The UMIST database for astrochemistry 2012
TL;DR: The UMIST Database for Astrochemistry (UDfaa) as mentioned in this paper contains 6173 gas-phase reactions involving 467 species, 47 of which are new to this release.
Abstract: We present the fifth release of the UMIST Database for Astrochemistry (UDfA). The new reaction network contains 6173 gas-phase reactions, involving 467 species, 47 of which are new to this release. We have updated rate coefficients across all reaction types. We have included 1171 new anion reactions and updated and reviewed all photorates. In addition to the usual reaction network, we also now include, for download, state-specific deuterated rate coefficients, deuterium exchange reactions and a list of surface binding energies for many neutral species. Where possible, we have referenced the original source of all new and existing data. We have tested the main reaction network using a dark cloud model and a carbon-rich circumstellar envelope model. We present and briefly discuss the results of these models.
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TL;DR: The Cologne Database for Molecular Spectroscopy (CDMS) as discussed by the authors provides line lists of mostly molecular species which are or may be observed in various astronomical sources (usually) by radio astronomical means.
510 citations
TL;DR: In this paper, a detailed analysis of the gas temperature determination from rotational spectra is performed, and a large range of conditions for which non-equilibrium occurs are identified.
Abstract: The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.
366 citations
Centre national de la recherche scientifique1, University of Virginia2, University of Paris-Sud3, University of Grenoble4, INAF5, Stockholm University6, Academia Sinica Institute of Astronomy and Astrophysics7, University of Franche-Comté8, University of Burgundy9, Argonne National Laboratory10, University of Rennes11, University of Gothenburg12, University of Cologne13, University of Montpellier14, University College London15
TL;DR: In this paper, the authors used a network of chemical reactions and associated rate coefficients to study the chemical composition of the gas and the ices in the interstellar medium, which is based on the network of reactions.
Abstract: Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reacti ...
343 citations
TL;DR: A collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved as mentioned in this paper.
Abstract: A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and for other wavelength-dependent radiation fields, including cool stellar and solar radiation, Lyman-α dominated radiation, and a cosmic-ray induced ultraviolet flux. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. Comparison with other databases such as PHIDRATES is made. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of these shielding types depends on the atom or molecule in question and the assumed dust optical properties. All of the new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of the calculated rates to variation of temperature and isotope, and uncertainties in measured or calculated cross sections, are tested and discussed. Tests were conducted on the new rates with an interstellar-cloud chemical model, and find general agreement (within a factor of two) in abundances obtained with the previous iteration of the Leiden database assuming an ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2 O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.
323 citations
TL;DR: The impact of low-energy cosmic rays on interstellar chemistry is a fast-developing topic, including the propagation of these particles into the clouds in which the chemistry occurs as mentioned in this paper.
Abstract: Cosmic-ray astrophysics has advanced rapidly in recent years, and its impact on other astronomical disciplines has broadened. Many new experiments measuring these particles, both directly in the atmosphere or space and indirectly via γ rays and synchrotron radiation, have widened the range and quality of the information available on their origin, propagation, and interactions. The impact of low-energy cosmic rays on interstellar chemistry is a fast-developing topic, including the propagation of these particles into the clouds in which the chemistry occurs. Cosmic rays, via their γ-ray production, also provide a powerful way to probe the gas content of the interstellar medium. Substantial advances have been made in the observations and modelling of the interplay between cosmic rays and the interstellar medium. Focusing on energies up to 1 TeV, these interrelating aspects are covered at various levels of detail, giving a guide to the state of the subject.
288 citations
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TL;DR: The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars as discussed by the authors, and the value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag).
Abstract: The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars.
