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.

IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud, which is only 140 parsecs away from Earth, making it the closest large star-forming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 AU is the distance from Earth to the Sun), and is infalling and rotating. It also has a protostellar disk with a radius of 90 AU (ref. 6), from which a planetary system is expected to form. The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the mid-plane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-C3H2 resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 AU), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.

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 .

Context. Water is a primordial species in the emergence of life, and comets may have brought a large fraction to Earth to form the oceans. To understand the evolution of water from the first stages of star formation to the formation of planets and comets, the HDO/H 2 O ratio is a powerful diagnostic. Aims. Our aim is to determine precisely the abundance distribution of HDO towards the low-mass protostar IRAS 16293-2422 and learn more about the water formation mechanisms by determining the HDO/H 2 O abundance ratio. Results. It is the first time that so many HDO and H 182 O transitions have been detected towards the same source with high spectral resolution. We derive an inner HDO abundance (T ≥ 100 K) of about 1.7 × 10 −7 and an outer HDO abundance (T < 100 K) of about 8 × 10 −11 . To reproduce the HDO absorption lines observed at 894 and 465 GHz, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. At a 3σ uncertainty, the HDO/H 2 O ratio is 1.4-5.8% in the hot corino, whereas it is 0.2-2.2% in the outer envelope. It is estimated at ∼4.8% in the added absorbing layer. Conclusions. Although it is clearly higher than the cosmic D/H abundance, the HDO/H 2 O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similarity of the ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol, which formed later once the CO molecules had depleted on the grains.

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.

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, interaction of the outflow with its surroundings or the impact of UV/X-rays inside the cavities. In this Letter we present the first interferometric observations of two complex molecules, CH 3 CN and HCOOCH 3 , towards the solar type protostar IRAS16293-2422. 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. Subject headings: ISM: abundances -ISM: individual (IRAS16293) -ISM: molecules -stars: formation 0 Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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.