Deuterated molecules in Orion KL from Herschel/HIFI

TL;DR

This study analyzes deuterated molecules in Orion KL using Herschel/HIFI data, providing the most precise constraints on deuterium fractionation in an interstellar cloud, revealing regional differences linked to temperature and chemistry.

Contribution

It offers the first comprehensive analysis of multiple deuterated molecules in Orion KL, combining data to refine deuterium fractionation estimates and compare hot core and ridge regions.

Findings

Deuterium fractionation ratios in Orion KL are (2--8) x 10^-3.

Ammonia is more deuterated than water in the Hot Core.

The Compact Ridge shows similar deuteration levels across molecules.

Abstract

We present a comprehensive study of the deuterated molecules detected in the fullband HIFI survey of the Orion KL region. Ammonia, formaldehyde, and methanol and their singly deuterated isotopologues are each detected through numerous transitions in this survey with a wide range in optical depths and excitation conditions. In conjunction with a recent study of the abundance of HDO and H$_2$O in Orion KL, this study yields the best constraints on deuterium fractionation in an interstellar molecular cloud to date. As previous studies have found, both the Hot Core and Compact Ridge regions within Orion KL contain significant abundances of deuterated molecules, suggesting an origin in cold grain mantles. In the Hot Core, we find that ammonia is roughly a factor of 2 more fractionated than water. In the Compact Ridge, meanwhile, we find similar deuterium fractionation in water, formaldehyde, and methanol, with D/H ratios of (2---8) $\times$ $10^{-3}$. The [CH$_2$DOH]/[CH$_3$OD] ratio in the Compact Ridge is found to be $1.2 \pm 0.3$. The Hot Core generally has lower deuterium fractionation than the Compact Ridge, suggesting a slightly warmer origin, or a greater contribution from warm gas phase chemistry.