2,625 citations
TL;DR: In this paper, the authors present a compilation of critically evaluated kinetic data on elementary homogeneous gas phase chemical reactions for use in modelling combustion processes Data sheets are presented for some 196 reactions each data sheet sets out relevant thermodynamic data, rate coefficient measurements, an assessment of the reliability of the data, references and recommended rate parameters Tables summarizing the preferred rate data are also given
Abstract: This compilation contains critically evaluated kinetic data on elementary homogeneous gas phase chemical reactions for use in modelling combustion processes Data sheets are presented for some 196 reactions Each data sheet sets out relevant thermodynamic data, rate coefficient measurements, an assessment of the reliability of the data, references, and recommended rate parameters Tables summarizing the preferred rate data are also given The reactions considered are limited largely to those involved in the combustion of methane and ethane in air but a few reactions relevant to the chemistry of exhaust gases and to the combustion of aromatic compounds are also included
1,986 citations
TL;DR: In this paper, the authors evaluated data on the kinetics and thermodynamic properties of species that are of importance in methanepyrolysis and combustion, including H, H2, O, O2, OH, HO2, CH2O, CH4, C2H6, HCHO, CO2, CO, HCO, CH3, CH5, CH6, CH7, CH8, CH9, CH10, CH11, CH12, CH13, CH14, CH15, CH16, CH17, CH
Abstract: This document contains evaluated data on the kinetics and thermodynamic properties of species that are of importance in methanepyrolysis and combustion. Specifically, the substances considered include H, H2, O, O2, OH, HO2, H2O2, H2O, CH4, C2H6, HCHO, CO2, CO, HCO, CH3, C2H5, C2H4, C2H3, C2H2, C2H, CH3CO, CH3O2, CH3O, singlet CH2, and triplet CH2. All possible reactions are considered. In arriving at recommended values, first preference is given to experimental measurements. Where data do not exist, a best possible estimate is given. In making extrapolations, extensive use is made RRKM calculations for the pressure dependence of unimolecular processes and the BEBO method for hydrogen transfer reactions. In the total absence of data, recourse is made to the principle of detailed balancing, thermokinetic estimates, or comparisons with analogous reactions. The temperature range covered is 300–2500 K and the density range 1×1016–1×1021 molecules/cm3. This data base forms a subset of the chemical kinetic data base for all combustion chemistry processes. Additions and revisions will be issued periodically.
1,803 citations
TL;DR: In this paper, a compilation of kinetic data on elementary, homogeneous, gas phase reactions of neutral species involved in combustion systems has been presented under the auspices of the European Community Energy Research and Development Program.
Abstract: This compilation updates and expands a previous evaluation of kinetic data on elementary, homogeneous, gas phase reactions of neutral species involved in combustion systems [J. Phys. Chem. Ref. Data 21, 411 (1992)]. The work has been carried out under the auspices of the European Community Energy Research and Development Program. Data sheets are presented for some 78 reactions and two tables in which preferred rate parameters are presented for reactions of ethyl, i‐propyl, t‐butyl, and allyl radicals are given. Each data sheet sets our relevant thermodynamic data, experimental kinetic data, references, and recommended rate parameters with their error limits. A table summarizing the recommended rate data is also given. The new reactions fall into two categories: first, to expand the previous compilation relating largely to the combustion in air of methane, ethane and aromatic compounds; and second, provide data for some of the key radicals involved in the combustion of higher alkanes.
1,454 citations
TL;DR: In this paper, a steady-state solution for the temperature of diffuse gas (including radiative cooling and recombination, cosmic ray or X-ray heating and ionization, grain photoelectric heating, and other heating mechanisms) is found.
Abstract: Photoelectric emission from interstellar grains is reexamined, and it is argued that some of the assumptions made by other authors lead to an overestimate of the heating rate associated with this process, particularly at temperatures T> or approx. =3000 K. Steady-state solutions for the temperature of diffuse gas (including radiative cooling and recombination, cosmic ray or X-ray heating and ionization, grain photoelectric heating, and other heating mechanisms) are found. Grains do not contribute significantly to the heating of the ''hot'' (Tapprox. =8000 K) phase, although they dominate the heating of the ''cold'' (Tapprox. =100 K) phase. The minimum pressure for which the ''cold'' phase can exist is sensitive to the choice of grain properties and grain abundance, and under some circumstances the coexistence of two distinct phases in pressure equilibrium is forbidden. A steady-state model with intercloud H I heated by soft X-rays and clouds heated by grain photoemission is in accord with some observations but lacks intermediate-temperature H I. The time-dependent cooling of a fossil H II region is calculated; grain photoelectric heating significantly prolongs the time required for the gas to cool. Fossil H II in the wakes of runaway O stars may produce significant amounts of themore » intermediate temperatue (500> or approx. =T> or approx. =3000 K) H I inferred from 21 cm observations.« less
1,348 citations