ALMA Observation of the 658 GHz Vibrationally Excited H$_{2}$O Maser in Orion KL Source I

Abstract

We present an observational study of the vibrationally excited H$_{2}$O line at 658 GHz ($\nu_{2}$=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, the 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 s$^{-1}$) bipolar outflow traced by 22 GHz H$_{2}$O masers, SiO masers, and thermal SiO lines. A velocity gradient can be seen perpendicular to the bipolar outflow. Overall spatial and velocity structure seems analogous to that of the 321 GHz H$_{2}$O maser line previously detected with ALMA and vibrationally excited SiO maser emission. The brightness temperature of the 658 GHz H$_{2}$O line is estimated to be higher than 2$\times$10$^{4}$ K, implying that it is emitted via maser action. Our results suggest that the 658 GHz H$_{2}$O 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 H$_{2}$O maser. We also search for two other H$_{2}$O 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.