817..168H

We present an observational study of the vibrationally excited H₂ O line at 658 GHz (v₂ = 1, l_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) 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 2x10⁴ 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 H₂ O maser. We also search for two other H₂O lines at 646 GHz (9_7,3-8_8,0 and -8_8,1), but they are not detected in Orion KL.