# Deuterated methanol toward NGC 7538-IRS1

## TL;DR

This study measures deuteration levels of methanol in NGC 7538-IRS1, finding low D/H ratios consistent with rapid pre-stellar formation, and reports a statistical deuteration ratio of CH$_2$DOH to CH$_3$OD.

## Contribution

First multi-transition analysis of deuterated methanol in a high-mass star-forming region, providing new insights into deuteration processes and ratios.

## Key findings

- D/H ratio does not exceed 1%
- CH$_2$DOH/CH$_3$OH ratio is about 3.2
- Deuteration consistent with rapid pre-stellar formation

## Abstract

We investigate the deuteration of methanol towards the high-mass star forming region NGC 7538-IRS1. We have carried out a multi-transition study of CH$_3$OH, $^{13}$CH$_3$OH and of the deuterated fllavors, CH$_2$DOH and CH$_3$OD, between 1.0--1.4 mm with the IRAM-30~m antenna. In total, 34 $^{13}$CH$_3$OH, 13 CH$_2$DOH lines and 20 CH$_3$OD lines spanning a wide range of upper-state energies (E$_{up}$) were detected. From the detected transitions, we estimate that the measured D/H does not exceed 1$\%$, with a measured CH$_2$DOH/CH$_3$OH and CH$_3$OD/CH$_3$OH of about (32$\pm$8)$\times$10$^{-4}$ and (10$\pm$4)$\times$10$^{-4}$, respectively. This finding is consistent with the hypothesis of a short-time scale formation during the pre-stellar phase. We find a relative abundance ratio CH$_2$DOH/CH$_3$OD of 3.2 $\pm$ 1.5. This result is consistent with a statistical deuteration. We cannot exclude H/D exchanges between water and methanol if water deuteration is of the order 0.1$\%$, as suggested by recent Herschel observations.