UMIST Rate file

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 (CHCN) are included.

2. The rate coefficient file

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

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, 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 is written in the format:


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:-

The ratefile is laid out as follows:-

Several of the reactions contained in 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 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,, contains 394 chemical species - note that it excludes electrons - in the reaction set. For brevity, some species are written in 'shorthand' - C5H4 represents CHCH. 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 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

4. Caveats

The file 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:-

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
P O Box 88
Manchester M60 1QD
Tel.: +44-(0)161-200-3677
FAX.: +44-(0)161-200-4303

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&AS;, 87, 585
Millar, TJ, Farquhar, PRA & Willacy, K, 1997, A&AS;, 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

Tom Millar,
Fri 14th May 1996