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 16293Ϫ2422. 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, CH 3 CHO, methyl formate, CH 3 OCHO, dimethyl ether, CH 3 OCH 3 , acetic acid, CH 3 COOH, methyl cyanide, CH 3 CN, ethyl cyanide, C 2 H 5 CN, and propyne, CH 3 CCH. 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.

Abstract. We report a theoretical study of sulphur chemistry, as applied to hot cores, where S-bearing molecular ratios have been previously proposed and used as chemical clocks. As in previous models, we follow the S-bearing molecular composition after the injection of grain mantle components into the gas phase. For this study, we developed a time-dependent chemical model with up-to-date reaction rate coefficients. We ran several cases, using different realistic chemical compositions for the grain mantles and for the gas prior to mantle evaporation. The modeling shows that S-bearing molecular ratios depend very critically on the gas temperature and density, the abundance of atomic oxygen, and, most importantly, on the form of sulphur injected in the gas phase, which is very poorly known. Consequently, ratios of S-bearing molecules cannot be easily used as chemical clocks. However, detailed observations and careful modeling of both physical and chemical structure can give hints on the source age and constrain the mantle composition (i.e. the form of sulphur in cold molecular clouds) and, thus, help to solve the mystery of the sulphur depletion. We analyse in detail the cases of Orion and IRAS 16293-2422. The comparison of the available observations with our model suggests that the majority of sulphur released from the mantles is mainly in, or soon converted into, atomic form.

Context. Despite the low cosmic abundance of deuterium (D/H ∼ 10 −5 ), high degrees of deuterium fractionation in molecules are observed in star-forming regions with enhancements that can reach 13 orders of magnitude, a level that current models have difficulty accounting for. Aims. Multi-isotopologue observations are a very powerful constraint for chemical models. The aim of our observations is to understand the processes that form the observed high abundances of methanol and formaldehyde in low-mass protostellar envelopes (gas-phase processes? chemistry on the grain surfaces?), as well as to better constrain the chemical models.Methods. With the IRAM 30 m single-dish telescope, we observed deuterated formaldehyde (HDCO and D 2 CO) and methanol (CH 2 DOH, CH 3 OD, and CHD 2 OH) towards a sample of seven low-mass class 0 protostars. Using population diagrams, we then derived the fractionation ratios of these species (abundance ratio between the deuterated molecule and its main isotopologue) and compared them to the predictions of grain chemistry models. Results. These protostars show a similar level of deuteration as in IRAS 16293−2422, where doubly-deuterated methanol -and even triply-deuterated methanol -were first detected. Our observations point to the formation of methanol on the grain surfaces, while formaldehyde formation cannot be fully pinned down. While none of the scenarii can be excluded (gas-phase or grain chemistry formation), they both seem to require abstraction reactions to reproduce the observed fractionations.

Abstract.We report the first detection of triply-deuterated methanol, with 12 observed transitions, towards the low-mass protostar IRAS 16293−2422, as well as multifrequency observations of 13 CH 3 OH, used to derive the column density of the main isotopomer CH 3 OH. The derived fractionation ratio [CD 3 OH]/[CH 3 OH] averaged on a 10 beam is 1.4%. Together with previous CH 2 DOH and CHD 2 OH observations, the present CD 3 OH observations are consistent with a formation of methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the accreting gas. Such a high atomic ratio can be reached in the framework of gas-phase chemical models including all deuterated isotopomers of H + 3 .

We report the detection of D2CO in a sample of starless dense cores, in which we previously measured the degree of CO depletion. The deuterium fractionation is found extremely high, [D2CO]/[H2CO] ~ 1-10 %, similar to that reported in low-mass protostars. This provides convincing evidence that D2CO is formed in the cold pre-stellar cores, and later desorbed when the gas warms up in protostars. We find that the cores with the highest CO depletions have also the largest [D2CO]/[H2CO] ratios, supporting the theoretical prediction that deuteration increases with increasing CO depletion.Comment: 11 pages, 2 figures, accepted by ApJ Letter

