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

Abstract

We present an observational study of the vibrationally excited H 2 O line at 658 GHz ( 2 n = 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 H 2 O 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 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×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.

ExoMol molecular line lists XXX: a complete high-accuracy line list for water

“…Apart from modelling hot objects, transitions involving highly excited energy levels may be important in environments far from thermodynamic equilibrium; for example, water fluorescence on comets can occur from very highly excited levels (Dello Russo et al 2004Russo et al , 2005Barber et al 2007); such observations are not always well-understood (Barber et al 2007). Similarly observations with the Atacama Large Millimeter Array (ALMA) are beginning to probe maser emission from vibrationally excited water (Hirota et al 2012(Hirota et al , 2016; modelling water maser emission requires very extensive transition datasets (Gray et al 2016). Finally, extensive water line lists are also important for many terrestrial applications, such as modelling and monitoring of emissions from combustion engines (Kranendonk et al 2007;Rein & Sanders 2010) or studying high-explosive blast waves (Carney et al 2011).…”

Alma Band 8 Continuum Emission From Orion Source I

“…High resolution VLBI observations reveal that the SiO masers trace a magneto-centrifugal disk wind from the surface of the circumstellar disk with 100 au scale (Matthews et al 2010). Vibrationally excited molecular lines such as H 2 O, SiO, CO, and SO at the excitation energies of 500-3500 K are also detected around Source I by the recent Atacama Large Millimeter/Submillimeter Array (ALMA) observations suggesting a hot molecular gas in the rotating disk and base of the outflow (Hirota et al 2012(Hirota et al , 2014(Hirota et al , 2016Plambeck & Wright 2016).…”

Disk-driven rotating bipolar outflow in Orion Source I

“…At the center of the outflow, vibrationally excited SiO masers trace a rotating outflow arising from the surface of a circumstellar disk with a radius of ∼50 au [11,12], consistent with a magneto-centrifugal disk wind [6,11]. Recent high-angular resolution (100-200 au) ALMA observations of various molecular lines at high-excitation energies of 500-3500 K [18,19,20,21] revealed a rotating hot molecular gas disk with an enclosed mass of (5−7)M ⊙ .…”