The Experts below are selected from a list of 153 Experts worldwide ranked by ideXlab platform
Walter W. Duley - One of the best experts on this subject based on the ideXlab platform.
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MOLECULAR CLUSTERS IN Interstellar Clouds
The Astrophysical Journal, 1996Co-Authors: Walter W. DuleyAbstract:Cluster ions of the type H+3 (H2)p, H3O+ (H2O)q, and mixed clusters of the type H3O+ (H2O)q(H2)p may be formed by gas-phase chemistry or by cosmic-ray-induced desorption from dust grains in dense Interstellar Clouds. An analysis of formation mechanisms leads to the prediction of an equilibrium abundance of H+3 (H2)p clusters, where p = 3-4, of ~10-10 n. The initial stage in the gas-phase formation of these cluster ions would be via radiative association of H+3 and H2 at a rate ~10-16 cm3 s-1. Desorption from H2 monolayers by H+3 or He+ collisions with grains leads to a similar production rate for H+3 (H2)p clusters. Such cluster ions have been observed in laboratory experiments on charged particle impact with solid H2 layers. Cosmic-ray sputtering of adsorbed layers on dust can form cluster ions via the creation of energetic ions such H+3 and H3O+. An equilibrium abundance of H+3 (H2)p clusters, independent of cloud density, of ~10-8 cm-3 is predicted due to cosmic-ray sputtering of adsorbed H2 molecules. Sputtering of ice layers by cosmic rays should produce a range of large cluster ions H3O+ (H2O)q in Interstellar Clouds. Laboratory data on sputtering of H2O with keV He+ ions shows that clusters with q 50 are possible. The fragmentation of such clusters on electron-ion recombination is likely to lead to a range of neutral clusters. The abundance of such clusters, which may be considered to be a population of very small grains, is predicted to be comparable to that of dust grains. These clusters can accrete other atomic and molecular species and may constitute a gas-phase route toward grain formation in dense Interstellar Clouds.
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IR emission from vibrationally excited molecules as a probe of chemistry in cold, dark Interstellar Clouds
Monthly Notices of the Royal Astronomical Society, 1992Co-Authors: Walter W. Duley, David A. WilliamsAbstract:Molecule formation in Interstellar Clouds can leave molecules such as H 2 , CH + , CH 2 + , CO, etc. in vibrationally excited states. In general, molecules lose this excitation by emission of IR radiation. The possibility of detecting this emission in dark Interstellar Clouds is discussed and it is shown that emission from vibrationally excited H 2 , H 3 + and CO should be detectable. Detection (or non-detection) of this radiation would provide important information on chemical reaction routes in dark Interstellar Clouds
Eric Herbst - One of the best experts on this subject based on the ideXlab platform.
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Chemical Kinetics in Interstellar Clouds
Reference Module in Chemistry Molecular Sciences and Chemical Engineering, 2015Co-Authors: Eric HerbstAbstract:The solar system is not the only place in the universe in which there is an active chemistry. Chemistry also occurs in giant Clouds of gas and dust particles that exist in the rarefied regions among the stars. In the so-called dense Interstellar Clouds, nearly 200 different molecules, most of them organic in nature, have been discovered in the gas. Molecules have also been discovered to exist in ice mantles that surround dust particles. In this article, we discuss the chemistry that produces these diverse molecules, especially in portions of dense Clouds that are collapsing to form new stars and planets.
