The UMIST Database for Astrochemistry 1995

1. Introduction

2. The rate coefficient file

I, R1, R2, P1, P2, P3, P4, , , , Note

where I is the reaction number, R1 and R2 are the reactants - R2 can be a cosmic-ray proton (CRP), a general interstellar ultraviolet photon (PHOTON), or a cosmic-ray-induced ultraviolet photon (CRPHOT) - and P1 to P4 are reaction products. A FORTRAN program, DELOAD, writes FORTRAN source code for the time-dependent, non-linear differential equations, based on the reaction ratefile and species set. The code writes the conservation equations (X) and the odes (YDOT) and must have the subroutine and variable declarations and an END statement added. The code, which was written by Dr Lida Nejad, has descriptions of its several modes of operation, and is available as the file deload95.f. In the implementation available here, a conservation equation is written only for electrons; all other species have an ode written. Because of this, the species file, spec94.data to be input to deload95.f does not contain the species ELECTR, which is declared in deload95.f. The resulting subroutine can then be used with an integrator such as the GEAR package. Each entry in rate95.data is written in the format:

1X,A4,1X,4(1A7,1X),A3,1X,A3,1PE8.2,1X,0PF5.2,1X,F8.1,A9

For each reaction, the rate coefficient can be calculated as:-

for two-body reactions, where is the gas kinetic temperature,

for direct cosmic-ray ionisation (R2 = CRP),

for interstellar photoreactions (R2 = PHOTON), where represents the rate in the unshielded interstellar UV radiation field, and is the parameter used to take into account the extinction of the UV radiation by dust particles, and

for cosmic-ray-induced photoreactions (R2 = CRPHOT), where is the grain albedo in the far UV, typically 0.6 at 150 nm, and is the probability per cosmic-ray ionisation that the appropriate photoreaction occurs.

The photorates have been determined using the Draine (1978) UV radiation field, while the CRP and CRPHOT reactions are given for a standard interstellar ionisation rate of 1.3 . These rates can be scaled easily for other choices of the fields.

'Note' is a nine-column entry which gives information on the type and source of the data and has the form:-

• Column 1: An 'M' here means that the rate coefficient has been measured in the laboratory or, in the case of radiative association, has been deduced with the aid of an experiment on the analogous three-body association. For ion-neutral reactions, the rate coefficients are generally independent of temperature. An important exception are those reactions in which the neutral molecule has a large, say greater than 1 Debye, permanent electric dipole moment. In such cases, the rate coefficients generally increase at low temperatures (Adams et al. 1985). The file dipole94.data gives electric dipole moments, where available, for the neutral species in the database.

  Neutral-neutral reactions are ususally studied experimentally at room temperature and above and therefore application of laboratory-determined rate coefficients to the low temperature environments of interstellar clouds is fraught with uncertainty. For example, it is possible that several reactions listed in the ratefile as not possessing an activation energy do, in fact, have small barriers (< 100 K) which are not evident in measurements done at room temperature and above. In addition, some experimental data are best characterised, in the temperature range over which they are studied, by a negative activation energy barrier. In order to prevent a serious overestimate of these rate coefficients at 10 K, we have generally preferred to adopt an alternative, although still accurate, form for the rate coefficients.

• Columns 2,3: A two-letter symbol for the type of reaction. The symbols used are: AD, associative detachment; CD, collisional dissociation; CH, chemi-ionisation; CI, carbon insertion, involving a reaction between a carbon atom and a hydrocarbon ion; CP, cosmic-ray-induced photoreaction; CO, positive ion-neutral reaction in which H and/or H are the only neutral products; CR, cosmic-ray ionisation; CT, charge transfer; DR, dissociative recombination with electrons; HA, hydrogen abstraction; MN, mutual neutralisation; NA, neutral radiative association; NE, neutral exchange; NI, negative ion-neutral; PD, photodissociation; PI, photoionisation; PM, photodetachment of electron; PN, positive ion-neutral; PT, proton transfer; RA, radiative association between a positive ion and a neutral molecule; RM, radiative electron (minus ion) attachment; RR, radiative recombination with electrons.

