Multifluid modelling of cometary coma for diverse range of parent volatile compositions

<p>Comets show a general diversity in their parent volatile composition, but in most cases H<sub>2</sub>O is observed to be the dominant volatile in terms of abundance. This is followed by CO and CO<sub>2</sub>, and trace amounts of other species such as CH<sub>4</sub>, CH<sub>3</sub>OH, O<sub>2</sub>, and NH<sub>3</sub> are also present. However, the observed ratio of n_x/H<sub>2</sub>O varies considerably from one comet to another (n_x represents any parent species other than water).</p><p>We aim to study how the chemistry and dynamics of the cometary coma changes for varying abundances of the major parent volatiles. We have constructed a fluid model, using the principles of conservation of mass, momentum and energy, for our study. Parent volatiles sublimating from the nucleus undergo photolytic reactions due to the solar UV radiation field, resulting in the formation of secondary neutral and ionic species and photoelectrons. Active chemistry occurs in the coma, and some of the chemical reactions taking place are ion-neutral rearrangement, charge exchange, dissociative recombination, electron impact dissociation and radiative de-excitation. The energy that is released due to these chemical reactions is non-uniformly distributed amongst all the species, resulting in different temperatures. Hence,  for a complete description of the coma, we have used a multifluid model whereby the neutrals, ions and electrons are considered as three separate fluids. Apart from chemical reactions, we have also considered the exchange of energy between the three fluids due to elastic and inelastic collisions.</p><p>We consider different initial compositions of the comet, and then use our model to generate the temperature and velocity profiles of the coma, for varying cometocentric distances. We also obtain the number density profiles of the different ionic and neutral species that are created in the coma. We see that changes in the initial parent volatile abundance will modify the temperature profile, and there are significant changes in the ionic abundances. Hence, the parent volatile composition of the comet drives the physico-chemical attributes of the coma.</p>

Reactive collision of electrons with CO+ in cometary coma

Context. In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional coma, inelastic collisions between thermal electrons and molecular ions result in recombination and vibrational excitation, the rates of these processes being particularly elevated due to the high charged particle densities in the inner region. Aims. This work addresses the dissociative recombination, vibrational excitation, and vibrational de-excitation of electrons with CO+ molecular cations. The aim of this study is to understand the importance of these reactive collisions in producing carbon and oxygen atoms in cometary activity. Methods. The cross-section calculations were based on multichannel quantum defect theory. The molecular data sets, used here to take into account the nuclear dynamics, were based on ab initio R-matrix approach. Results. The cross-sections for the dissociative recombination, vibrational excitation, and vibrational de-excitation processes, for the six lowest vibrational levels of CO+ – relevant for the electronic temperatures observed in comets – are computed, as well as their corresponding Maxwell rate coefficients. Moreover, final state distributions for different dissociation pathways are presented. Conclusions. Among all reactive collisions taking place between low-energy electrons and CO+, the dissociative recombination is the most important process at electronic temperatures characterizing the comets. We have shown that this process can be a major source of O(3P), O(1D), O(1S), C(3P) and C(1D) produced in the cometary coma at small cometocentric distances.