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 Chemical Models of Collapsing Envelopes

2000 ◽  
Vol 197 ◽  
pp. 51-60
Author(s):  
Edwin A. Bergin
Keyword(s):  
Star Formation ◽  
Gas Phase ◽  
Chemical Evolution ◽  
Molecular Ions ◽  
Polar Molecules ◽  
Mass Star ◽  
Chemical Models ◽  
Gas Phase Molecules ◽  
Low Mass

We discuss recent models of chemical evolution in the developing and collapsing protostellar envelopes associated with low-mass star formation. In particular, the effects of depletion of gas-phase molecules onto grain surfaces is considered. We show that during the middle to late evolutionary stages, prior to the formation of a protostar, various species selectively deplete from the gas phase. The principal pattern of selective depletions is the depletion of sulfur-bearing molecules relative to nitrogen-bearing species: NH3 and N2H+. This pattern is shown to be insensitive to the details of the dynamics and marginally sensitive to whether the grain mantle is dominated by polar or non-polar molecules. Based on these results we suggest that molecular ions are good tracers of collapsing envelopes. The effects of coupling chemistry and dynamics on the resulting physical evolution are also examined. Particular attention is paid to comparisons between models and observations.

scholarly journals Evolutionary Models of Interstellar Chemistry

1987 ◽  
Vol 120 ◽  
pp. 259-272
Author(s):  
Sheo S. Prasad
Keyword(s):  
Evolutionary Models ◽  
Evolutionary Sequence ◽  
Dynamical Models ◽  
Interstellar Chemistry ◽  
Interstellar Clouds ◽  
The Way

The goal of evolutionary models of interstellar chemistry is to understand how interstellar clouds came to be the way they are, how they will change with time, and to place them in an evolutionary sequence with other celestial objects such as stars. To this end, we present an improved Mark II version of our earlier model of chemistry in dynamically evolving clouds. The Mark II model suggests that the conventional elemental C/O ratio less than one can explain the observed abundances of CI and the non-detection of O2 in dense clouds. Coupled chemical-dynamical models seem to have the potential to generate many observable discriminators of the evolutionary tracks. This is exciting, because, in general, purely dynamical models do not yield enough verifiable discriminators of the predicted tracks.

scholarly journals Molecules in Comets: An ISM-Solar System Connection?

2000 ◽  
Vol 197 ◽  
pp. 447-460
Author(s):  
William M. Irvine ◽  
Edwin A. Bergin
Keyword(s):  
Solar Nebula ◽  
Isotopic Fractionation ◽  
Interstellar Matter ◽  
Interstellar Chemistry ◽  
Interstellar Clouds ◽  
Cometary Nuclei ◽  
Chemical Models ◽  
Nuclear Composition ◽  
Relative Abundances ◽  
Cometary Comae

Our knowledge of the volatile composition of comets has advanced considerably since the last IAU Astrochemistry Symposium, in large part due to the apparition of comet Hale-Bopp and its study with both new ground-based instruments and from spacecraft. Some 23 or 24 coma molecules are now known which are probably, at least in part, volatile constituents of the nucleus. Relative abundances have been measured for rarer isotopomers of molecules containing 13C, 15N, 34S, and D, and significant isotopic fractionation is observed for D-containing species. There are striking similarities in both relative abundances of molecular constituents and in isotopic fractionation between material in dense interstellar clouds and that in cometary comae. Whether this indicates that cometary nuclei consist of relatively unprocessed interstellar matter is less clear, since the observed coma composition is not simply related to the nuclear composition, and since recent chemical models of the outer solar nebula mimic interstellar chemistry in important respects.

scholarly journals Rapid compressions in a captive bubble apparatus are isothermal

2003 ◽  
Vol 95 (5) ◽  
pp. 1896-1900
Author(s):  
Wenfei Yan ◽  
Stephen B. Hall
Keyword(s):  
Hydrostatic Pressure ◽  
Gas Phase ◽  
Pulmonary Surfactant ◽  
First Order ◽  
First Order Kinetics ◽  
Gas Phase Molecules ◽  
Actual Change ◽  
Phase Temperature ◽  
Two Phases ◽  
Interfacial Films