• Column 4: A label is used to denote the relevance of a reaction. In many instances the user will be interested in using only a subset of the reaction file rate95.data, and may want, for ease of computation and analysis, to neglect unimportant reactions. Certain types of reaction may be excluded from a particular model. For example, reactions with large activation energy barriers, which are important in shocked gas, and photoreactions can be neglected in models of cold, dark clouds. In addition, however, there are reactions whose neglect will not seriously compromise the results of most models. These are labelled with an 'E' in this column. This category is most used for ions, such as NH and HO, which have rapid reactions with H. The low abundance of all other species X relative to H in many astronomical regions, means that all ion-X reactions can be excluded. This criterion is only useful if H is the dominant form of hydrogen. If a reaction should be included in any comprehensive model it is labelled with an 'A'.

• Column 5: A digit is used to represent the accuracy of the data. We have used such labelling mainly for the ion-neutral and photoreactions, although sometimes the accuracy of the formula given may be restricted to a particular range of temperature. The following scheme has been used:
  • 1. Error < 25 %
  • 2. Error < 50 %
  • 3. Error to within a factor of 2
  • 4. Error within an order of magnitude
  • 5. Highly uncertain
  We have not attempted to give the accuracy of the unmeasured reactions. The ion-neutral rate coefficients should be accurate to a factor of two, but it is possible that some of the reactions included here have different products or are unreactive.

• Columns 6-9: A label to the source of the data. Labels of the form '8010' refer to the labelling scheme of Anich and Huntress (1986). An explanation of the letter codes, such as RJ91, are listed at the end of the rate file, rate95.data and full references are given by, most recently, Millar, Farquhar and Willacy (1997). The label 'NIST', used for many of the neutral-neutral reactions, refers to the National Institute of Standards and Technology Chemical Kinetics Database - Ver. 6.0 (Mallard et al. 1994).

The ratefile is laid out as follows:-

• Reactions 1 - 394. Neutral-neutral reactions.
• Reactions 395 - 3197. Ion-neutral reactions, including reactions of positive and negative ions.
• Reactions 3198 - 3634. Electron reactions, ncluding radiative electron attachment of H, C, O and S, as well as radiative and dissociative recombination of positive ions.
• Reactions 3635 - 3645. Ionisation by cosmic-ray protons.
• Reactions 3646 - 3795. Photoreactions driven by the interstellar UV radiation field.
• Reactions 3796 - 3864. Cosmic-ray-induced photoreactions.

Several of the reactions contained in rate95.data may be endoergic. H-H Lee (Ohio State University) has kindly made available his data on the heats of formation of astrophysically interesting molecules and this file is accessible in heats94.data. We have used this file to delete several highly endoergic ion-neutral reactions present in previous releases of the reaction file.

3. The species file

The dipole moments of the 137 neutral molecules in the species set are given, where available, in the file dipole94.data. The rate file discriminates between some isomers, such as HCN and HNC, as well as CHOH and CHOCH, and the dipole moments are listed for each isomer. In those cases where the chemistry in the ratefile does not differentiate, for example the various forms of linear and cyclic CH, H2CCC and cyclic CH, and CH (HCCCCH) and HCCCC, the dipole moments of each form are listed in dipole94.data.

4. Caveats

• The radiative association rate coefficients are valid only for the 10-50 K temperature range, in general. At higher temperatures different values, and different temperature dependencies, can apply.
• The rate coefficients of the collisional dissociation (CD) reactions are dependent both on density and temperature and reference should be made to the original papers, (Roberge and Dalgarno 1982; Dove and Mandy 1986), for details.
• Some of the more important ion-neutral reactions, such as the N - H and CH - H systems, have rate coefficients which are not easily parameterised in the form given here. Such reactions are discussed in detail by Millar et al. (1991).
• The dissociative recombination of H with electrons contains to be a subject of great debate and its rate coefficient is expected to remain time-dependent for the foreseeable future, although hopefully with less strong a dependence as in the past.
• Photoprocesses are dependent on the detailed radiative transfer of UV photons which depends, in turn, on the poorly known properties of the dust grains. Thus, both the pre-exponential () and exponential () factors can be different for different dust properties. In some cases a bi-exponential formula is to be preferred. Van Dishoeck (1988) discusses these points in detail.

5. Updates

6. Contact point

Professor T J Millar
Department of Physics
UMIST
P O Box 88
Manchester M60 1QD
England
Tel.: +44-(0)161-200-3677
FAX.: +44-(0)161-200-4303
E-mail: Tom.Millar@umist.ac.uk

from whom copies of the paper by Millar, Farquhar and Willacy (1997) can also be requested.

Authors of all publications arising from this data should acknowledge the primary sources, in this case Millar et al. (1991) and Millar, Farquhar and Willacy (1997).

7. References

T J Millar, P R A Farquhar and K Willacy

14th May 1996