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Dissociative Recombination in Interstellar Clouds
Dissociative Recombination of Molecular Ions with Electrons, 2003Co-Authors: Eric HerbstAbstract:Dissociative recombination has a universality far greater than most researchers in the field realize — it occurs in Interstellar matter, the ubiquitous material that exists among the stars of our own galaxy and most others. The Interstellar medium is not uniform but is Condensed into relatively dense regions known as Interstellar Clouds, which can be many light years in extent and contain many solar masses of material. The physical conditions in Interstellar Clouds comprise a wide range of temperature and density; the matter consists of a gaseous component, which contains roughly 99% of the mass, and a particulate component of tiny dust particles perhaps 0.1µ in radius.1
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radiative electron attachment to small linear carbon clusters and its significance for the chemistry of diffuse Interstellar Clouds
International Journal of Mass Spectrometry, 2000Co-Authors: R Terzieva, Eric HerbstAbstract:Abstract Recent spectroscopic studies of carbon chain anions in the gas phase, and more specifically of C7−, have brought insight into the long-standing mystery of the unexplained diffuse Interstellar bands. Previously, negative ions had not been considered highly abundant in Interstellar Clouds, and the question of efficient mechanisms leading to their formation had not been investigated in great detail. This work presents a statistical calculation of the rate coefficients for radiative attachment of an electron to small linear carbon clusters containing 4–9 atoms. We conclude that for molecules with 6 or more C atoms, the attachment occurs on every collision at the low temperatures of diffuse Interstellar Clouds.
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fractional abundances of molecules in dense Interstellar Clouds a compendium of recent model results
Astronomy & Astrophysics Supplement Series, 1996Co-Authors: H H Lee, R P A Bettens, Eric HerbstAbstract:In this paper we present calculated fractional abundances in dense Interstellar Clouds for selected atomic and molecular species using three different homogeneous, pseudo-time-dependent models discussed by Bettens, Lee, & Herbst (1995): the new standard model, the new neutral-neutral model, and model 4. We have run each model with 3 different hydrogen densities – 103 , 104 , and 105 cm-3 – and two temperatures – 10 K and 50 K. “Low metal” elemental abundances have been used for all three models; the new standard model has also been run with “high metal” abundances.
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models of gas grain chemistry in dense Interstellar Clouds with complex organic molecules
Astrophysical Journal Supplement Series, 1992Co-Authors: Tatsuhiko I Hasegawa, Eric Herbst, Chun M LeungAbstract:Models of the chemistry of dense Interstellar Clouds are presented in which both gas-phase and grain-surface chemistry occur. The dust grain and gas temperatures are fixed at 10 K, and the gas density n = n(H) + 2n(H 2 ) remains approximately at 2 × 10 4 cm -3 in these models, which are designed primarily to represent the chemistry occurring in dark Clouds. We utilize previous ideas on what constitutes the most likely reactions to occur on large classical grains. Grain reactions that produce molecules as complex as those in our previous models of gas-phase chemistry are included to help elucidate the role of grains in the synthesis of organic molecules
Javier Ballesterosparedes - One of the best experts on this subject based on the ideXlab platform.
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six myths on the virial theorem for Interstellar Clouds
Monthly Notices of the Royal Astronomical Society, 2006Co-Authors: Javier BallesterosparedesAbstract:The Interstellar medium is highly dynamic and turbulent. However, little or no attention has been paid in the literature to the implications that this fact has on the validity of at least six common assumptions on the virial theorem (VT), which are as follows. (i) The only role of turbulent motions within a cloud is to provide support against collapse; (ii) the surface terms are negligible compared to the volumetric ones; (iii) the gravitational term is a binding source for the Clouds since it can be approximated by the gravitational energy; (iv) the sign of the second time-derivative of the moment of inertia determines whether the cloud is contracting ( ¨ I 0); (v) Interstellar Clouds are in virial equilibrium (VE) and (vi) Larson’s relations (mean density‐size and velocity dispersion‐size) are the observational proof that Clouds are in VE. However, turbulent, supersonic Interstellar Clouds cannot fulfil these assumptions because turbulent fragmentation will induce flux of mass, moment and energy between the Clouds and their environment, and will favour local collapse while it may also disrupt the Clouds within a dynamical time-scale. It is argued that although the observational and numerical evidence suggests that Interstellar Clouds are not in VE, the so-called ‘virial mass’ estimations, which should actually be called ‘energy-equipartition mass’ estimations, are good order of magnitude estimations of the actual mass of the Clouds just because observational surveys will tend to detect Interstellar Clouds appearing to be close to energy equipartition. Similarly, order of magnitude estimations of the energy content of the Clouds is reasonable. However, since Clouds are actually out of VE, as suggested by asymmetrical line profiles, they should be transient entities. These results are compatible with observationally based estimations for rapid star formation, and call into question the models for the star formation efficiency based on Clouds being in VE.
