Effects of large rate coefficients for ion-polar neutral reactions on chemical models of dense interstellar clouds

1986 ◽  
Vol 310 ◽  
pp. 378 ◽  
Author(s):  
Eric Herbst ◽  
Chun Ming Leung
Keyword(s):  
Rate Coefficients ◽  
Interstellar Clouds ◽  
Chemical Models

scholarly journals Chemistry in Dense Interstellar Clouds – Data Requirements

1989 ◽  
Vol 8 ◽  
pp. 369-374 ◽  
Author(s):  
T. J. Millar
Keyword(s):  
Experimental Data ◽  
Observational Studies ◽  
Rate Coefficients ◽  
Interstellar Clouds ◽  
Chemical Models ◽  
Data Requirements

ABSTRACTChemical models of dense interstellar clouds are reviewed with particular emphasis on recent results. The need for theoretical and experimental data on rate coefficients is pointed out and some observational studies are suggested.

scholarly journals Evolutionary Models of Interstellar Chemistry

1992 ◽  
Vol 150 ◽  
pp. 205-210
Author(s):  
Sheo S. Prasad
Keyword(s):  
Gas Phase ◽  
Chemical Processes ◽  
Evolutionary Models ◽  
Evolutionary Perspective ◽  
Interstellar Chemistry ◽  
Interstellar Clouds ◽  
Dynamical Equilibrium ◽  
Chemical Models ◽  
Gas Phase Molecules ◽  
Fixed Condition

Evolutionary chemical models are ultimately unavoidable for a full understanding of interstellar clouds. They include not only the chemical processes but also the dynamical processes by which the modeled object came to be the way it is. From an evolutionary perspective, dark cores may be ephemeral objects and dynamical equilibrium an exception rather than norm. Evolutionary models have numerous advantages over “classical” fixed condition equilibrium models. They have the potential to provide more elegant explanations for the observed inter-cloud and intra-cloud chemical differences. The problem of the depletion of gas phase molecules by condensation onto the grain may also be less serious in evolutionary models. Hence, these models should be actively developed.

scholarly journals Recent Advances in the studies of Reaction Rates relevant to Interstellar Chemistry

1987 ◽  
Vol 120 ◽  
pp. 1-18
Author(s):  
Nigel G. Adams ◽  
David Smith
Keyword(s):  
Laboratory Data ◽  
Dissociative Recombination ◽  
Reaction Rates ◽  
Rate Coefficients ◽  
Current Status ◽  
Laboratory Measurements ◽  
Interstellar Molecules ◽  
Interstellar Clouds ◽  
Radiative Association ◽  
Molecular Synthesis

The current status of laboratory measurements of the rate coefficients for ionic reactions involved in interstellar molecular synthesis is discussed and the experimental techniques used to acquire such data are briefly described. Examples are given of laboratory data which are being obtained at temperatures close to those of interstellar clouds. Particular attention is given to the results of recent theoretical and experimental work which show that the rate coefficients for the binary reactions of ions with polar molecules at low temperatures are much larger than previously assumed. It is shown how these new developments in experiment and theory are reconciling the differences between predicted and observed abundances for some interstellar molecules. Also briefly discussed are: - the phenomenon of isotope exchange in ion/neutral reactions which explains the apparent enrichment of heavy isotopes in some interstellar molecules, the role of atoms in molecular synthesis, some studies of ion/neutral reactions pertaining to shocked regions of interstellar clouds, ternary association reactions and the analogous radiative association reactions, and recent new laboratory measurements of dissociative recombination coefficients. Finally, some guidance is offered in the proper choice of critical kinetic data for use in interstellar chemical modelling and some further requirements and likely future developments are mentioned.

scholarly journals 14N/15N ratio measurements in prestellar cores with N2H+: new evidence of 15N-antifractionation

2018 ◽  
Vol 617 ◽  
pp. A7 ◽  
Author(s):  
E. Redaelli ◽  
L. Bizzocchi ◽  
P. Caselli ◽  
J. Harju ◽  
A. Chacón-Tanarro ◽  
...  
Keyword(s):  
Core Sample ◽  
Rate Coefficients ◽  
Chemical Models ◽  
Prestellar Cores ◽  
Bona Fide ◽  
Molecular Tracer ◽  
Non Local ◽  
The One ◽  
Collisional Rate Coefficients ◽  
High Level

Context. The 15N fractionation has been observed to show large variations among astrophysical sources, depending both on the type of target and on the molecular tracer used. These variations cannot be reproduced by the current chemical models. Aims. Until now, the 14N/15N ratio in N2H+ has been accurately measured in only one prestellar source, L1544, where strong levels of fractionation, with depletion in 15N, are found (14N/15N ≈ 1000). In this paper, we extend the sample to three more bona fide prestellar cores, in order to understand if the antifractionation in N2H+ is a common feature of this kind of source. Methods. We observed N2H+, N15NH+, and 15NNH+ in L183, L429, and L694-2 with the IRAM 30 m telescope. We modelled the emission with a non-local radiative transfer code in order to obtain accurate estimates of the molecular column densities, including the one for the optically thick N2H+. We used the most recent collisional rate coefficients available, and with these we also re-analysed the L1544 spectra previously published. Results. The obtained isotopic ratios are in the range 580–770 and significantly differ with the value, predicted by the most recent chemical models, of ≈440, close to the protosolar value. Our prestellar core sample shows a high level of depletion of 15N in diazenylium, as previously found in L1544. A revision of the N chemical networks is needed in order to explain these results.

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