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.

We present IRAM 30 m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293-2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293-2422. We interpret the data by means of the Ceccarelli et al. (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance x_in ^HDO = 1 × 10^-7 and an outer abundance x_out ^HDO ≤ 1 × 10^-9 (3σ). This result is in favor of HDO enhancement due to ice evaporation from the grains in the inner envelope. The deuteration ratio HDO/H₂O is found to be f_in = 3% and f_out ≤ 0.2% (3σ) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H₂O ices consistently derive a H₂O ice abundance of several 10^-5 to 10^-4, some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293-2422.

We present the first results of a systematic (sub)millimeter unbiased spectral line survey performed with the IRAM-30m and JCMT radiotele-scopes towards the low-mass protostar IRAS 16293-2422 (IRAS16293). When completed this survey will cover most of the frequency band attainable with the 30m (80-280 GHz) and with the JCMT in the 350 GHz band (335-365 GHz), i.e. 201 GHz.

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.

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.

While warm dense gas is prevalent around low-mass protostars, the presence of complex saturated molecules - the chemical inventory characteristic of hot cores - has remained elusive in such environments. Here we report the results of an IRAM 30 m study of the molecular composition associated with the low-mass protostar IRAS 16293-2422. Our observations highlight an extremely rich organic inventory in this source with abundant amounts of complex O- and N-bearing molecules such as formic acid, HCOOH, acetaldehyde, CH₃CHO, methyl formate, CH₃OCHO, dimethyl ether, CH₃OCH₃, acetic acid, CH₃COOH, methyl cyanide, CH₃CN, ethyl cyanide, C₂H₅CN, and propyne, CH₃CCH. We compare the composition of the hot core around this low-mass young stellar object with those around massive protostars and address the chemical processes involved in molecular complexity in regions of star formation.