Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular deuterium fractionation has been found in the environments of low-mass star-forming regions and, in particular, the Class 0 protostar IRAS 16293-2422. Aims. The key program Chemical HErschel Surveys of Star forming regions (CHESS) aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI (Heterodyne Instrument for Far-Infrared) instrument provide a unique opportunity to observe the fundamental 1_1,1-0_0,0 transition of ortho-D₂O at 607 GHz and the higher energy 2_1,2-1 _0,1 transition of para-D₂O at 898 GHz, both of which are inaccessible from the ground. Methods. The ortho-D₂O transition at 607 GHz was previously detected. We present in this paper the first tentative detection for the para-D₂O transition at 898 GHz. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of D₂O with the same method that was used to reproduce the HDO and H₂¹⁸O line profiles in IRAS 16293-2422. Results. As for HDO, the absorption component seen on the D₂O lines can only be reproduced by adding an external absorbing layer, possibly created by the photodesorption of the ices at the edges of the molecular cloud. The D ₂O column density is found to be about 2.5 × 10¹² cm^-2 in this added layer, leading to a D₂O/H₂O ratio of about 0.5%. At a 3σ uncertainty, upper limits of 0.03% and 0.2% are obtained for this ratio in the hot corino and the colder envelope of IRAS 16293-2422, respectively. Conclusions. The deuterium fractionation derived in our study suggests that the ices present in IRAS 16293-2422 formed on warm dust grains (~15-20 K) in dense (~10⁴-5 × 10⁴ cm ^-3) translucent clouds. These results allow us to address the earliest phases of star formation and the conditions in which ices form.

Complex organic molecules have previously been discovered in solar-type protostars, raising the questions of where and how they form in the envelope. Possible formation mechanisms include grain mantle evaporation, the interaction of the outflow with its surroundings, and/or the impact of UV/X-rays inside the cavities. In this Letter we present the first interferometric observations of two complex molecules, CH₃CN and HCOOCH₃ toward the solar-type protostar IRAS 16293-2422. The images show that the emission originates from two compact regions centered on the two components of the binary system. We discuss how these results favor the grain mantle evaporation scenario, and we investigate the implications of these observations for the chemical composition and physical and dynamical state of the two components.

We report the detection of complex molecules (HCOOCH₃, HCOOH, and CH₃CN), signposts of a hot core-like region, toward the low-mass Class 0 source NGC 1333 IRAS 4A. This is the second low-mass protostar in which such complex molecules have been searched for and reported, the other source being IRAS 16293-2422. It is therefore likely that compact (a few tens of AU) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles and where complex molecules are found, are common in low-mass Class 0 sources. Given that the chemical formation timescale is much shorter than the gas hot-core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first-generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second-generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar-type protostars, suggesting perhaps a common origin.