The UMIST Database for Astrochemistry 2006

We present a new version of the UMIST Database for Astrochemistry, the fourth such version to be released to the public. The current version contains some 4573 binary gas-phase reactions, an increase of 10% from the previous (1999) version, among 420 species, of which 23 are new to the database. Major updates have been made to ion-neutral reactions, neutral-neutral reactions, particularly at low temperature, and dissociative recombination reactions. We have included for the first time the interstellar chemistry of fluorine. In addition to the usual database, we have also released a reaction set in which the effects of dipole-enhanced ion-neutral rate coefficients are included. These two reactions sets have been used in a dark cloud model and the results of these models are presented and discussed briefly. The database and associated software are available on the World Wide Web at www.udfa.net. Tables 1, 2, 4 and 9 are only available in electronic form at http://www.aanda.org

The UMIST database for astrochemistry 1999

We report a new version of the UMIST database for astrochemistry. The previous (1995) version has been updated and its format has been revised. The database contains the rate coefficients, temperature ranges and - where available - the temperature dependence of 4113 gas-phase reactions important in astrophysical environments. The data involve 396 species and 12 elements. We have also tabulated permanent electric dipole moments of the neutral species and heats of formation. A new table lists the photo process cross sections (ionisation, dissociation, fragmentation) for a few species for which these quantities have been measured. Data for Deuterium fractionation are given in a separate table. Finally, a new online Java applet for data extraction has been created and its use is explained in detail. The detailed new datafiles and associated software are available on the World Wide Web at http://www.rate99.co.uk.

EXPERIMENTAL EVIDENCE FOR RADIATIVE ATTACHMENT IN ASTROCHEMISTRY FROM ELECTRON ATTACHMENT TO NCCCCN

Electron attachment to NCCCCN, dicyanoacetylene (2-butynedinitrile), has been observed. Metastable parent anions, NCCCCN-*, with microsecond or longer lifetimes are formed close to 0 eV electron energy with a cross section of >= 0.25 angstrom(2). The stability of NCCCCN suggests that radiative attachment to NCCCCN and similar linear carbon chain molecules may be an important mechanism for the formation of negatively charged molecular ions in astrophysical environments. CCCN- and CN- fragment anions are formed at similar to 3 and similar to 6 eV.

The UMIST database for astrochemistry 1995

We report the release of a new version of the UMIST database for astrochemistry. The database contains the rate coefficients of 3864 gas-phase reactions important in interstellar and circumstellar chemistry and involves 395 species and 12 elements. The previous (1990) version of this database has been widely used by modellers. In addition to the rate coefficients, we also tabulate permanent electric dipole moments of the neutral species and heats of formation. A numerical model of the chemical evolution of a dark cloud is calculated and important differences to that calculated with the previous database noted.

GAS-PHASE REACTIONS AND RATE COEFFICIENTS FOR USE IN ASTROCHEMISTRY - THE UMIST RATEFILE

We present the rate coefficients of 2880 gas-phase reactions among 313 species involving 12 elements for use in astrochemical models. We describe the motivation behind this work and the caveats which attach to the data in general as well as to specific reactions. We give the permanent electric dipole moments of nearly all the 112 neutral molecules contained in the data set, so that rate coefficients can be calculated at the low temperatures of dark interstellar dust clouds. We have used the data to calculate the pseudo-time-dependent chemical evolution of a dark, dense interstellar cloud and present both early time and steady-state abundances for all 313 species.

The UMIST Database for Astrochemistry 2012

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.

