Observations of Water Vapor toward Orion BN/KL

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Abstract

We have obtained spectra of the rotational ground-state 110-101 556.936 GHz ortho-H216O and 110-101 547.676 GHz ortho-H218O transitions toward Orion BN/KL using the Submillimeter Wave Astronomy Satellite (SWAS). The ortho-H216O spectrum shows strong evidence for both a broad (Δv 48 km s-1) and a narrow (Δv 7.5 km s-1) component, while the ortho-H218O shows evidence for only a broad (Δv 24 km s-1) component. The broad component emission in both ortho-H216O and ortho-H218O arises primarily from gas heated within the low- and high-velocity outflows and shocked gas surrounding IRc2 in which the ortho-H216O and ortho-H218O fractional abundances are estimated to be 3.5 × 10-4 and 7 × 10-7, respectively. This finding provides further confirmation that water is efficiently and abundantly produced within warm shock-heated gas. We estimate that the hot core plus the compact ridge contribute 10% to the ortho-H216O integrated intensity within the SWAS beam. The narrow component seen in the ortho-H216O spectrum is best fitted by ortho-water emission from the extended ridge (ER) and the higher temperature core of the extended ridge (CER) with a common fractional abundance of 3.3 × 10-8. The absence of any discernible narrow component in the ortho-H218O spectrum is used to set 3 σ upper limits on the ortho-water fractional abundance within the ER of 7 × 10-8 and within the CER of 5.2 × 10-7. This implies that within the dense extended quiescent region, gas-phase water is neither a major repository of oxygen nor a major coolant in Orion BN/KL.

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