Captive bubbles are commonly used to determine how interfacial films of pulmonary surfactant respond to changes in surface area, achieved by varying hydrostatic pressure. Although assumed to be isothermal, the gas phase temperature (Tg) would increase by >100°C during compression from 1 to 3 atm if the process were adiabatic. To determine the actual change in temperature, we monitored pressure (P) and volume (V) during compressions lasting <1 s for bubbles with and without interfacial films and used P · V to evaluate Tg. P · V fell during and after the rapid compressions, consistent with reductions in n, the moles of gas phase molecules, because of increasing solubility in the subphase at higher P. As expected for a process with first-order kinetics, during 1 h after the rapid compression P · V decreased along a simple exponential curve. The temporal variation of n moles of gas was determined from P · V >10 min after the compression when the two phases should be isothermal. Back extrapolation of n then allowed calculation of Tg from P · V immediately after the compression. Our results indicate that for bubbles with or without interfacial films compressed to >3 atm within 1 s, the change in Tg is <2°C.

scholarly journals Standard Molar Enthalpies of Formation of Halomethanes Based on Quantum Chemical Computations

2020 ◽  
Author(s):  
Konstantinos Kalamatianos
Keyword(s):  
Gas Phase ◽  
Quantum Chemical ◽  
Halogen Bond ◽  
Chemical Processes ◽  
Enthalpies Of Formation ◽  
Isodesmic Reaction ◽  
Gold Standard Method ◽  
Ozone Destruction ◽  
Quantum Chemical Computations ◽  
Reaction Schemes

Accurate calculations of standard molar enthalpies of formation (ΔΗf°)m(g) and carbon-halogen bond dissociation enthalpies, BDE, of a variety of halomethanes with relevance on several atmospheric chemical processes and particularly to ozone destruction, were performed in the gas phase at 298.15 K. The (ΔΗf°)m(g) of the radicals formed through bond dissociations have also been computed. Ab initio computational methods and isodesmic reaction schemes were used. It is found that for the large majority of these species, the gold standard method of quantum chemistry (CCSD(T)) and even MP2 are capable to predict enthalpy values nearing chemical accuracy provided that isodesmic reaction schemes are used. New estimates for standard molar enthalpies of formation and BDE are suggested including for species that to our knowledge there are no experimental (ΔΗf°)m(g) (CHCl2Br, CHBr2Cl, CHBrCl, CHICl, CHIBr) or BDE values (CHCl2Br, CHBr2Cl, CHBrCl, CHICl, CHIBr) available in the literature. The method and calculational procedures presented may profitably be used to obtain accurate (ΔΗf°)m(g) and BDE values for these species.

scholarly journals The Gas-phase Chemical Evolution of Dark Clouds

1992 ◽  
Vol 45 (4) ◽  
pp. 451
Author(s):  
RPA Bettens
Keyword(s):  
Gas Phase ◽  
Chemical Species ◽  
Dynamical Evolution ◽  
Constant Density ◽  
Dark Cloud ◽  
Dark Clouds ◽  
Chemical Models ◽  
Almost All ◽  
Present Understanding ◽  
Phase Chemical

A rich chemistry exists within dark clouds. In the most chemically studied dark cloud, Taurus molecular cloud one (TMC-l), more than 40 molecules have been detected. In this paper I look at the current isochoric, i.e. constant density, isothermal time-dependent gas-phase chemical models of dark clouds such as TMC-l and very briefly outline the present understanding of the chemistry of these objects. The above chemical models agree very well with the observed abundances of almost all chemical species at times earlier than steady state, i.e. earlier than thirty million years. However, the models are fraught with uncertainty and are not physically realistic representations of the full dynamical evolution of dark clouds from a more diffuse state. Nevertheless the agreement with observation is striking.

scholarly journals Molecules in nearby and primordial supernovae

2008 ◽  
Vol 4 (S251) ◽  
pp. 221-226
Author(s):  
Isabelle Cherchneff ◽  
Simon Lilly
Keyword(s):  
Gas Phase ◽  
Early Universe ◽  
Organic Molecules ◽  
Molecular Form ◽  
Chemical Kinetic ◽  
Kinetic Approach ◽  
Dust Formation ◽  
Core Collapse ◽  
Chemical Models ◽  
Upper Limits

AbstractWe present new chemical models of supernova (SN) ejecta based on a chemical kinetic approach. We focus on the formation of inorganic and organic molecules including gas phase dust precursors, and consider zero-metallicity progenitor, massive supernovae and nearby core-collapse supernovae such as SN1987A. We find that both types are forming large amounts of molecules in their ejecta at times as early as 200 days after explosion. Upper limits on the dust formation budget are derived. Our results on dust precursors do not agree with existing studies on dust condensation in SN ejecta. We conclude that PMSNe could be the first non-primodial molecule providers in the early universe, ejecting up to 34% of their progenitor mass under molecular form to the pristine, local gas.

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