The isotopic ratio of nitrogen measured in primitive Solar System bodies shows a broad range of values, the origin of which remains unknown. One key question is whether these isotopic reservoirs of ...nitrogen predate the comet formation stage or are posterior to it. Another central question is elucidating the processes that can produce the observed variations in the 14N/15N isotopic ratio. Disks that orbit pre-main-sequence (T Tauri) stars provide unique opportunities for observing the chemical content of analogs of the protosolar nebula and therefore for building a comprehensive scenario that can explain the origin of nitrogen in the Solar System and in planet-forming disks. With ALMA, it has become possible to measure isotopic ratios of nitrogen-bearing species in such environments. We present spectrally and spatially resolved observations of the hyperfine structure of the 4−3 rotational transition of HCN and its main isotopologs H13CN and HC15N in the disk orbiting the 8 Myr old T Tauri star TW Hya. The sensitivity allows directly measuring the HCN/H13CN and HCN/HC15N abundance ratios with minimal assumptions. Averaged spatially over the disks, the ratios are 86 ± 4 and 223 ± 21, respectively. The latter value is significantly lower than the CN/C15N ratio of 323 ± 30 in this disk and thus provides the first evidence that two isotopic reservoirs of nitrogen are present in a disk at the stage of giant planet and comet formation. Furthermore, we find clear evidence for an increase in the ratio of HCN to HC15N with radius. The ratio in the outer disk, at 45 au, is 339 ± 28, in excellent agreement with direct measurements in the local interstellar medium, and with the bulk nitrogen isotopic ratio predicted from galactic evolution calculations. In the comet formation region at r = 20 au, the ratio is a factor ≈3 lower, 121 ± 11. This radial increase qualitatively agrees with the scenario in which selective photodissociation of N2 is the dominant fractionation process. However, our isotopic ratios and kinetic temperature of the HCN-emitting layers quantitatively disagree with models of nitrogen chemistry in disks.
Abstract
Complex organic molecules have been observed for decades in the interstellar medium. Some of them might be considered as small bricks of the macromolecules at the base of terrestrial life. ...It is hence particularly important to understand organic chemistry in Solar-like star-forming regions. In this article, we present a new observational project: Seeds Of Life In Space (SOLIS). This is a Large Project using the IRAM-NOEMA interferometer, and its scope is to image the emission of several crucial organic molecules in a sample of Solar-like star-forming regions in different evolutionary stages and environments. Here we report the first SOLIS results, obtained from analyzing the spectra of different regions of the Class 0 source NGC 1333-IRAS4A, the protocluster OMC-2 FIR4, and the shock site L1157-B1. The different regions were identified based on the images of formamide (NH
2
CHO) and cyanodiacetylene (HC
5
N) lines. We discuss the observed large diversity in the molecular and organic content, both on large (3000–10,000 au) and relatively small (300–1000 au) scales. Finally, we derive upper limits to the methoxy fractional abundance in the three observed regions of the same order of magnitude of that measured in a few cold prestellar objects, namely
–10
−11
with respect to H
2
molecules.
The advent of large instantaneous bandwidth receivers and high spectral resolution spectrometers on (sub-)millimeter telescopes has opened up the possibilities for unbiased spectral surveys. Because ...of the large amount of data they contain, any analysis of these surveys requires dedicated software tools. Here we present an extension of the widely used CLASS software that we developed to that purpose. This extension, named Weeds, allows for searches in atomic and molecular lines databases (e.g. JPL or CDMS) that may be accessed over the internet using a virtual observatory (VO) compliant protocol. The package permits a quick navigation across a spectral survey to search for lines of a given species. Weeds is also capable of modeling a spectrum, as often needed for line identification. We expect that Weeds will be useful for analyzing and interpreting the spectral surveys that will be done with the HIFI instrument onboard Herschel, but also observations carried-out with ground based millimeter and sub-millimeter telescopes and interferometers, such as IRAM-30 m and Plateau de Bure, CARMA, SMA, eVLA, and ALMA.
