3 The reaction set

Table 8 contains all the reactions and associated rate coefficients, and is available online at
http://www.rate99.co.uk
http://www.edpsciences.org

   
3.1 The new entry format

The number of observed and predicted astrophysical species increases steadily with time, and with it, the size of their formula (e.g. ethyl methyl ether, C₂H₅OCH₃, has been discussed as a possible interstellar molecule). To account for this, we have altered the format for the names of the species by increasing by one letter, making them 8-character strings. Also, the smallest products in a four-product reaction are mostly H and/or He, therefore only 4 characters have been allocated for the last two products. The necessity to consider termolecular reactions for high density environments means that a third species must be included on the reactant side of these reactions. To account for that, each reaction now comprises three reactants and four products. Our new reaction format reads:

I, R1, R2, R3, P1, P2, P3, P4, $\alpha$, $\beta$, $\gamma$, flags

where I is the reaction number, R1 to R3 are the reactants, P1 to P4 are the products, and $\alpha$, $\beta$ and $\gamma$ are the constants used to determine the rate coefficients. The series of flags is a string of 16 characters and/or digits that store respectively:

• the kind of data: measured M, estimated E, calculated C or literature search L, with format A1. Here "literature search'' means that the given datum is a compilation of several other data (measured and/or calculated). The sources of these data are mainly Baulch et al. (1992) and the NIST database (Mallard et al. 1998);
• the lowest and highest temperatures defining the temperature range, format 2(I5). Each temperature is given as an integer number of kelvins in the range 10 < T < 41 000 K;
• the error on the rate value, format A1. The following scheme has been used:
  - "A''. Error < 25%
  - "B''. Error < 50%
  - "C''. Error within a factor of 2
  - "D''. Error within an order of magnitude
  - "E''. Highly uncertain;
• the reference code, format A4. The references are listed in Table 4.

The full entry format in Fortran is correspondingly written as:
I4, 5(1X, A8), 2(1X, A4), 1X, 1PE8.2, 3X, 0PF5.2, 2X, 0PF8.1, A1, 2(I5), A1, A4.

   
3.2 Calculation of the rates from $\alpha$, $\beta$ and $\gamma$

For two- or three-body reactions, the rate coefficient is given by:

$$k = \alpha \; (T/300)^{\beta} \; \exp(-\gamma /T) \qquad \mbox{cm$^3$ s$^{-1}$ }$$ (1)

where T is the gas temperature.
For direct cosmic-ray ionisation (R2 = CRP):

$$k = \alpha \qquad \mbox{s$^{-1}$ }$$ (2)

whereas for cosmic-ray-induced photoreactions (R2 = CRPHOT):

$$k = \alpha \; (T/300)^{\beta} \; \gamma /(1-\omega) \qquad \mbox{s$^{-1}$ }$$ (3)

where $\alpha$ is the cosmic-ray ionisation rate, $\gamma$ is the probability per cosmic-ray ionisation that the appropriate photoreaction takes place, and $\omega$ is the dust grain albedo in the far ultraviolet (typically 0.6 at 150 nm). We note that because CO is destroyed by the line absorption, its rate of destruction is sensitive to its rotational level populations. To account for this we have included a temperature-dependence in the calculation of the rate coefficient.

For interstellar photoreactions (R2 = PHOTON), the rate is derived as:

$$k = \alpha \; \exp(-\gamma A_{\rm V}) \qquad \mbox{s$^{-1}$ }$$ (4)

where $\alpha$ represents the rate in the unshielded interstellar ultraviolet radiation field, $A_{\rm V}$ is the extinction at visible wavelengths caused by interstellar dust, and $\gamma$ is the parameter used to take into account the increased extinction of dust at ultraviolet wavelengths.

   
3.3 General form of the reaction set

We have re-organised the order of reactions in this release. The reactions are divided into 14 categories or types, which are grouped together in the database. Table 8 summarizes these categories, along with their position in the database. Within each category, the reactions are listed by increasing total molar mass (total mass of the reactants).

• Photoprocesses: the temperature flags of the photoprocesses, which are just an artifact from the data revision software, are, of course, irrelevant. Also, the unshielded rates shown are valid only for the ISM UV field (from Draine 1978). These photorates can be rescaled to a stellar radiation field or calculated from cross sections when available (see Table 7 and Eq. (5));
• Neutral-neutral reactions: the reactions that were already present in the former UMIST Database have had their temperature dependence reviewed and temperature range defined when known. When not known, an arbitrary range of 10 to 300 K has been attributed, since these reactions were originally defined for these low temperatures;
• Cosmic-Ray reactions: reactions with CRPHOT (cosmic ray photons) and CRP (cosmic ray protons) have been left unchanged;
• Sundries: this section gathers all the reactions that cannot be classified by any of the other types because they are a combination of at least two different types.

3.4 Alterations present in this release

• A major change brought to the database is the inclusion of the temperature dependence and temperature range for all the reactions. The maximum temperature range used in the database has been arbitrarily defined from 10 to 41 000 K. Where explicit information is not available, the rate coefficients have been attributed a range of 10-300 K. However, if their "$\gamma$'' Arrhenius coefficient is too large, the lowest temperature has been defined as $T_{\rm l} = \gamma / 30$ (in K), and the largest temperature $T_{\rm u}$ has been arbitrarily taken as 41 000 K (the value of such rates does not change significantly at higher temperatures), unless the rate's value becomes unphysical, in which case an appropriate upper temperature has been determined. Many Ion-Neutral and Ion-Ion reactions remain constant whatever the temperature, and where this is the case they have been arbitrarily defined from 10 to 41 000 K;
• Two new reaction types have been added, namely Termolecular reactions and Collider reactions, both of which become important at high density, typically above 10¹⁰ cm^-3. Termolecular reactions are catalysed bimolecular reactions and the catalyst is named "M''. The nature of the third body is not important in general because it is only used as a de-excitation energy carrier:
  
  $$\mbox{X + Y + M $\rightarrow$\space XY + M}.$$
  
  
  Collider reactions are collision-induced dissociations and the collider is also named "M'' as its nature does not significantly alter the rate of the process:
  
  $$\mbox{XY + M $\rightarrow$\space X + Y + M};$$
  
  

• Because the reactions were chosen to be appropriate not only for the cold ISM, less discrimination has been operated so novel reactions have been added to all the types. The net result is a richer chemistry with multi-product reactions;

• Tables of cross section data appropriate for photo processes have been gathered and are included in the electronic tables to allow study of chemistry in a variety of radiation fields. Table 7 gives further information on these cross sections.