Astrochemistry

In the last 40 years a wide range of molecules, including neutrals, cations and anions, containing up to 13 atoms—in addition to detections of C₆₀ and C₇₀ — have been found in the harsh environment of the interstellar medium. The exquisite sensitivity and very high spectral and, more recently, spatial resolution, of modern telescopes has enabled the physics of star formation to be probed through rotational line emission. In this article, I review the basic properties of interstellar clouds and the processes that initiate the chemistry and generate chemical complexity, particularly in regions of star and planet formation. Our understanding of astrochemistry has evolved over the years. Before 1990, the general consensus was that molecules were formed in binary, gas-phase, or volume, reactions, most importantly ion-neutral reactions despite the very low ionization in clouds. Since then, observations have indicated unambiguously that there is also a contribution from surface processes, particularly on the icy mantles that form around refractory grain cores in cold, dense gas. The balance between these two processes depends on particular physical conditions and can vary during the life cycle of a particular volume of interstellar cloud.The complex chemistry that occurs in space is driven mostly through interaction of thegas with cosmic ray protons, a source of ionization that enables a rich ion-neutral chemistry. In addition, I show that the interaction between the gas and the dust in cold, dense regionsalso leads to additional chemical complexity through reactions that take place in ices at onlya few tens of degrees above absolute zero. Although densities are low compared to those in terrestrial environments, the extremely long life times of interstellar clouds and their enormous sizes, enable complex molecules to be synthesised and detected. I show that in some instances, particularly in reactions involving deuterium, the rotational populations of reactants, together with spin-selection rules, can determine the detailed abundances. Although the review is mainly focused on regions associated with star formation, I also consider chemistry in other interesting astronomical regions — in the early Universe and in the envelopes formed by mass loss during the final stages of stellar evolution.

Dissociative Recombination of Protonated Formic Acid: Implications for Molecular Cloud and Cometary Chemistry

At the heavy ion storage ring CRYRING in Stockholm, Sweden, we have investigated the dissociative recombination of DCOOD2+ at low relative kinetic energies, from ~1 meV to 1 eV. The thermal rate coefficient has been found to follow the expression k(T) = 8.43 × 10-7 (T/300)^-0.78 cm3 s-1 for electron temperatures, T, ranging from ~10 to ~1000 K. The branching fractions of the reaction have been studied at ~2 meV relative kinetic energy. It has been found that ~87% of the reactions involve breaking a bond between heavy atoms. In only 13% of the reactions do the heavy atoms remain in the same product fragment. This puts limits on the gas-phase production of formic acid, observed in both molecular clouds and cometary comae. Using the experimental results in chemical models of the dark cloud, TMC-1, and using the latest release of the UMIST Database for Astrochemistry improves the agreement with observations for the abundance of formic acid. Our results also strengthen the assumption that formic acid is a component of cometary ices.

Chemical Processes in Protoplanetary Disks

We have developed a high-resolution combined physical and chemical model of a protoplanetary disk surrounding a typical T Tauri star. Our aims were to use our model to calculate the chemical structure of disks on small scales (submilliarcsecond in the inner disk for objects at the distance of Taurus, ~140 pc) to investigate the various chemical processes thought to be important in disks and to determine potential molecular tracers of each process. Our gas-phase network was extracted from the UMIST Database for Astrochemistry to which we added gas–grain interactions including freezeout and thermal and non-thermal desorption (cosmic-ray-induced desorption, photodesorption, and X-ray desorption), and a grain-surface network. We find that cosmic-ray-induced desorption has the least effect on our disk chemical structure while photodesorption has a significant effect, enhancing the abundances of most gas-phase molecules throughout the disk and affecting the abundances and distribution of HCN, CN, and CS, in particular. In the outer disk, we also see enhancements in the abundances of H2O and CO2. X-ray desorption is a potentially powerful mechanism in disks, acting to homogenize the fractional abundances of gas-phase species across the depth and increasing the column densities of most molecules, although there remain significant uncertainties in the rates adopted for this process. The addition of grain-surface chemistry enhances the fractional abundances of several small complex organic molecules including CH3OH, HCOOCH3, and CH3OCH3 to potentially observable values (i.e., a fractional abundance of greater than 10-11).

Chemical evolution of protoplanetary disks - the effects of viscous accretion, turbulent mixing and disk winds

We calculate the chemical evolution of protoplanetary disks considering radial viscous accretion, vertical turbulent mixing, and vertical disk winds. We study the effects on the disk chemical structure when different models for the formation of molecular hydrogen on dust grains are adopted. Our gas-phase chemistry is extracted from the UMIST Database for Astrochemistry (Rate06) to which we have added detailed gas-grain interactions. We use our chemical model results to generate synthetic near- and mid-infrared local thermodynamic equilibrium line emission spectra and compare these with recent Spitzer observations. Our results show that if H2 formation on warm grains is taken into consideration, the H2O and OH abundances in the disk surface increase significantly. We find that the radial accretion flow strongly influences the molecular abundances, with those in the cold midplane layers particularly affected. On the other hand, we show that diffusive turbulent mixing affects the disk chemistry in the warm molecular layers, influencing the line emission from the disk and subsequently improving agreement with observations. We find that NH3, CH3OH, C2H2, and sulfur-containing species are greatly enhanced by the inclusion of turbulent mixing. We demonstrate that disk winds potentially affect the disk chemistry and the resulting molecular line emission in a manner similar to that found when mixing is included.

