(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap

Context. The discovery of amino acids in meteorites fallen to Earth and the detection of glycine, the simplest of them, in samples returned from a comet to Earth strongly suggest that the chemistry of the interstellar medium is capable of producing such complex organic molecules and that they may be widespread in our Galaxy. Aims. Our goal is to investigate the degree of chemical complexity that can be reached in the interstellar medium, in particular in dense star-forming regions. Methods. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M), two regions where high-mass stars are formed, with the IRAM 30 m telescope in the 3 mm atmospheric transmission window. Partial surveys at 2 and 1.3 mm were performed in parallel. The spectra were analyzed with a simple radiative transfer model that assumes local thermodynamic equilibrium but takes optical depth effects into account. Results. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected at 3 mm toward Sgr B2(N) and (M), i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of two over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward Sgr B2(N) and (M) are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. In addition, we report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively, up to a vibration energy of 1400 cm −1 (2000 K). Excitation temperatures and column densities were derived for each species but should be used with caution. The rotation temperatures of the detected complex molecules typically range from ∼50 to 200 K. Among the detected molecules, aminoacetonitrile, n-propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were five rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within twelve new vibrationally or torsionally excited states of known molecules. Absorption features produced by diffuse clouds along the line of sight are detected in transitions with low rotation quantum numbers of many simple molecules and are modeled with ∼30-40 velocity components with typical linewidths of ∼3-5 km s −1 . Conclusions. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vibrationally or torsionally excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. Significant progress in extending the inventory of complex organic molecules in Sgr B2(N) and deriving tighter constraints on their location, origin, and abundance is expected in the near future thanks to an ongoing spectral line survey at 3 mm with ALMA in its cycles 0 and 1. The present single-dish surve...

Context. Amino acids are building blocks of proteins and therefore key ingredients for the origin of life. The simplest amino acid, glycine (NH 2 CH 2 COOH), has long been searched for in the interstellar medium but has not been unambiguously detected so far. At the same time, more and more complex molecules have been newly found toward the prolific Galactic center source Sagittarius B2. Aims. Since the search for glycine has turned out to be extremely difficult, we aimed at detecting a chemically related species (possibly a direct precursor), amino acetonitrile (NH 2 CH 2 CN). Methods. With the IRAM 30 m telescope we carried out a complete line survey of the hot core regions Sgr B2(N) and (M) in the 3 mm range, plus partial surveys at 2 and 1.3 mm. We analyzed our 30 m line survey in the LTE approximation and modeled the emission of all known molecules simultaneously. We identified spectral features at the frequencies predicted for amino acetonitrile lines having intensities compatible with a unique rotation temperature. We also used the Very Large Array to look for cold, extended emission from amino acetonitrile. Results. We detected amino acetonitrile in Sgr B2(N) in our 30 m telescope line survey and conducted confirmatory observations of selected lines with the IRAM Plateau de Bure and the Australia Telescope Compact Array interferometers. The emission arises from a known hot core, the Large Molecule Heimat, and is compact with a source diameter of 2 (0.08 pc). We derived a column density of 2.8 × 10 16 cm −2 , a temperature of 100 K, and a linewidth of 7 km s −1 . Based on the simultaneously observed continuum emission, we calculated a density of 1.7 × 10 8 cm −3 , a mass of 2340 M , and an amino acetonitrile fractional abundance of 2.2 × 10 −9 . The high abundance and temperature may indicate that amino acetonitrile is formed by grain surface chemistry. We did not detect any hot, compact amino acetonitrile emission toward Sgr B2(M) or any cold, extended emission toward Sgr B2, with column-density upper limits of 6 × 10 15 and 3 × 10 12−14 cm −2 , respectively. Conclusions. Based on our amino acetonitrile detection toward Sgr B2(N) and a comparison to the pair methylcyanide/acetic acid both detected in this source, we suggest that the column density of both glycine conformers in Sgr B2(N) is well below the best upper limits published recently by other authors, and probably below the confusion limit in the 1−3 mm range.

Context. In recent years, organic molecules of increasing complexity have been found toward the prolific Galactic center source Sagittarius B2. Aims. We wish to explore the degree of complexity that the interstellar chemistry can reach in star-forming regions. Methods. We carried out a complete line survey of the hot cores Sgr B2(N) and (M) with the IRAM 30 m telescope in the 3 mm range, plus partial surveys at 2 and 1.3 mm. We analyzed this spectral survey in the local thermodynamical equilibrium approximation. We modeled the emission of all known molecules simultaneously, which allows us to search for less abundant, more complex molecules. We compared the derived column densities with the predictions of a coupled gas-phase and grain-surface chemical code.Results. We report the first detection in space of ethyl formate (C 2 H 5 OCHO) and n-propyl cyanide (C 3 H 7 CN) toward Sgr B2(N). The detection of n-propyl cyanide is based on refined spectroscopic parameters derived from combined analyses of available laboratory spectroscopic data. For each molecule, we identified spectral features at the predicted frequencies having intensities compatible with a unique rotation temperature. For an assumed source size of 3 , our modeling yields a column density of 5.4 × 10 16 cm −2 , a temperature of 100 K, and a linewidth of 7 km s −1 for ethyl formate. n-Propyl cyanide is detected with two velocity components having column densities of 1.5 × 10 16 cm −2 and 6.6 × 10 15 cm −2 , respectively, for a source size of 3 , a temperature of 150 K, and a linewidth of 7 km s −1 . The abundances of ethyl formate and n-propyl cyanide relative to H 2 are estimated to be 3.6 × 10 −9 and 1.0 × 10 −9 , respectively. We derived column density ratios of 0.8/15/1 for the related species t-HCOOH/CH 3 OCHO/C 2 H 5 OCHO and 108/80/1 for CH 3 CN/C 2 H 5 CN/C 3 H 7 CN. Our chemical modeling reproduces these ratios reasonably well. It suggests that the sequential, piecewise construction of ethyl and n-propyl cyanide from their constituent functional groups on the grain surfaces is their most likely formation route. Ethyl formate is primarily formed on the grains by adding CH 3 to functional-group radicals derived from methyl formate, although ethanol may also be a precursor. Conclusions. The detection in Sgr B2(N) of the next stage of complexity in two classes of complex molecule, esters and alkyl cyanides, suggests that greater complexity in other classes of molecule may be present in the interstellar medium.

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 .