The subject matter of this Ph.D. thesis is the production and application of the Barber-Tennyson line list, BT2, a computed list of H2160 transition frequencies and intensities. BT2 was produced using a discrete variable representation two-step approach for solving the rotation-vibration nuclear motions (Tennyson et al., 2004, Comp. Phys. Comm., 163, 85). It is the most complete water line list in existence, comprising over 500 million transitions ( 65% more than any other list) and it is also the most accurate (over 90% of all known experimental energy levels are within 0.3 cm-1 of the BT2 values). Its accuracy has been confirmed by extensive testing against astronomical and laboratory data. The line list has already found application in a wide range of astrophysical en vironments. It has been used to identify individual water lines in: the spectra of comets, sunspots, cool stars, brown dwarfs and the enigmatic object V838 Mon, and in a number of cases physical parameters have been derived from the intensities of the lines. The line list has also been used to model the atmospheres of cool stars and brown dwarfs and the chemistry of the circumstellar molecular shell ejected by V838 Mon. In addition to our own work with BT2, we have supplied various data, de rived from BT2, to other astrophysical research groups including: N. Dello Russo and co-workers for cometary applications J.M.C. Rawlings for modelling the time- dependent chemistry of the shell ejected by V838 Mon and P. Hauschildt's stellar atmospheric modelling group, which has included the complete BT2 line list in its 'PHOENIX' database. Practical applications of BT2 are not confined to the field of astronomy. The line list is now the preferred reference tool for many groups engaged in the identification of water lines in high temperature laboratory torch spectra, and we have been pleased to supply them with appropriate BT2 spectral data. The final chapter of the thesis looks to the future, which includes further appli cations of BT2 and the production of an NH3 line list.

The observational analysis of astrophysical plasmas relies on accurate calculations of the atomic processes involved. The recombination spectra of singly ionised oxygen (O il) and carbon (C il) present excellent tools for investigating regions such as planetary nebulae and H II regions. In this thesis, detailed treatments of the recombination processes of both O II and C II are presented. Using the R-matrix solution to the close coupling equations, I present the results of accurate photoionisation calculations. Bound state energy levels are determined and oscillator strengths calculated for both species. Recombination coefficients were evalu ated for low n and 1, for C II in LS-coupling, and 0 II in intermediate coupling, taking particular care to treat resonances effectively. Sample photoionisation cross-sections are presented for both species, and compared to previous work. A complete radiative-cascade model is treated for both species, in order to determine line emissivities under nebular conditions at a wide range of temperatures and densities. Collisional effects are treated for C II, along with, for the first time, the effects of high temperature dielectronic recombination, allowing the modelling of regions of much higher electron temperature than previous work. The O II calculations were performed under intermediate coupling for the first time, allowing the effects of non-statistical popula tions of the parent ion fine-structure levels and dielectronic recombination onto bound states within this fine-structure to be taken into account in line emissivities. Detailed comparison with previous theoretical work was made for both species. The application of the C II and 0 n recombination spectra to determining tempera ture and densities from the observed spectra of a number of ionised nebulae is considered. The potential for using the new recombination spectra as diagnostic tools to solve some of the key problems in the study of ionised nebulae is demonstrated.

This thesis is concerned with the spectroscopy of hot, hydrogen-rich white dwarfs, and the development of instrumentation with which to further their study. It begins with a review of white dwarf research, and a summary of results from some notable Extreme-Ultraviolet (EUV) missions. The absence of helium in hot hydrogen-rich white dwarfs is introduced, and the need to precisely determine their composition is explained. These issues provide motivation for the Joint Astrophysical Plasmadynamic Experiment (J-PEX). Using normal incidence, multilayer coated optics and a high resolution focal plane detector, this spectrometer offers a substantial improvement in effective area and spectral resolution over current instrumentation. The design and development of a microchannel plate (MCP) detector for J-PEX is discussed, and a study of MCP sensitivity enhancement processes presented. A CsI photocathode is found to offer superior performance in the 225 - 245 Å band covered by J-PEX. The low quantum efficiencies recently measured for MCPs are also discussed. A preliminary analysis of data from the first successful flight of J-PEX, on board a sounding rocket, is described. Techniques are devised to overcome uncertainties in wavelength calibration, leading to production of the highest resolution EUV spectrum currently available for an astronomical object. These data reveal the presence of helium along the line of sight to the white dwarf G191-B2B, and prove the value of the J-PEX design. Data from the Hubble Space Telescope and the International Ultraviolet Explorer are used to search for white dwarfs with highly ionised, non-photospheric absorption features. Three new identifications are made, in the white dwarfs REJ 1738+665, REJ 0558+165 and WD 2218+706. Possible explanations, including absorption by planetary nebulæ, are suggested. The spectra are of longer wavelength, but higher resolution than currently obtainable by J-PEX, and complement the latter instrument by resolving multiple velocity components along the line-of-sight. The importance of these measurements in the context of J-PEX results, is demonstrated.

