Rotational spectroscopy of CH$_3$OD with a reanalysis of CH$_3$OD toward IRAS 16293$-$2422

TL;DR

This study provides a detailed spectroscopic model and line list for CH$_3$OD, enabling accurate radio astronomical detection and analysis, and reveals significant deuteration differences in methanol around IRAS 16293--2422.

Contribution

It offers a new comprehensive spectroscopic line list for CH$_3$OD up to 1.35 THz, improving detection accuracy and understanding of deuteration in star-forming regions.

Findings

Line list accurate up to 1.35 THz for CH$_3$OD

Deuteration in methanol differs by a factor of ~7.5 between functional groups

Enhanced understanding of vibration-torsion-rotation interactions in isotopologs

Abstract

We have started a measurement campaign of numerous methanol isotopologs in low-lying torsional states in order to provide extensive line lists for radio astronomical observations from an adequate spectroscopic model and to investigate how the intricate vibration-torsion-rotation interactions manifest themselves in the spectra of different isotopic species. After CD$_3$OH and CD$_3$OD, we turn our focus to CH$_3$OD, which is an important species for studying deuteration in prestellar cores and envelopes that enshroud protostars. Notably, deuteration is frequently viewed as a diagnostic tool for star formation. The measurements used in this study were obtained in two spectroscopic laboratories and cover large fractions of the 34 GHz--1.35 THz range. As done in previous studies, we employed a torsion-rotation Hamiltonian model for our analysis that is based on the rho-axis method. The resulting model describes the ground and first excited torsional states of CH$_3$OD well up to quantum numbers $J \leqslant 51$ and $K_a \leqslant 18$. We derived a line list for radio astronomical observations from this model that is accurate up to at least 1.35~THz and should be sufficient for all types of radio astronomical searches for this methanol isotopolog in these two lowest torsional states. This line list was applied to a reinvestigation of CH$_3$OD in data from the Protostellar Interferometric Line Survey of IRAS 16293--2422 obtained with the Atacama Large Millimeter/submillimeter Array. The new accurately determined value for the column density of CH$_3$OD implies that the deuteration in methanol differs in its two functional groups by a factor of $\sim$7.5.