Molecules In Dense Interstellar Clouds Research Articles

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Understanding the abundances of molecules in dense interstellar clouds requires knowledge of the rates of gas-phase reactions between uncharged species. However, because of the low temperatures within these clouds, reactions with an activation barrier were considered too slow to play an important role. Here we show that, despite the presence of a barrier, the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol--one of the most abundant organic molecules in space--is almost two orders of magnitude larger at 63 K than previously measured at ∼200 K. We also observe the formation of the methoxy radical product, which was recently detected in space. These results are interpreted by the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum-mechanical tunnelling to form products. We postulate that this tunnelling mechanism for the oxidation of organic molecules by OH is widespread in low-temperature interstellar environments.

The chemistry in circumstellar envelopes of evolved stars: following the origin of the elements to the origin of life.

Mass loss from evolved stars results in the formation of unusual chemical laboratories: circumstellar envelopes. Such envelopes are found around carbon- and oxygen-rich asymptotic giant branch stars and red supergiants. As the gaseous material of the envelope flows from the star, the resulting temperature and density gradients create a complex chemical environment involving hot, thermodynamically controlled synthesis, molecule "freeze-out," shock-initiated reactions, and photochemistry governed by radical mechanisms. In the circumstellar envelope of the carbon-rich star IRC+10216, >50 different chemical compounds have been identified, including such exotic species as C(8)H, C(3)S, SiC(3), and AlNC. The chemistry here is dominated by molecules containing long carbon chains, silicon, and metals such as magnesium, sodium, and aluminum, which makes it quite distinct from that found in molecular clouds. The molecular composition of the oxygen-rich counterparts is not nearly as well explored, although recent studies of VY Canis Majoris have resulted in the identification of HCO(+), SO(2), and even NaCl in this object, suggesting chemical complexity here as well. As these envelopes evolve into planetary nebulae with a hot, exposed central star, synthesis of molecular ions becomes important, as indicated by studies of NGC 7027. Numerous species such as HCO(+), HCN, and CCH are found in old planetary nebulae such as the Helix. This "survivor" molecular material may be linked to the variety of compounds found recently in diffuse clouds. Organic molecules in dense interstellar clouds may ultimately be traced back to carbon-rich fragments originally formed in circumstellar shells.

Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth

▪ Abstract Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observational results from the ground and space, as well as laboratory simulation experiments and new methods for theoretical modeling, this review recapitulates the inventory and distribution of organic molecules in different environments. The evolution, survival, transport, and transformation of organics is monitored, from molecular clouds and the diffuse interstellar medium to their incorporation into solar system material such as comets and meteorites. We constrain gas phase and grain surface formation pathways to organic molecules in dense interstellar clouds, using recent observations with the Infrared Space Observatory (ISO) and ground-based radiotelescopes. The main spectroscopic evidence for carbonaceous compounds in the diffuse interstellar medium is discussed (UV bump at 2200 Å, diffuse interstellar bands, extended red emission, and infrared absorption and emission bands). We critically review the signatures and unsolved problemsrelated to the main organic components suggested to be present in the diffuse gas, such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, diamonds, and carbonaceous solids. We also briefly discuss the circumstellar formation of organics around late-typestars. In the solar system, space missions to comet Halley and observations of the bright comets Hyakutake and Hale-Bopp have recently allowed a reexamination of the organic chemistry of dust and volatiles in long-period comets. We review the advances in this area and also discuss progress being made in elucidating the complex organic inventory of carbonaceous meteorites. The knowledge of organic chemistry in molecular clouds, comets, and meteorites and their common link provides constraints for the processes that lead to the origin, evolution, and distribution of life in the Galaxy.

Reaction kinetics at very low temperatures: laboratory studies and interstellar chemistry.

Studies of gas-phase processes at temperatures down to 10 K have recently blossomed, largely through application of the CRESU (cinétique de réaction en ecoulement supersonique uniforme) technique. The results are of considerable relevance to the synthesis of molecules in dense interstellar clouds, demonstrating that the models developed to explain the observed molecular abundances must be expanded to include reactions between electrically neutral species. In addition, the experimental results have stimulated theoretical efforts to describe the factors that control the rates of such low-temperature reactions. In this Account, the CRESU method is described and the relevance of the results discussed.

Fractional abundances of molecules in dense interstellar clouds: A compendium of recent model results

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.

Calculations concerning the reaction C + H3+ --> CH(+) + H2.

The ion-molecule reaction C + H3+ --> CH(+) + H2 has not been studied in the laboratory but is thought to be important in the gas phase synthesis of organic molecules in dense interstellar clouds. We have studied this reaction theoretically by performing quantum chemical ab initio calculations on the potential surface. We find that there is no activation barrier to the reaction and that it proceeds smoothly to the first excited electronic state of CH+. The rate coefficient as a function of temperature can then be estimated using the proper long-range potentials. The rate coefficient at 10 K is calculated to be 2.9 x 10(-9) cm3 s-1.