Clusters of galaxies are a long-expected source class of diffuse γ-ray emission, but a detection of such is yet to be made. Point-like signals from their dominant mem- bers are observed, and provide fresh insight into the processes involved in feedback from active galaxies, which suppress cooling in cluster cores, and inject non-thermal particles into the intracluster medium. The case for multi-scale γ-ray emission via leptonic and hadronic channels subse- quent to cosmic-ray acceleration, dark matter annihilation, and processes driven by the active nuclei of brightest cluster galaxies (BCGs), motivates the work. A temporal analysis of the source associated with the BCG NGC1275 is carried out; the high-energy (HE) emission (observed with Fermi) is compared with 1.3mm SMA data. No correlation is found on short timescales, however a gradual rising trend is common in both light curves, in agreement with the historical activity of the source, and suggestive of long-term variation in the fuelling of the black hole. Very-high-energy emission recorded with H.E.S.S. over the past decade from the BCG M87 is analysed spectrally, and compared with the HE activity of the source, and with NGC1275. Three samples of candidate HE sources are constructed: 114 galaxy clusters con- taining a radio-bright BCG (motivated by the detection of NGC1275 and M87), 105 hosting diffuse radio synchrotron emission, and the 90 most promising targets for dark matter annihilation signatures. Samplewise, candidate fields output from standard Fermi analyses are stacked. The upper limit on the emission within the stacked target region is at least an order-of-magnitude more constraining than lim- its on individual clusters in each case. A subset of local clusters are investigated as potential extended sources, yielding hints of diffuse emission from Perseus, and several individual fields are found to contain significant emission.

Data from MESSENGER and the GOES solar activity archive are used to deduce properties of Mercury and its X-ray environment in preparation for the forthcoming ESA/JAXA BepiColombo mission to Mercury. Data from MESSENGER’s X-ray spectrometer operations at Mercury exhibit background X-ray enhancements which are identified as the results of astrophysical sources crossing the instrument field of view and of electron bombardment of the X-ray spectrometer causing fluorescence within the instrument. These electron enhancements are analysed and mapped, leading to a three-dimensional representation of low-energy electron populations in the Mercury environment, the distributions of which are associated with features of Mercury’s magnetosphere, including the sunward magnetopause and magnetic equator. The population is characterised, producing estimates of low-energy electron number densities which are consistent with previous observations. Thus, the X-ray spectrometer functions as a proxy detector for electrons with energies below the threshold of dedicated electron detectors. Variations in seasonal X-ray background flux observed by the MESSENGER X-ray spectrometer are investigated, with a modulation visible with a periodicity of one Mercury year. Several telemetry parameters, including temperature, show corresponding variations. The origin of the variation in background flux is identified as the calculation in spacecraft telemetry relating instrument live time to instrument integration time, which varies on the same yearly period. The GOES solar activity archive is used to generate a model of solar X-ray flux levels, with the intention of providing an estimate of the levels of X-ray flux the BepiColombo mission will encounter, and subsequently the volume and rate of data that the MIXS instrument will produce. This model is used to predict the likelihood of the instrument experiencing data loss events by exceeding the data storage available, and how this likelihood varies with the level of solar flux and the onboard storage space that is allocated to the instrument.