The ratio between the two stable isotopes of nitrogen, 14N and 15N, is well measured in the terrestrial atmosphere (~272), and for the pre-solar nebula (~441, deduced from the solar wind). ...Interestingly, some pristine solar system materials show enrichments in 15N with respect to the pre-solar nebula value. However, it is not yet clear if and how these enrichments are linked to the past chemical history because we have only a limited number of measurements in dense star-forming regions. In this respect, dense cores, which are believed to be the precursors of clusters and also contain intermediate- and high-mass stars, are important targets because the solar system was probably born within a rich stellar cluster, and such clusters are formed in high-mass star-forming regions. The number of observations in such high-mass dense cores has remained limited so far. In this work, we show the results of IRAM-30 m observations of the J = 1−0 rotational transition of the molecules HCN and HNC and their 15N-bearing counterparts towards 27 intermediate- and high-mass dense cores that are divided almost equally into three evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact Hii regions. We have also observed the DNC(2–1) rotational transition in order to search for a relation between the isotopic ratios D/H and 14N/15N. We derive average 14N/15N ratios of 359 ± 16 in HCN and of 438 ± 21 in HNC, with a dispersion of about 150–200. We find no trend of the 14N/15N ratio with evolutionary stage. This result agrees with what has been found for N2H+ and its isotopologues in the same sources, although the 14N/15N ratios from N2H+ show a higher dispersion than in HCN/HNC, and on average, their uncertainties are larger as well. Moreover, we have found no correlation between D/H and 14N/15N in HNC. These findings indicate that (1) the chemical evolution does not seem to play a role in the fractionation of nitrogen, and that (2) the fractionation of hydrogen and nitrogen in these objects is not related.
Isotopic ratios are keys to understanding the origin and early evolution of the solar system in the context of Galactic nucleosynthesis. The large range of measured 14N/15N isotopic ratios in the ...solar system reflects distinct reservoirs of nitrogen whose origins remain to be determined. We have directly measured a C14N/C15N abundance ratio of 323 ± 30 in the disk orbiting the nearby young star TW Hya. This value, which is in good agreement with nitrogen isotopic ratios measured for prestellar cores, likely reflects the primary present-day reservoir of nitrogen in the solar neighbourhood. These results support models invoking novae as primary 15N sources as well as outward migration of the Sun over its lifetime, and suggest that comets sampled a secondary, 15N-rich reservoir during solar system formation.
Abstract
HCN and HNC are two fundamental molecules in the dense interstellar medium. The HNC/HCN abundance ratio depends on the kinetic temperature and can be used to explore the physical and ...chemical conditions of star-forming regions. Modelling of HCN and HNC emissions from interstellar clouds requires to model their collisional and radiative excitations. We report the calculation of the HCN and HNC excitation rate coefficients among the first 26 rotational levels due to H2 collisions, for temperatures ranging from 5 to 500 K, using the exact close coupling and the approximate coupled states methods. We found a propensity for even Δj transitions in the case of HCN–para-H2 collisions, whereas a propensity for odd Δj transitions is observed in the case of HNC–para-H2 collisions. For collisions with ortho-H2, both molecules show a propensity rule favouring transitions with odd Δj. The rate coefficients for HCN and HNC differ significantly, showing clearly that the collisional excitation of the two isomers is different, especially for para-H2. We also evaluate the impact of these new data on the astrophysical modelling through radiative transfer calculations. It is shown that specific calculations have to be performed for the two isomers and that the HNC/HCN abundance ratio in cold molecular clouds cannot be estimated from line intensity ratio. Finally, observations of the two isotopologues H13CN and HN13C towards a sample of prestellar cores are presented, and the larger excitation temperature of HN13C is well reproduced by our excitation model.