The in situ formation of large molecules in dense interstellar clouds

view Abstract Citations (41) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The In Situ Formation of Large Molecules in Dense Interstellar Clouds Herbst, Eric Abstract A simple model has been developed to estimate the abundances of large hydrocarbon molecules produced by gas-phase, ion-molecule reactions in dense interstellar clouds. Hydrocarbons through 30 carbon atoms in size are considered in the model. Molecular complexity is built up via synthetic reactions involving smaller hydrocarbon neutral and ions. A quasi-time-dependent model has been utilized in which the abundances of hydrocarbons with fewer than 10 carbon atoms are fixed at values calculated in a previous model. The time dependence of the abundances of the more complex hydrocarbons is then followed. The results show that, with favorable values for important rate coefficients, it is possible to produce sizeable abundances of hydrocarbon molecules exceeding about 20 carbon atoms in size an in a time scale of not greater than 10 to the 6th yr. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169546 Bibcode: 1991ApJ...366..133H Keywords: Astronomical Models; Interstellar Chemistry; Interstellar Matter; Molecular Clouds; Abundance; Computational Astrophysics; Hydrocarbons; Astrophysics; INTERSTELLAR: ABUNDANCES; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES full text sources ADS |

Microwave Spectroscopy of Astrophysical Molecules

ABSTRACTRecent detections of new molecules in dense interstellar clouds, first detections of certain chemical elements in interstellar molecules, and new information on isotopic fractionation of hydrogen in the interstellar medium are discussed in the context of the need for new laboratory data on transition rest frequencies, reaction rates, and branching ratios.

Can gas phase reactions produce complex oxygen-containing molecules in dense interstellar clouds? - A revision of some important radiative association rate coefficients

view Abstract Citations (55) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Can Gas Phase Reactions Produce Complex Oxygen-containing Molecules in Dense Interstellar Clouds? A Revision of Some Important Radiative Association Rate Coefficients Herbst, Eric Abstract In light of new laboratory measurements and theoretical techniques, previously suggested gas phase syntheses of dimethyl ether (CH3OCH3), ethyl alcohol (C2H5OH), and acetaldehyde (CH3CHO) in dense interstellar clouds have been reexamined. Several radiative association rate coefficients leading to the precursor ions to these species are now calculated to be significantly smaller than previous estimates. On the other hand, a new and rapid synthesis of the precursor ion to acetaldehyde appears to be viable. The diminution in the values of selected radiative association rate coefficients raises the possibility that normal gas phase syntheses under ambient dense cloud conditions may not account fully for the observed abundances of ethyl alcohol and, especially, dimethyl ether. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/165026 Bibcode: 1987ApJ...313..867H Keywords: Association Reactions; Interstellar Chemistry; Molecular Clouds; Reaction Kinetics; Vapor Phases; Acetaldehyde; Ethers; Ethyl Alcohol; Methyl Compounds; Oxygen; Astrophysics; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES full text sources ADS | data products SIMBAD (2)

Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry: model update and sensitivity analysis

Les calculs concernent une liste de 1958 reactions qui impliquent 206 especes incluant un echantillon representatif de molecules complexes. L'etude chimique est faite en fonction de l'extinction visuelle, de la temperature de nuage et du rapport des abondances carbone/oxygene

Collisional excitation of molecules in dense interstellar clouds

Interpretation of radiofrequency and microwave observations of interstellar molecules requires knowledge of state-to-state collisional excitation rate constants. Although the required rates generally cannot be measured experimentally, they can be calculated theoretically. Computational methods are summarized and a survey of the availability of relevant values is presented. Accuracy of computed values as determined from comparison with related experimental rates is discussed briefly.

Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry - A pseudo time-dependent calculation

view Abstract Citations (336) References (80) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry: a pseudo time-dependent calculation. Leung, C. M. ; Herbst, E. ; Huebner, W. F. Abstract A pseudo-time-dependent numerical model of the gas-phase chemistry of dense interstellar clouds has been developed, in order to determine whether gas-phase processes can account for the high degree of chemical activity found in the course of recent observations. The model includes 200 species and over 1800 gas-phase reactions. To simulate chemical processes in the clouds, an initial set of chemical abundances is allowed to evolve under constant physical conditions and in the absence of photodissociations. The results show that under constant conditions complex molecular species are produced in significant amounts, but not under the steady-state conditions which exist when C I abundance is a small fraction of the CO abundance. It is found that the abundances calculated by the model often exceeded observed values. It is suggested that this discrepancy is due to the difference in the time of maximum complex molecular abundances and the time when steady-state is reached (not less than 10 to the 7th years). Some of the observational implications of the data are discussed, and a table listing the calculated abundances is provided. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1984 DOI: 10.1086/190982 Bibcode: 1984ApJS...56..231L Keywords: Interstellar Chemistry; Interstellar Gas; Molecular Clouds; Molecular Gases; Synthesis (Chemistry); Abundance; Reaction Kinetics; Time Dependence; Astrophysics full text sources ADS | data products SIMBAD (2)

An ion-molecule scheme for the synthesis of hydrocarbon-chain and organonitrogen molecules in dense interstellar clouds

A general scheme is proposed for the synthesis of large molecules in dense interstellar clouds based on rates of ion-molecule reactions measured in our laboratory and elsewhere. C--C bond formation is dominated by reactions involving the must abundant ions C/sup +/, CH/sub 3//sup +/, C/sub 2/H/sub 2//sup +/, and C/sub 2/H/sub 3//sup +/ and the most abundant neutrals C/sub 2/H/sub 2/ and CH/sub x/, where 1

The Formation and Depletion of Molecules in Dense Interstellar Clouds

The Formation and Depletion of Molecules in Dense Interstellar Clouds