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Astron. Astrophys. 354, L6-L8 (2000)

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2. Reaction network

We take the UMIST database (Millar, Farquhar & Willacy 1997; hereafter referred to as MFW97) as our basis of chemical reactants and reactions, but added several new reactions such as synthesis of amino acids (alanine and glycine), hydroxy-acids (glycolic and lactic acids), DNA base (adenine, see, Volkenshtein 1983), urea synthesis etc. These new reactions make the total number of species to be 421. The rate co-efficients of these additional reactions are difficult to find, especially in the environs of a molecular cloud. To use UMIST database, the rate constant for a two body reaction is written as (MFW97),

[EQUATION]

where, [FORMULA], [FORMULA] and [FORMULA] are constants and T is the temperature. Amino acid synthesis rate was estimated from Fig. 8 of Schulte & Shock (1995). Urea synthesis rate is kept comparable to the rates given in UMIST table. The rate constants were taken to be [FORMULA], [FORMULA] for each two-body reactions. The rate constants for adenine synthesis was chosen to be similar to other two body reactions [[FORMULA] [FORMULA] for each HCN addition in the chain [FORMULA] (adenine)]. This should provide a sufficiently good estimate for the final abundance of these pre-biotic molecules.

Initial composition of the cloud before the simulation begins is kept to be the same as in MFW97, and formation of [FORMULA] is included using the grain-surface reaction with rates as in MFW97. The initial mass fractions are taken to be the same as in MFW97 (but converted to mass fractions), i.e., H:He:C:N:O:Na:Mg:Si:P:S:Cl:Fe = 0.64:0.35897:[FORMULA]:[FORMULA]:[FORMULA]:[FORMULA]: [FORMULA]:[FORMULA]:[FORMULA]:[FORMULA]:[FORMULA]:[FORMULA].

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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