On the Relationship between CH$_3$OD Abundance and Temperature in the Orion KL Nebula

TL;DR

This study investigates how CH$_{3}$OD abundance varies with temperature in Orion KL, revealing chemical processes affecting deuterated methanol and supporting D-H exchange in icy grain mantles, with implications for star formation chemistry.

Contribution

The paper presents high-resolution ALMA observations linking CH$_{3}$OD abundance to temperature, providing new insights into deuterium chemistry and D-H exchange in star-forming regions.

Findings

CH$_{3}$OD column densities vary with temperature in Orion KL.

Evidence supports D-H exchange at methanol's hydroxyl site.

Enhanced CH$_{3}$OD is localized to the Hot Core-SW region.

Abstract

The relative abundances of singly-deuterated methanol isotopologues, [CH$_{2}$DOH]/[CH$_{3}$OD], in star-forming regions deviate from the statistically expected ratio of 3. In Orion KL, the nearest high-mass star-forming region to Earth, the singly-deuterated methanol ratio is about 1, and the cause for this observation has been explored through theory for nearly three decades. We present high-angular resolution observations of Orion KL using the Atacama Large Millimeter/submillimeter Array to map small-scale changes in CH$_{3}$OD column density across the nebula, which provide a new avenue to examine the deuterium chemistry during star and planet formation. By considering how CH$_{3}$OD column densities vary with temperature, we find evidence of chemical processes that can significantly alter the observed column densities. The astronomical data are compared with existing theoretical work and support D-H exchange between CH$_{3}$OH and heavy water (i.e., HDO and D$_{2}$O) at methanol's hydroxyl site in the icy mantles of dust grains. The enhanced CH$_{3}$OD column densities are localized to the Hot Core-SW region, a pattern that may be linked to the coupled evolution of ice mantel chemistry and star formation in giant molecular clouds. This work provides new perspectives on deuterated methanol chemistry in Orion KL and informs considerations that may guide future theoretical, experimental, and observational work.