Ortho-to-para ratio of interstellar heavy water

Abstract

<i>Context. <i/>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.<i>Aims. <i/>The CHESS (Chemical <i>HErschel<i/> Surveys of Star forming regions) key program aims to study the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the <i>Herschel<i/>/HIFI instrument provide a unique opportunity to observe the fundamental 1- transition of the ortho-D<sub>2<sub/>O molecule, which is inaccessible from the ground, and determine the ortho-to-para D<sub>2<sub/>O ratio. <i>Methods. <i/>We detected the fundamental transition of the ortho-D<sub>2<sub/>O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in absorption with a line opacity of 0.62 <i>±<i/> 0.11 (1<i>σ<i/>). From the previous ground-based observations of the fundamental 1 – 1 transition of para-D<sub>2<sub/>O seen in absorption at 316.80 GHz, we estimate a line opacity of 0.26 <i>±<i/> 0.05 (1<i>σ<i/>).<i>Results. <i/> 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 D<sub>2<sub/>O ratio to be lower than 2.6 at a 3<i>σ<i/> level of uncertainty, which should be compared with the thermal equilibrium value of 2:1.