The first detection of the 232 GHz vibrationally excited H2O maser in Orion KL with ALMA

We investigated the ALMA science verification data of Orion KL and found a spectral signature of the vibrationally excited H2O maser line at 232.68670 GHz (nu2=1, 5,5,0-6,4,3). This line has been detected in circumstellar envelopes of late-type stars so far but not in young stellar objects including Orion KL. Thus, this is the first detection of the 232 GHz vibrationally excited H2O maser in star-forming regions. The distribution of the 232 GHz maser is concentrated at the position of the radio Source I, which is remarkably different from other molecular lines. The spectrum shows a double-peak structure at the peak velocities of -2.1 and 13.3 km s-1. It appears to be consistent with the 22 GHz H2O masers and 43 GHz SiO masers observed around Source I. Thus, the 232 GHz H2O maser around Source I would be excited by the internal heating by an embedded protostar, being associated with either the root of the outflows/jets or the circumstellar disk around Source I, as traced by the 22 GHz H2O masers or 43 GHz SiO masers, respectively.
Comment: 11 pages, 4 figures, accepted for publication in ApJL