D

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Fig. 43. Methanol lines observed in comet Hyakutake [28].

4.5 OH Maser Emission

The excitation of OH radio lines is the only case of stimulated emission in the cometary coma. The 18-cm lines arise from excitation of the ^-doublet in the molecular ground state by solar UV radiation, building up an inversion population. Because transitions between the ^-doublets are inhibited, the resulting maser transitions are very sensitive to quenching by collisions of OH with neutrals and ions in the coma. The radius up to which quenching needs to be considered in calculations of the OH excitation depends on the activity of a comet. It is around 105 km for comets with production rates near 1029 s_1. Proper treatment of collisional quenching is difficult because colli-sional cross-sections and ion densities are often not sufficiently known in the coma. The quenching of the OH population is therefore usually approximated by appropriate scaling laws, unless it can be measured by spatial mapping the OH distribution in the coma [44,90,91,190].

4.6 X-ray Emission

The first observations of a comet with an X-ray telescope lead to the discovery of X-ray radiation of comets [144]. Since then, many model attempts and further observations of several comets have been made (e.g., [6, 96,135,136, 142]) to explain the cause of the emission, and it has become a new field of cometary science. See [143] for an overview.

5 Chemical Processes in the Coma

The composition of the cometary coma changes with increasing nucleocentric distance because of a number of chemical reactions. Reactions occur between neutral gas molecules, neutral and ionic species, with the solar wind particles, and the solar radiation field (Fig. 14). Table 2 provides an overview of the different types of reactions considered in chemical models of cometary comae and gives an example for each reaction type. More detailed discussions also including reactions of minor importance can be found in [194], [110], and [184].

To model the composition of the coma, a set of equations including the relevant chemical reactions has to be solved. The density of a given species, ni,

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Fig. 43. Methanol lines observed in comet Hyakutake [28].

Table 2. Types of chemical reactions in cometary comae

Reaction type

Example

Photodissociation Photoionisation Photodissociative ionisation Electron impact dissociation Electron impact ionisation Dissociative electron recombination C2H+ + e ^ C2 + H

H2O + hv ^ OH + H H2O + hv ^ H2O+ CO2 ^ O + CO+ + e C2H2 + e ^ C2 + H2 + e

Charge exchange Neutral-neutral reactions 3-Body reactions

varies by the sum of its chemical formation and loss processes. In the simple case of a reaction of two species, A + B ^ C + D, the change of their number densities by this reaction is:

dnA dnB dnc dnu

Here, k is the reaction rate. In general form, for a total number of s species in a reaction network, this can be written as (see also [194]):

Here, q is the number of chemical reactions and Vj is the stoichiometric coefficient of species i in reaction j, which is positive for products and negative for reactants. The reaction order mj is equal to | Vj | if negative and zero otherwise.

The rate coefficients for collisional reactions, kj, are usually expressed by the Arrhenius-law:

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