Ortho-to-para ratio of interstellar heavy water

TL;DR

This study measures the ortho-to-para ratio of heavy water (D2O) in a protostar environment, revealing it is lower than the thermal equilibrium value, which informs our understanding of interstellar chemistry.

Contribution

First measurement of the ortho-to-para D2O ratio in a protostellar envelope using Herschel data, providing new insights into molecular spin-state distributions in space.

Findings

Ortho-D2O transition detected in absorption at 607.35 GHz.

Estimated ortho-to-para D2O ratio is less than 2.6 at 3 sigma.

The ratio is lower than the thermal equilibrium value of 2:1.

Abstract

Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular D/H ratios have been found in the environments of low-mass star forming regions, and in particular the Class 0 protostar IRAS 16293-2422. The CHESS (Chemical HErschel Surveys of Star forming regions) Key Program aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the HIFI instrument provide a unique opportunity to observe the fundamental 1,1,1 - 0,0,0 transition of the ortho-D2O molecule, inaccessible from the ground, and to determine the ortho-to-para D2O ratio. We have detected the fundamental transition of the ortho-D2O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in absorption with a line opacity of 0.62 +/- 0.11 (1 sigma). From the previous ground-based observations of the fundamental 1,1,0 - 1,0,1 transition of para-D2O seen in absorption at 316.80 GHz we estimate a line opacity of 0.26 +/- 0.05 (1 sigma). We show that the observed absorption is caused by the cold gas in the envelope of the protostar. Using these new observations, we estimate for the first time the ortho to para D2O ratio to be lower than 2.6 at a 3 sigma level of uncertainty, to be compared with the thermal equilibrium value of 2:1.