The abundance of H2O and HDO in Orion KL from Herschel/HIFI

TL;DR

This study uses Herschel/HIFI and ALMA data to measure water and deuterated water abundances in Orion KL, revealing high H2O levels in a small hot clump and varying HDO/H2O ratios across different regions, influenced by shock chemistry.

Contribution

It provides detailed measurements of H2O and HDO abundances in Orion KL's various components using high-resolution spectral data, highlighting the spatial distribution and chemical processes affecting water deuteration.

Findings

High H2O abundance in a small hot clump near the Hot Core.

HDO/H2O ratio of 0.003 in the hot component.

Lower HDO/H2O ratio in outflowing gas, likely due to shock chemistry.

Abstract

Using a broadband, high spectral resolution survey toward Orion KL acquired with Herschel/HIFI as part of the HEXOS key program, we derive the abundances of H$_2$O and HDO in the different spatial/velocity components associated with this massive star-forming region: the Hot Core, Compact Ridge, and Plateau. A total of 20 transitions of H$_2$$^{18}$O, 14 of H$_2$$^{17}$O, 37 of HD$^{16}$O, 6 of HD$^{18}$O, and 6 of D$_2$O are used in the analysis, spanning from ground state transitions to over 1200 K in upper-state energy. Low-excitation lines are detected in multiple components, but the highest-excitation lines ($E_u >$ 500 K) are well modeled as emitting from a small ($\sim 2"$) clump with a high abundance of H$_2$O ($\chi = 6.5 \times 10^{-4}$ relative to H$_2$) and a HDO/H$_2$O ratio of 0.003. Using high spatial resolution ($1.5" \times 1.1"$) images of two transitions of HDO measured by ALMA as part of its science verification phase, we identify this component as located near, but not directly coincident with, known continuum sources in the Hot Core region. Significant HDO/H$_2$O fractionation is also seen in the Compact Ridge and Plateau components. The outflowing gas, observed with both emission and absorption components, has a lower HDO/H$_2$O ratio than the compact components in Orion KL, which we propose could be due to modification by gas-phase shock chemistry.