Successive deuteration in low-mass star-forming regions: the case of D$_{2}$-methanol (CHD$_{2}$OH) in IRAS 16293-2422

TL;DR

This study provides a comprehensive spectroscopic dataset for CHD$_{2}$OH, enabling accurate measurement of deuteration levels in IRAS 16293-2422, revealing similar D/H ratios to other low-mass star-forming regions and insights into astrochemical processes.

Contribution

The paper introduces a new spectroscopic line list and analysis method for CHD$_{2}$OH, facilitating precise deuteration measurements in low-mass protostars.

Findings

High D/H ratios of ~7.6% in IRAS 16293-2422 A and B.

Deuteration levels are comparable to other low-mass star-forming regions.

Deuteration is likely driven by H-D substitution reactions.

Abstract

Accurate quantification of the column density of di-deuterated methanol is a key missing puzzle piece in the otherwise thoroughly constrained family of D-bearing methanol in the deeply embedded low-mass protostellar system and astrochemical template source IRAS16293-2422. A spectroscopic dataset for astrophysical purposes is built for CHD$_{2}$OH and made publicly available to facilitate accurate characterization of this species in astrochemical surveys. The newly computed line list and partition function are used to search for CHD$_{2}$OH towards IRAS16293-2422 A and B in data from ALMA-PILS. Only non-blended, optically thin lines of CHD$_{2}$OH are used for the synthetic spectral fitting. The constructed spectroscopic database contains line frequencies and strengths for 7417 transitions in the 0 to 500 GHz frequency range. ALMA-PILS observations in the 329-363 GHz range are used to identify 105 unique, non-blended, optically thin line frequencies of CHD$_{2}$OH for synthetic spectral fitting. The derived excitation temperatures and column densities yield high D/H ratios of CHD$_{2}$OH in IRAS 16293-2422 A and B of 7.5$\pm$1.1% and 7.7$\pm$1.2%, respectively. Deuteration in IRAS 16293-2422 is not higher than in other low-mass star-forming regions. Di-deuterated molecules consistently have higher D/H ratios than their monodeuterated counterparts in all low-mass protostars, which may be a natural consequence of H-D substitution reactions as seen in laboratory experiments. The Solar System's natal cloud, as traced by comet 67P/Churyumov-Gerasimenko, may have had a lower initial abundance of D, been warmer than the cloud of IRAS16293-2422, or been partially reprocessed. In combination with accurate spectroscopy, a careful spectral analysis, and a consideration of the underlying assumptions, successive deuteration is a robust window on the physicochemical provenance of star-forming systems.