The analysis of spectra is at the forefront of astrophysics research since it can provide us with a rich amount of information on the structure of the universe. The spectral modelling packages, which aid this analysis, such as Cloudy, are heavily reliant upon the atomic data available for each ion. In this work, the atomic data, both radiative and collisional, for Sc II, Mn V, S III and Mg VIII have been advanced. At each stage, the accuracy of the data have been carefully assessed via theoretical and observational comparisons. All the data have now been incorporated into Cloudy's new database - Stout, making it widely available to the astrophysics community. For Sc II, collision strengths and A-values were determined using the RMATRXII and CIV3 codes respectively, for an 89 level model. A Cloudy model was created for the Strontium filament of Eta -Carinae and the theoretical line intensities were compared with HST spectra. Two models, consisting of 125 levels, were created for Mg VIII. The models were used to determine the best set of transition probabilities and collision strengths for use in the modelling of the solar corona. The theoretical line intensities were generated and compared with the SERTS-89 and SUMER spectra. An existing S III collision strength model was extended to enable an examination of allowed emission lines from the 10 plasma torus. The theoretical spectrum was produced using Cloudy and was compared with the FUSE and EUVE spectra. Four models, consisting of 80 levels, were created for Mn V. The different models were constructed to enable a comparison ofthe Breit-Pauli, ICFT and DARC R-matrix codes. A new set of A-values was also produced using CIV3. A Cloudy model was created for NGC 7009 and t he identification of 14 [Mn V] emission lines in the spectrum was investigated.

In this thesis the formation of molecular species in cold icy mantles, typical of those in the interstellar medium (ISM) has been studied. The construction of an ultra high vacuum system for the formation and containment of these astrophysical ice analogues is described. The method of preparation of these ices is detailed and analysis methodologies are discussed. VUV spectra of molecular ices (e.g. SO2, CH3NH2, OCS) measured on the UV1 beam line of the Astrid Synchrotron facility at the University of Aarhus in Denmark, are presented. Molecular synthesis (e.g. CO3, CO, H2CO3) induced by ion irradiated CO2/H 2O ices using facilities at Queens University Belfast are also reported. In particular this thesis focuses on the irradiation studies of the binary ice mixture of Methylamine (CH3NH2) and Carbon Dioxide (CO2). The results of irradiation of CH3NH2 and CO2 binary ice mixtures by ultraviolet photons, and electrons are presented. The apparent production of the amino acid - glycine - in our irradiated samples is discussed together with possible production processes.

Scientific curiosity and thirst for knowledge have driven human progress and helped mankind develop an always deeper understanding of our world. New - and sometimes controversial - ideas have been proposed in order to explain its mysteries. Looking for alternative perspectives has proven essential, particularly when the current paradigms do not give complete satisfaction. Such is the case for three important questions in modern astrophysics and cosmology, which we intend to investigate: the Paradox of Youth, Dark Matter and Dark Energy. We will first explore a new possible explanation to the paradoxical observation of young, massive stars near the centre of our galaxy. Hosting a solar mass black hole might allow them to spare some of their burning material, giving them enough time to travel from further galactic distances. In the second part of our thesis, we will study Dark Matter accretion onto neutron stars. Considering different types of Dark Matter particles, we will determine the consequences of their capture by these very dense objects and look for observable signatures. Finally, our current views on Dark Energy will be examined through its role in void models. Among them, the Swiss-Cheese Universe and central void models will be analysed in order to evaluate the impact of inhomogeneities on the determination of cosmological parameters. All three parts are similarly structured: conceptual introduction, analytical and computational considerations, commented results and conclusive remarks. Bibliographies are given separatly after each part. Lists of figures, tables and abbreviations can be found at the end of the document.

The interstellar medium (ISM) is the region of space between the stars, where star and planet formation occurs. Molecular hydrogen in the ISM initiates all the chemistry that occurs in these regions and without it, no stars or planets could form. The molecules formed by the chemistry initiated by H2 provide cooling mechanisms for the huge molecular clouds which collapse to form stars. It is well known that the abundance of molecular hydrogen in the ISM is too high for it to form through gas phase processes alone and hence, the widely accepted theory is that H2 forms through heterogeneous catalysis on the surfaces of interstellar dust grains. These grains make up approximately 1% of the mass of the ISM and are thought to be carbonaceous or silicate in nature. Despite its importance, only recently have laboratory experiments been set-up to study the formation of molecular hydrogen on interstellar grain analogues in detail. This thesis presents results from an experiment designed to determine what happens to the energy released on the formation of H2, under conditions similar to those of the ISM. The experiment involves using an atom source, ultrahigh vacuum chamber and cryogenic cooling methods in order to reach the temperatures and pressures of interstellar space. The laser technique of Resonance Enhanced Multiphoton Ionisation (REMPI) is employed to look at the internal energy distribution of newly formed H2 and HD molecules from a graphite surface. Improvements to an existing experiment are described, including the introduction of a second atom source to study HD formation. New results are presented, including the first observations of molecular hydrogen formed ro-vibrationally excited in states v" = 1 and v" = 2. The results are placed in context with the results of other experiments to form H2 under ISM conditions, and the astrophysical implications are discussed.

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