Aims. The condensation of diffuse gas into molecular clouds and dense cores occurs at a rate driven largely by turbulent dissipation. This process still has to be caught in action and characterized. ...Methods. We observed a mosaic of 13 fields with the IRAM-PdB interferometer (PdBI) to search for small-scale structure in the 12CO(1-0) line emission of the turbulent and translucent environment of a low-mass dense core in the Polaris Flare. The large size of the mosaic (1' $\times$ 2') compared to the resolution (4'') is unprecedented in the study of the small-scale structure of diffuse molecular gas. Results. The interferometer data uncover eight weak and elongated structures with thicknesses as small as ≈3 mpc (600 AU) and lengths up to 70 mpc, close to the size of the mosaic. These are not filaments because once merged with short-spacings data, the PdBI-structures appear to be the sharp edges, in space and velocity-space, of larger-scale structures. Six out of eight form quasi-parallel pairs at different velocities and different position angles. This cannot be the result of chance alignment. The velocity-shears estimated for the three pairs include the highest values ever measured in regions that do not form stars (up to 780 km s-1 pc-1). The CO column density of the PdBI-structures is in the range $N({\rm CO)}$ = 1014 to 1015 cm-2 and their H2 density, estimated in several ways, does not exceed a few 103 cm-3. Because the larger scale structures have sharp edges (with little or no overlap for those that are pairs), they have to be thin layers of CO emission. We call them SEE(D)S for sharp-edged extended (double) structures. These edges mark a transition, on the milliparsec scale, between a CO-rich component and a gas undetected in the 12CO(1-0) line because of its low CO abundance, presumably the cold neutral medium. Conclusions. We propose that these SEE(D)S are the first directly-detected manifestations of the intermittency of interstellar turbulence. The large velocity-shears reveal an intense straining field, responsible for a local dissipation rate several orders of magnitude above average, possibly at the origin of the thin CO layers.
We present a suite of Atacama Large Millimeter Array (ALMA) interferometric molecular line and continuum images that elucidate, on linear size scales of ∼30-40 au, the chemical structure of the ...nearby, evolved, protoplanetary disk orbiting the close binary system V4046 Sgr. The observations were undertaken in the 1.1-1.4 mm wavelength range (ALMA Bands 6 and 7) with antenna configurations involving maximum baselines of several hundred meters, yielding subarcsecond-resolution images in more than a dozen molecular species and isotopologues. Isotopologues of CO and HCN display centrally peaked morphologies of integrated emission-line intensity, whereas the line emission from complex nitrile group molecules (HC3N, CH3CN), deuterated molecules (DCN, DCO+), hydrocarbons (as traced by C2H), and potential CO ice line tracers (N2H+, and H2CO) appears as a sequence of sharp and diffuse rings. The dimensions and morphologies of HC3N and CH3CN emission are suggestive of photodesorption of organic ices from the surfaces of dust grains, while the sequence of increasing radius of peak intensity represented by DCN (smallest), DCO+, N2H+, and H2CO (largest) is qualitatively consistent with the expected decline of midplane gas temperature with increasing disk radius. Empirical modeling indicates that the sharp-edged C2H emission ring lies at relatively deep disk layers, leaving open the question of the origin of C2H abundance enhancements in evolved disks. This study of the "molecular anatomy" of V4046 Sgr should serve as motivation for additional subarcsecond ALMA molecular line imaging surveys of nearby, evolved protoplanetary disks aimed at addressing major uncertainties in protoplanetary disk physical and chemical structure and molecular production pathways.
ABSTRACT
The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different ...nuclear-spin states of the hydrides of carbon, nitrogen, oxygen, and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involving up to seven protons. The abundances and ortho-to-para ratios (OPRs) of gas-phase water and water ions (H2O+ and H3O+) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a pre-stellar core. The model incorporates the freezing of the molecules on to grains, simple grain surface chemistry, and cosmic ray induced and direct desorption of ices. The predicted OPRs are found to deviate significantly from both thermal and statistical values and to be independent of temperature below ∼30 K. The OPR of H2O is shown to lie between 1.5 and 2.6, depending on the spin state of H2, in good agreement with values derived in translucent clouds with relatively high extinction. In the pre-stellar core-collapse calculations, the OPR of H2O is shown to reach the statistical value of 3 in regions with severe depletion (nH > 107 cm−3). We conclude that a low water OPR (≲ 2.5) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H2) ≲ 1. Available OPR measurements in protoplanetary discs and comets are finally discussed.
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