Dust is believed to play a significant role in the evolution of interstellar clouds and hence in processes such as star formation. The physics involved is similar to that responsible for terrestrial aerosols. Certain chemical reactions in interstellar conditions may only occur on the surface of a host particle and are not viable purely in the gas phase. The traditionally used rate equations approach to describe these reactions fail to account for the statistical fluctuations in the reactant populations, which would be significant in situations where the mean population may be well below unity. This can easily occur in interstellar conditions and quite often in reactions catalysed by terrestrial aerosols. This thesis considers a master equation approach that provides a stochastic description of heterogeneous chemical kinetics and demonstrates that classical kinetics may have been overestimating the reaction rates by one order of magnitude under interstellar conditions. The same idea can be extended to study mantle growth on dust surfaces. Traditionally, this is described using a classical description of nucleation kinetics, generally suitable for large systems. Again, this can be unreliable for heterogeneous nucleation taking place on small particles under low vapour concentration where the mean population of adsorbed nucleating species could be of order unity or less. The the-sis explores a stochastic description of heterogeneous nucleation kinetics and solves the arising equations numerically to demonstrate that the stochastic nucleation rate could be significantly different from that derived using the traditional approach. The chemical composition of interstellar dust has for long puzzled experts. The key to determining this lies in an accurate description of the physical processes underlying the formation of these particles. Magnesium oxide is considered to be one of the major candidates as the primary nucleating material, but recently doubts have been cast over this. However, the models employed in reaching that conclusion seem to be rather inaccurate. The thesis attempts to calculate free energies of molecular clusters using newly designed potential models for MgO. It is found that MgO is probably not the primary nucleating dust species in stellar winds.

In this thesis, the results of a series of experimental studies of electron induced anion formation are presented for a number of molecular targets. The motivation of this work was the discovery of anions in interstellar space and in planetary atmospheres raising the question of how such species are formed. Molecular anions are commonly formed by electron impact via the Dissociative Electron Attachment (DEA) and Dipolar Dissociation (DD) with the simultaneous formation of neutral often reactive species (in case of DEA) and cationic fragments (in case of DD). These fragments may then undergo reactions leading to the chemical synthesis of larger stable products. Thus electron induced chemistry may provide a hitherto unexplored route of molecular synthesis and aid in our understanding of the role of anions in interstellar and atmospheric chemistry. .

In the last few decades, astronomers have found that interstellar clouds are chemical factories, in which atoms are processed into more complex chemical species in reactions energized by starlight and fast particles. Many of the identified molecules are formed in networks of ion-molecule and neutral-neutral reactions and, at the low temperatures in the interstellar medium (-15 K), can accrete on the surface of interstellar dust grains to form ices. However, it is widely speculated that some of these interstellar ices can only form sufficiently rapidly if the more abundant atoms, and carbon monoxide, are hydrogenated on the surface of dust grains. Hence, there is an urgent need for data concerning gas-grain interactions, especially with regard to whether addition reactions can take place on dust grains. This thesis presents detailed information on the adsorption and desorption of water, methanol and ammonia on suitable dust grain analogue surfaces, using a combination of temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). All three adsorbates show evidence for molecular adsorption in a physisorbed state. In addition, various degrees of hydrogen bonding are observed in the multilayer. Crystallisation is also observed to take place during the desorption of the ices in both RAIRS and TPD studies. Detailed analysis of the TPD spectra has been performed for all adsorption systems to give desorption energies, desorption orders and pre-exponential factors. The information obtained from these experiments has been incorporated into simple simulations under astrochemically relevant conditions (i.e. low heating rates and appropriate ice thicknesses). These simulations can be directly incorporated into astronomical models. This in turn helps to lead to a greater understanding of star formation, and hence of the Universe in which we live.

