2001ApJ...562..799P

We present the first detection of two submillimeter lines of HDO in the KL region of Orion: J_Ka,Kb=2_1,2⟶1_1,1 (848.9619 GHz), and 1_1,1⟶0_0,0 (893.6387 GHz). The first line has been mapped at 10" angular resolution. These transitions involve some of the lowest energy levels of HDO and have the shortest wavelengths accessible from the ground. Therefore, they provide a perfect tool to complement previous works that made use of millimeter HDO transitions involving similar energy levels (1_1,0⟶1_1,1 at 80.6 GHz, 2_1,1⟶2_1,2 at 241.6 GHz, and others). The two submillimeter lines arise from the moderate expanding material or ``Plateau'' (v<sub>LSR</sub>∼9 km.s<sup>–1</sup>, Δv≥20 km.s<sup>–1</sup>). The emission is very compact in both HDO transitions (no more extended than ∼40"-45") with similar intensities, line shapes, and line widths. The Hot Core seems completely hidden in our data in contrast with the majority of other millimeter-wave observations. This fact can only be explained if the Hot Core is embedded or behind the region of the outflow. The high line opacity of the submillimeter HDO lines would then hide the Hot Core emission. A comparison with our previously published high angular resolution para-H<sub>2</sub>O data (3<sub>1,3</sub>⟶2<sub>2,0</sub> at 183.31 GHz, and 5<sub>1,5</sub>⟶4<sub>2,2</sub> at 325.15 GHz) can be performed to derive the HDO/H<sub>2</sub>O ratio in the ``Plateau'' warm molecular environment. We have found this ratio to be in the range 0.004 to 0.01. Such a high value, taking into account that the kinetic temperature exceeds 150 K, clearly supports the idea that the observed HDO has recently evaporated from dust grain mantles.