Tautomeric equilibria in relation to pi-electron delocalization.

We have imaged emission from the millimeter lines of eight molecules—C2H, C 34 S, N2H + ,C H3OH, HNCO, HNC, HC3N, and SO—in the central half-kiloparsec of the nearby spiral galaxy IC 342. The 5 00 (� 50 pc) resolution images were made with the Owens Valley Millimeter Array. Using these and previously published CO and HCN images, we obtain a picture of the chemistry within the nuclear region on the size scales of individual giant molecular clouds. Bright emission is detected from all but SO. There are marked differences in morphology for the different molecules. A principal-component analysis is performed to quantify similarities and differences among the images. This analysis reveals that while all molecules are to zeroth order correlated, that is, that they are all found in dense molecular clouds, there are three distinct groups of molecules distinguished by the location of their emission within the nuclear region. N2H + ,C 18 O, HNC, and HCN are widespread and bright, good overall tracers of dense molecular gas. C2 Ha nd C 34 S, tracers of photodissociation region chemistry, originate exclusively from the central 50–100 pc region, where radiation fields are high. The third group of molecules, CH3OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by the processing of grain mantles. HC3N is observed to correlate tightly with 3 mm continuum emission, demonstrating that the young starbursts are the sites of the warmest and densest molecular gas. We compare our HNC images with the HCN images of Downes and coworkers to produce the first highresolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity across the nucleus, and the correlation of both of these gas tracers with star formation is excellent. The ratio exhibits no obvious correlation with gas temperature or star formation strength. Subject headings: astrochemistry — galaxies: individual (IC 342) — galaxies: ISM — galaxies: starburst — radio lines: galaxies

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Spatially resolved chemistry in nearby galaxies. I. The center of IC 342

We have observed 10 molecular species of "intermediate" complexity (four to seven atoms) in three translucent clouds and in TMC-1 and L183. Of these species HNCO, HOCO+, H2CCO, CCH, and CH3CCH are detected in all five objects, while HCOOH, CH3CHO, CH3CN, CH2NH, and VyCN are detected in fewer objects. These species are chosen because they are expected to mark the transition between simpler species that are formed in the gas phase and complex hydrogenated species [NH2CHO, EtOH, EtCN, (CH3)2O, and CH3OCHO] that are believed to form by grain chemistry. The C/O ratio and the metal abundances are determining factors in the abundances of most of the species. The gas-phase chemistry of HNCO is set out for the first time. HNCO, H2CCO, CCH, and CH3CCH are successfully modeled for a wide range of C/O, metals, and cloud conditions. HOCO+, HCOOH, CH3CN, and VyCN are well modeled under fewer conditions, while CH3CHO and CH2NH are fitted in only a small range of conditions. For most species, the models tend to underestimate the observed abundances. By introducing or modifying a number of gas-phase reactions in the New Standard Model we are able to explain the abundances of most species for specific sets of physical conditions (density, temperature, extinction), abundances (metals, C/O), and time epoch. The parameter sets for the different species are largely non overlapping. However, we have been able to find a single, unique set of parameters including a specific epoch for each of TMC-1, L183, and the translucent clouds that explains eight of the 10 species. The exceptions are CH2NH, which requires higher C/O than other species, and CH3CHO, which is possibly modeled in dark clouds but not in translucent objects. For the eight well-fitted species, C/O=0.4 is favored, and the time epoch is 5(5)±4(5) yr for each type of object. In this exercise we cannot distinguish between high and low metals. Previously studied species (HCN, C3H2, CCS, and HC3N) fit the same set of conditions. It is possible that all 10 species may be explained by gas-phase chemistry, perhaps when suitable neutral-neutral reactions are found for species such as CH3CHO and CH2NH. Until then, grain processes cannot be ruled out for these and other species such as HOCO+ (formed from desorbed CO2) or VyCN (highly saturated). Ortho/para and E/A ratios are found to lie within the thermal limits for all cases except O/P (H2CCO), whose value of 5.9 in TMC-1 exceeds the limit of 3.0, which confirms an earlier result by Ohishi. This result cannot be explained by grain interactions. Nevertheless, in the species of intermediate complexity, we may be encountering the boundary between gas and grain chemistry for partly hydrogen-saturated species. The transition to grain chemistry seems clearly to occur at the next level of complexity, which includes NH2CHO, EtOH, EtCN, (CH3)2O, and CH3OCHO, species we have failed to detect after exhaustive searches in translucent and dark clouds.

The Physics and Chemistry of Small Translucent Molecular Clouds. XII. More Complex Species Explainable by Gas-Phase Processes

2 Collisional excitation 7 2.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1.2 Potential energy surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1.3 Scattering Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2 H2, CO and H2O molecules as benchmark systems . . . . . . . . . . . . . . . . . 19 2.2.1 H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 ∗To whom correspondence should be addressed †Laboratoire Univers et Théories, Observatoire de Paris, 92190, Meudon, France ‡LOMC UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 540, 76058, Le Havre, France

Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes

Molecular excitation in the interstellar medium: recent advances in collisional, radiative, and chemical processes.

We have conducted a survey of HCN and HNC (two rotational transitions each) in our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. Both species are seen in all 38 objects. HCNH+ has been searched in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ~ r-α structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperfine splitting (hfs) of HCN. It is shown that serious errors are made in deriving HCN abundances by methods that ignore the hfs. Both HCN and HNC abundances are high, typically 1(-8) in most sources. The chemically important ratio HCN/HNC is found to be ~2.5 if these species are spatially centrally peaked and ~6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2-2.7 mag range (edge to center), thus displaying the same characteristic transition between diffuse and dense cloud chemistry as do most other species we have studied. HCN/HNC decreases with increasing extinction to a value of 1.3 at Av0 ~ 10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chemistry models have been carried out: a full model using the Standard Model rate file and comprising 409 species (by Lee and Herbst), and a simplified model comprising 21 nitrogen-bearing species for conditions relevant to translucent clouds. Good agreement between observations and chemistry models is achieved throughout the translucent extinction range. Important conclusions are that (1) neutral-neutral reactions such as N + CH2 dominate the chemistry of HCN; (2) low ion-polar reaction rates are strongly favored over high ones; (3) the reaction C+ + NH3 → H2NC+ → HNC is unimportant, thus largely uncoupling the CN and NH chemistries; (4) the ratio HCN/HNC is not a particularly important diagnostic of the CN chemistry; (5) model NH3 abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N+H+3→NH+2 is permitted at Langevin rate.

http://iopscience.iop.org/article/10.1086/304228/pdf

Y. C. Minh

Papers

The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC