The UMIST Database for Astrochemistry 1995

1. Introduction

Chemical modelling of interstellar clouds, shocked gas and circumstellar
regions require reaction rate coefficients of up to several thousand
reactions over the temperature range 10 - 2000 K, or so. The latest version
of the UMIST database, RATE95, provides the rate coefficients of 3864
gas-phase reactions among 395 species involving the 12 elements H, He, C, N,
O, Na, Mg, Si, P, S, Cl and Fe. With the exception of the elements Na, Mg
and Fe, which are included in the ratefile to produce a source of ionisation
in dark clouds, the chemistries of the other elements are fairly
comprehensive. Neutral molecules containing up to 12 atoms (CH_3_C_7_N) 
are included.

2. The rate coefficient file

The file rate95.data contains the reactions and rate coefficients. Each
entry in the file has the form:

I, R1, R2, P1, P2, P3, P4, {alpha} {beta} {gamma} 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:-

k={alpha}(T/300)^{beta}^exp(-{gamma}/T) cm^3^s^-1^

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

k = {alpha}s^-1^

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

k = {alpha}exp(-{gamma}A_V_)s^-1^

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

k = {alpha}{gamma}(1-{omega})s^-1^

for cosmic-ray-induced photoreactions (R2 = CRPHOT), where {omega} is the
grain albedo in the far UV, typically 0.6 at 150 nm, and {gamma} 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 1.3x10^-17^s^-1^. 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_2_
     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 H_2_O^+^, which have rapid
     reactions with H_2_. The low abundance of all other species X
     relative to H_2_ in many astronomical regions, means that all ion-X
     reactions can be excluded. This criterion is only useful if H_2_ 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:
        o 1. Error < 25 %
        o 2. Error < 50 %
        o 3. Error to within a factor of 2
        o 4. Error within an order of magnitude
        o 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

A separate file, species94.data, contains 394 chemical species - note that
it excludes electrons - in the reaction set. For brevity, some species are
written in 'shorthand' - C5H4 represents CH_3_C_4_H. The notation
used is given in Millar et al. (1991), Farquhar and Millar (1993) and
Millar, Farquhar and Willacy (1997).

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
C_2_H_5_OH and CH_3_OCH_3_ 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 
C_3_H, H2CCC and cyclic C_3_H_2_ and C_4_H_2_ (HCCCCH)
and H_2_CCCC, the dipole moments of each form are listed in
dipole94.data.

4. Caveats

The file rate95.data contains data on several thousand reactions. The data
are presented in a very simplified format and the user should be aware that
in some cases the parameters fit the data only under special conditions.
Some of these are:-

   * 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_2_ and C_2_H_2_^+^ - H_2_ 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_3_^+^ 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 ({alpha} and exponential 
     ({gamma} 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

Major updates to the data occur on roughly a two-year period. The next major
revision should be released in early 1997.

6. Contact point

Questions, comments, corrections and suggestions for improvement and
inclusion to the database should be directed to:

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

Adams, NG, Smith, D & Clary, DC, 1985, ApJ, 296, L31
Anicich, VG & Huntress, WT, Jr., 1986, ApJSS, 62, 553
Dove, JE & Mandy, ME, 1986, ApJ, 311, L93
Mallard, WG et al., 1994, NIST Chemical Kinetic Database - Ver 6.0
Millar, TJ, Rawlings, JMC, Bennett, A, Brown, PD & Charnley, SB, 1991,
A&ampAS, 87, 585
Millar, TJ, Farquhar, PRA & Willacy, K, 1997, A&ampAS, 121, 139
Roberge, WG & Dalgarno, A, 1982, ApJ, 255, 176
van Dishoeck, EF, 1988, in Rate Coefficients in Astrochemistry, (eds.) TJ
Millar & DA Williams (Kluwer: Dordrecht), p. 49

T J Millar, P R A Farquhar and K Willacy

14th May 1996

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Tom Millar, Tom.Millar@umist.ac.uk
Fri 14th May 1996