The work presented here looks at several aspects of searching for continuous gravitational waves from pulsars, often referred to simply as continuous waves or CWs. This begins with an examination of noise in the current generation of laser interferometer gravitational wave detectors in the region below ~100 Hz. This frequency region is of particular interest with regards to CW detection as two prime sources for a first CW detection, the Crab and Vela pulsars, are expected to emit CWs in this frequency range. The Crab pulsar's frequency lies very close to a strong noise line due to the 60 Hz mains electricity in the LIGO detectors. The types of noise generally present in this region are discussed. Also presented are investigations into the noise features present in the LIGO S6 data and the Virgo VSR2 data using a program called Fscan. A particular noise feature present during VSR2 was discovered with the use of Fscan, which I report on and show how it degrades the sensitivity of searches for CWs from the Vela pulsar using this data. I next present search results for CWs from the Vela pulsar using VSR2 and VSR4 data. Whilst these searches did not find any evidence for gravitational waves being present in the data, they were able to place upper limits on the strength of gravitational wave emission from Vela lower than the upper limit set by the pulsars spin-down, making it only the second pulsar for which this milestone has been achieved. The lowest upper limit derived from these searches confines the spin-down energy lost from Vela due to gravitational waves as just 9% of Vela's total spin-down energy. The data from VSR2 and VSR4 are also examined, analysis of hardware injections in these datasets verify the calibration of the data and the search method. Similar results are also presented for a search for CWs from the Crab pulsar, where data from VSR2, VSR3, VSR4, S5 and S6 are combined to produce an upper limit on the gravitational wave (GW) amplitude lower than has been previously possible, representing 0.5% of the energy lost by the pulsar as seen through its spin-down. The same search method is also applied to analyse data for another 110 known pulsars, with five of these being gamma-ray pulsars that have been timed by the Fermi satellite. GWs from the pulsars timed by Fermi are expected at frequencies below 40 Hz, the LIGO detectors are not calibrated below these frequencies but the Virgo detector is. Hence the data used to search for GWs from these pulsars is the Virgo VSR4 data. The other 105 pulsars were analysed using out of date ephemerides obtained for the LIGO S5 run and the data analysed was from the LIGO S6 run, hence the results obtained for these pulsars are presented as an indication of what results can be expected with updated ephemerides only. For these 110 pulsars the spin-down limits were not able to be beaten, although there are a few pulsars for which this may be able to be achieved with an analysis combining all the possible datasets, in particular J1913+1011. The final part of this thesis reports extensions to the search method used for the analyses previously described. The first way in which this search method is extended is the use of a nested sampling algorithm to perform the parameter estimation stage of the analysis which was previously preformed using a MCMC. The nested sampling code also allows for model selection through the computation of the Bayesian evidence, I present results from characterisation tests of this nested sampling search code that demonstrate the equivalence of its results to those from the MCMC and grid based codes. The other extension to the search method looks at a new CW emission mechanism from a neutron star with a pinned superfluid core that is misaligned from the star's principle axes. This emission model predicts CWs at both the stars spin frequency f and twice its spin frequency 2f, providing an extra data channel with which to perform a search when compared to the triaxial rotator model which only emits at 2f. I present the development of a search for the emission from this new model, tests of the algorithm developed using simulated data, and results from a search using actual data from the VSR4 run for CWs from the Crab pulsar. The testing of the search algorithm shows that the posterior for the model is sufficiently complex to inhibit useful parameter estimation, but that the computation of the Bayesian evidence allows one to distinguish between this model and the triaxial rotator given a low SNR signal in the f data channel.

This thesis concerns the analysis of continuous gravitational waves from neutron stars with non-axisymmetric rotational motion using data from ground-based interferometric gravitational wave detectors, the development of a computationally efficient algorithm for analysis of this data and the use of this algorithm in follow up searches, which were previously too computationally expensive to consider.