Abstract. We report the first detection of doubly-deuterated methanol (CHD 2 OH), as well as firm detections of the two singly-deuterated isotopomers of methanol (CH 2 DOH and CH 3 OD), towards the solar-type protostar IRAS 16293−2422. From the present multifrequency observations, we derive the following abundance ratios:The total abundance of the deuterated forms of methanol is greater than that of its normal hydrogenated counterpart in the circumstellar material of IRAS 16293−2422, a circumstance not previously encountered. Formaldehyde, which is thought to be the chemical precursor of methanol, possesses a much lower fraction of deuterated isotopomers (∼20%) with respect to the main isotopic form in IRAS 16293−2422. The observed fractionation of methanol and formaldehyde provides a severe challenge to both gas-phase and grain-surface models of deuteration. Two examples of the latter model are roughly in agreement with our observations of CHD 2 OH and CH 2 DOH if the accreting gas has a large (0.2-0.3) atomic D/H ratio. However, no gas-phase model predicts such a high atomic D/H ratio, and hence some key ingredient seems to be missing.

Abstract. We present a survey of the formaldehyde emission in a sample of eight Class 0 protostars obtained with the IRAM and JCMT millimeter telescopes. The range of energies of the observed transitions allows us to probe the physical and chemical conditions across the protostellar envelopes. The data have been analyzed with three different methods with increasing level of sophistication. We first analyze the observed emission in the LTE approximation, and derive rotational temperatures between 11 and 40 K, and column densities between 1 and 20 × 10 13 cm −2 . Second, we use a LVG code and derive higher kinetic temperatures, between 30 and 90 K, consistent with subthermally populated levels and densities from 1 to 6 × 10 5 cm −3 . The column densities from the LVG modeling are within a factor of 10 with respect to those derived in the LTE approximation. Finally, we analyze the observations based upon detailed models for the envelopes surrounding the protostars, using temperature and density profiles previously derived from continuum observations. We approximate the formaldehyde abundance across the envelope with a jump function, the jump occurring when the dust temperature reaches 100 K, the evaporation temperature of the grain mantles. The observed formaldehyde emission is well reproduced only if there is a jump of more than two orders of magnitude, in four sources. In the remaining four sources the data are consistent with a formaldehyde abundance jump, but the evidence is more marginal (≤2 σ). The inferred inner H 2 CO abundance varies between 1 × 10 −8 and 6 × 10 −6 . The absolute values of the jump in the H 2 CO abundance are uncertain by about one order of magnitude, because of the uncertainties in the density, ortho to para ratio, temperature and velocity profiles of the inner region, as well as the evaporation temperature of the ices. We discuss the implications of these jumps for our understanding of the origin and evolution of ices in low mass star forming regions. Finally, we give predictions for the submillimeter H 2 CO lines, which are particularly sensitive to the abundance jumps.

Context. Unbiased spectral surveys are powerful tools to study the chemistry and the physics of star forming regions, because they can provide a complete census of the molecular content and the observed lines probe the physical structure of the source. Aims. While unbiased surveys at the millimeter and sub-millimeter wavelengths observable from ground-based telescopes have previously been performed towards several high mass protostars, very little exists on low mass protostars, which are believed to resemble our own Sun's progenitor. To help fill up this gap in our understanding, we carried out a complete spectral survey of the bands at 3, 2, 1, and 0.9 mm towards the solar type protostar IRAS 16293-2422. Methods. The observations covered a range of about 200 GHz and were obtained with the IRAM-30 m and JCMT-15 m telescopes during about 300 h of observations. Particular attention was devoted to the inter-calibration of the acquired spectra with previous observations. All the lines detected with more than 3σ confidence-interval certainty and free from obvious blending effects were fitted with Gaussians to estimate their basic kinematic properties. Results. More than 4000 lines were detected (with σ ≥ 3) and identified, yielding a line density of approximatively 20 lines per GHz, comparable to previous surveys in massive hot cores. The vast majority (about two-thirds) of the lines are weak and produced by complex organic molecules. The analysis of the profiles of more than 1000 lines belonging to 70 species firmly establishes the presence of two distinct velocity components associated with the two objects, A and B, forming the IRAS 16293-2422 binary system. In the source A, the line widths of several species increase with the upper level energy of the transition, a behavior compatible with gas infalling towards a ∼1 M object. The source B, which does not show this effect, might have a much lower central mass of ∼0.1 M . The difference in the rest velocities of both objects is consistent with the hypothesis that the source B rotates around the source A. Conclusions. This spectral survey, although obtained with single-dish telescopes at a low spatial resolution, allows us to separate the emission from two different components, thanks to the large number of lines detected. The data of the survey are public and can be retrieved on the TIMASSS web site .