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A parameter based study has been carried out in order to investigate advection in photodissociation regions (PDRs) by using plane-parallel, semi-infinite assumption. The PDR material has been assumed to be advected from the molecular cloud towards the cloud surface with an initial flow velocity of 1 km s(^-1). The flow velocity, number density and mass density of the gas have been computed self-consistently as a function of visual extinction A(_u). The total cloud size has been assumed to be A(_u) = 10 mag. The models have been constructed for the radiation and density parameters, x = 10, 10(^3), 10(^5) with respect to Draine field and n(_H) = 10(^4), 10(^5), 10(^6) cm(^-3) respectively. In addition to 9 models characterized by these parameters, another model with X = 10(^2) and n (_H) = 10(^4) cm (^-3) has been studied in order to compare the results to the previously obtained by Bergin et al. (2003). The rotational line intensities of 0-H(_2)O and p-H(_2)O have been computed. The effects of advection in PDR structure, abundance profiles and line intensities of H(_2)O have been investigated. The lower density, lower radiation models among the models studied have been found to be effected by advection, significantly. H(_2)O abundance profile has been found to be effected by advection even in the models in which the PDR structure and the location of H/H(_2) transition zone are the least effected compared to the other models. The intensity of o-H(_2) O have been found to be sensitive to the order of magnitude of the flow velocity. It is concluded that the comparison of these results to the data from future observations with Herschel HIFI can provide information about the characteristics of the flow in nonequilibrium PDRs. This thesis includes observational work of depletion of NO in pre-protostellar cores, L1544 and L183. NO has been found to show depletion characteristics intermediate between the C-containing and N-containing species.

This thesis contains the results from a study of the optical emission lines of various samples of active galactic nuclei (AGN). We have used three separate techniques to determine the precise kinematics and the relative location of the emission line regions of AGN: a detailed study of a small sample (10) of AGN, a study of the general emission line properties of two larger samples, and the interpretation of the results from observations using numeric photoionisation models. Although we present results on both the broad line and narrow line regions of AGN the main drive behind this work is the study of the poorly defined intermediate line region using emission from highly ionised species (high ionisation emission lines). Our results derived from observations indicate that a significant proportion of the emission from these species is produced within the central void of the putative dusty torus and that emission from more highly ionised species increases at radii closer to the central engine of the AGN. Furthermore, we find strong evidence that the intermediate region traced by these high ionisation emission lines is generally outflowing from the central engine but there is little evidence of any significant acceleration or deceleration of this material between the inner and outer regions traced by the higher and lower ionisation lines, respectively. Our results derived from numerical photoionisation codes support a model in which this outflowing material responsible for the high ionisation line emission is launched from the inner edge of the the dusty torus.

The primary idea behind the work in this thesis is that accretion discs interacting with astrophysical bodies, from planets to supermassive black holes (SMBHs), can strongly affect the dynamical behaviour of those bodies. While this idea is by no means new, observational and theoretical developments in recent years provide fresh motivation to consider this effect across a number of astrophysical contexts. Of particular relevance to the work here are three observational measurements which I attempt to reconcile with theory by invoking interactions with accretion discs. Firstly, observations of giant exoplanets show that they prefer to inhabit eccentric orbits, which is unexpected given the predictions of planet formation theory. Conversely, Kepler’s discovery of planets with low eccentricities around moderately eccentric binaries goes against theoretical expectation that their orbits should be eccentric. In galactic centres, binary supermassive black holes are not observed despite theoretical expectations that their evolution should drive them to ~ parsec separations and leave them there. In this thesis I use high-resolution smoothed particle hydrodynamics simulations to investigate each of these problems involving planet- and binary-disc interactions. I show that these interactions are unable to solve the problem of eccentric giant exoplanets, but that they can cause damping of circumbinary planetary eccentricity and so are able to explain Kepler’s circumbinary planets. I use this latter to place a limit on the surface density in which Kepler-16b in particular can have formed. I also show that a disc formed from a gas cloud moving prograde with respect to a SMBH binary will fragment to form stars sooner than a similar retrograde disc. Consequently, a retrograde disc is able to drive stronger binary evolution than is the prograde disc. Allowing that a large number of such encounters would be expected in the aftermath of the galaxy merger that formed the binary, this process may be able to solve the ‘last parsec problem.’