CH abundance gradient in TMC-1

Aims. The aim of this study is to examine if the well-known chemical gradient in TMC-1 is reflected in the amount of rudimentary forms of carbon available in the gas-phase. As a tracer we use the CH radical which is supposed to be well correlated with carbon atoms and simple hydrocarbon ions. Methods. We observed the 9-cm ?-doubling lines of CH along the dense filament of TMC-1. The CH column densities were compared with the total H2 column densities derived using the 2MASS NIR data and previously published SCUBA maps and with OH column densities derived using previous observations with Effelsberg. We also modelled the chemical evolution of TMC-1 adopting physical conditions typical of dark clouds using the UMIST Database for Astrochemistry gas-phase reaction network to aid the interpretation of the observed OH/CH abundance ratios. Results. The CH column density has a clear peak in the vicinity of the cyanopolyyne maximum of TMC-1. The fractional CH abundance relative to H2 increases steadily from the northwestern end of the filament where it lies around 1.0 × 10-8 , to the southeast where it reaches a value of 2.0 × 10-8. The OH and CH column densities are well correlated, and we obtained OH/CH abundance ratios of ~16–20. These values are clearly larger than what has been measured recently in diffuse interstellar gas and is likely to be related to C to CO conversion at higher densities. The good correlation between CH and OH can be explained by similar production and destruction pathways. We suggest that the observed CH and OH abundance gradients are mainly due to enhanced abundances in a low-density envelope which becomes more prominent in the southeastern part and seems to continue beyond the dense filament. Conclusions. An extensive envelope probably signifies an early stage of dynamical evolution, and conforms with the detection of a large CH abundance in the southeastern part of the cloud. The implied presence of other simple forms of carbon in the gas phase provides a natural explanation for the observation of “early-type” molecules in this region.

Performance of density functionals for calculating barrier heights of chemical reactions relevant to astrophysics

The performance of 39 different LDA, GGA, meta-GGA, and hybrid density functionals has been evaluated, for calculating forward and reverse barrier heights of 10 gas-phase reactions involving hydrogen. The reactions are all relevant to astrochemistry. Special focus is put on the applicability of DFT for calculating the rates of corresponding surface hydrogenation reactions that are relevant to the chemistry of ice-coated interstellar grains. General trends in the performance of the density functionals for reactions involving H atoms, H-2, and OH are discussed. The OH+CO reaction is shown to be a very problematic case for DFT. The best overall performance is found for the hybrid density functionals, such as MPW1K, B97-1, B97-2, and B1B95. For several reactions, the HCTH GGA functionals and the VS98 and OLAP3 meta-GGA functionals also give results that are almost as good as those of the hybrid functionals.

Molecular line emission from a protoplanetary disk irradiated externally by a nearby massive star

Star formation often occurs within or nearby stellar clusters. Irradiation by nearby massive stars can photoevaporate protoplanetary disks around young stars (so-called proplyds) which raises questions regarding the ability of planet formation to take place in these environments. We investigate the two-dimensional physical and chemical structure of a protoplanetary disk surrounding a low-mass (T Tauri) star which is irradiated by a nearby massive O-type star to determine the survivability and observability of molecules in proplyds. Compared with an isolated star-disk system, the gas temperature ranges from a factor of a few (in the disk midplane) to around two orders of magnitude (in the disk surface) higher in the irradiated disk. Although the UV flux in the outer disk, in particular, is several orders of magnitude higher, the surface density of the disk is sufficient for effective shielding of the disk midplane so that the disk remains predominantly molecular in nature. We also find that non-volatile molecules, such as HCN and H2O, are able to freeze out onto dust grains in the disk midplane so that the formation of icy planetesimals, e.g., comets, may also be possible in proplyds. We have calculated the molecular line emission from the disk assuming LTE and determined that multiple transitions of atomic carbon, CO (and isotopologues, 13CO and C18O), HCO+, CN, and HCN may be observable with ALMA, allowing characterization of the gas column density, temperature, and optical depth in proplyds at the distance of Orion (˜400 pc).