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six myths on the virial theorem for Interstellar Clouds
arXiv: Astrophysics, 2006Co-Authors: Javier BallesterosparedesAbstract:It has been paid little or no attention to the implications that turbulent fragmentation has on the validity of at least six common assumptions on the Virial Theorem (VT), which are: (i) the only role of turbulent motions within a cloud is to provide support against collapse, (ii) the surface terms are negligible compared to the volumetric ones, (iii) the gravitational term is a binding source for the Clouds, (iv) the sign of the second-time derivative of the moment of inertia determines whether the cloud is contracting or expanding, (v) Interstellar Clouds are in Virial Equilibrium (VE), and (vi) Larson's (1981) relations are the observational proof that Clouds are in VE. Interstellar Clouds cannot fulfill these assumptions, however, because turbulent fragmentation will induce flux of mass, moment and energy between the Clouds and their environment, and will favor local collapse while may disrupt the Clouds within a dynamical timescale. It is argued that, although the observational and numerical evidence suggests that Interstellar Clouds are not in VE, the so-called ``Virial Mass'' estimations, which actually should be called ``energy-equipartition mass'' estimations, are good order-of magnitude estimations of the actual mass of the Clouds just because observational surveys will tend to detect Interstellar Clouds appearing to be close to energy equipartition. However, since Clouds are actually out of VE, as suggested by asymmetrical line profiles, they should be transient entities. These results are compatible with observationally-based estimations for rapid star formation. , and call into question the models for the star formation efficiency based on Clouds being in VE.
Chun M Leung - One of the best experts on this subject based on the ideXlab platform.
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models of gas grain chemistry in dense Interstellar Clouds with complex organic molecules
Astrophysical Journal Supplement Series, 1992Co-Authors: Tatsuhiko I Hasegawa, Eric Herbst, Chun M LeungAbstract:Models of the chemistry of dense Interstellar Clouds are presented in which both gas-phase and grain-surface chemistry occur. The dust grain and gas temperatures are fixed at 10 K, and the gas density n = n(H) + 2n(H 2 ) remains approximately at 2 × 10 4 cm -3 in these models, which are designed primarily to represent the chemistry occurring in dark Clouds. We utilize previous ideas on what constitutes the most likely reactions to occur on large classical grains. Grain reactions that produce molecules as complex as those in our previous models of gas-phase chemistry are included to help elucidate the role of grains in the synthesis of organic molecules
Kevin M. Hickson - One of the best experts on this subject based on the ideXlab platform.
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Elemental nitrogen partitioning in dense Interstellar Clouds
Proceedings of the National Academy of Sciences of the United States of America, 2012Co-Authors: Julien Daranlot, U. Hincelin, Astrid Bergeat, Michel Costes, Jean-christophe Loison, Valentine Wakelam, Kevin M. HicksonAbstract:Many chemical models of dense Interstellar Clouds predict that the majority of gas-phase elemental nitrogen should be present as N2, with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N2 is difficult to detect spectroscopically through infrared or millimeter-wavelength transitions. Therefore, its abundance is often inferred indirectly through its reaction product N2H+. Two main formation mechanisms, each involving two radical-radical reactions, are the source of N2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction, down to 56 K. The measured rate constants for this reaction, and those recently determined for two other reactions implicated in N2 formation, are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of Interstellar nitrogen present as N2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N2 formation are inefficient, we argue that N2 does not represent the main reservoir species for Interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH3 should be present on Interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.