The process of converting gas into stars underpins much of astrophysics, yet many fundamental questions surrounding this process remain unanswered. For example -- how sensitive is star formation to the local environmental conditions? How do massive and dense stellar clusters form, and how does this crowded environment influence the stars that form within it? How do the most massive stars form and is there an upper limit to the stellar initial mass function (IMF)? Answering questions such as these is crucial if we are to construct an end-to-end model of how stars form across the full range of conditions found throughout the Universe. The research described in this thesis presents a study that utilises a multi-scale approach to identifying and characterising the early precursors to young massive clusters and high-mass proto-stars, with a specific focus on the extreme environment in the inner few hundred parsecs of the Milky Way -- the Central Molecular Zone (CMZ). The primary sources of interest that are studied in detail belong to the Galactic centre dust ridge -- a group of six high-mass (M ~ 10^(4-5) Msun), dense (R ~ 1-3 pc, n > 10^(4) cm^(-3)), and quiescent molecular clouds. These properties make these clouds ideal candidates for representing the earliest stages of high-mass star and cluster formation. The research presented makes use of single-dish and interferometric far-infrared and (sub-)millimetre observations to study their global and small-scale properties. A comparison of the known young massive clusters (YMCs) and their likely progenitors (the dust ridge clouds) in the CMZ shows that the stellar content of YMCs is much more dense and centrally concentrated than the gas in the clouds. If these clouds are truly precursors to massive clusters, the resultant stellar population would have to undergo significant dynamical evolution to reach central densities that are typical of YMCs. This suggests that YMCs in the CMZ are unlikely to form monolithically. Extending this study to include YMCs in the Galactic disc again shows that the known population of YMC precursor clouds throughout the Galaxy are not sufficiently dense or central concentrated that they could form a cluster that then expands due to gas expulsion. The data also reveal an evolutionary trend, in which clouds contract and accrete gas towards their central regions along with concurrent star formation. This is argued to favour a conveyor-belt mode of YMC formation and is again not consistent with a monolithic formation event. High angular resolution observations of the dust ridge clouds with the Submillimeter Array are presented. They reveal an embedded population of compact and massive cores, ranging from ~ 50 - 2150 Msun within radii of ~ 0.1 - 0.25 pc. These are likely formation sites of high-mass stars and clusters, and are strong candidates for representing the initial conditions of extremely massive stars. Two of these cores are found to be young, high-mass proto-stars, while the remaining 13 are quiescent. Comparing these cores with high-mass proto-stars in the Galactic disc, along with models in which star formation is regulated by turbulence, shows that these cores are consistent with the idea that the critical density threshold for star formation is greater in the turbulent environment at the Galactic centre.

The aim of this thesis is to study stellar evolution and asteroseimology of red-giant stars mainly from a modelling point of view, in particular the impact on core-convective-burning stars of adopting different mixing schemes. Thanks to NASA space telescope Kepler, asteroseismology of thousands of giants provided us new information related to their internal structure, that can be used for finding constraints on their cores. I used several stellar evolution codes (MESA, BaSTI, and PARSEC) to investigate the effect of different mixing schemes in the helium-core-burning stars. Comparing them with observed stars, I concluded that standard stellar models, largely used in literature, cannot describe the combined observed distribution of luminosity and period spacing. I then proposed as solution a penetrative convection model with moderate overshooting parameter. Additional tests on Kepler's open clusters (NGC6791 and NGC6819) and secondary clump stars, allowed me to revised to my mixing model.

First, we investigated the ν max scaling relation, a widely-used equation that states that the frequency of maximum amplitude in a power spectrum scales with a combination of surface gravity and effective temperature. We tested how well the oscillations of cool main-sequence and sub-giant stars follow this relation, using a ensemble of asteroseismic targets observed by Kepler. We then tested seismic scaling relations in a small group of 10 bright red-giant stars observed by Kepler. These giants, some of the brightest observed in the Kepler field, have precise values of parallaxes. We compared the measured distances with inferences made using asteroseismic parameters. We also combined high-quality spectroscopic data with seismic constraints to determine their evolutionary phase. We compared the observed surface abundances of lithium and carbon with models that account for additional mixing processes in redgiants. Finally, we analyzed a group of 13 stars observed by Kepler, and use asteroseismic tools to extract modelindependent information about their internal regions. Our objective is to detect the so-called acoustic glitches, characterized as departures from the uniform frequency spacings predicted by the asymptotic relation. Such departures originate in regions where there is an abrupt change in the stratification of the star.

Globular clusters were among the first luminous objects to form in the Universe. They are dense collections of hundreds of thousands of stars. Globular cluster formation is a major unsolved problem in astrophysics. A new constraint on the problem came from the discovery of unexpected star-to-star variations in the abundances of some light elements. These abundance variations (or multiple stellar populations) are ubiquitous to all globular clusters studied to date. The pursuit to explain this longstanding prob- lem using these new constraints (i.e. the abundance variations), has reinvigorated the study of globular clusters, and at the same time has challenged our understanding of nucleosynthesis and stellar evolution. Several scenarios have been put forward to explain the presence of multiple stellar populations in globular clusters, nearly all requiring multiple generations of stars. The basic hypothesis in these models is that a second generation of stars is born during the early life of the globular cluster from the chemically-processed ejecta of some first generation stars in order to account for the signature multiple stellar populations observed in old globular clusters today. Many of these scenarios are mutually exclusive. Therefore, to determine which of them fits the current evidence the best became the priority of globular cluster studies. Modern observational facilities cannot resolve the globular cluster formation process in the early Universe. However, none of the scenarios for the origin of globular cluster and their multiple stellar populations make any distinctions between star/cluster formation at the present day and earlier epochs of the universe. Accordingly, the processes invoked in these scenarios can, in principle, be constrained by studies of the formation of young massive star clusters in the local Universe, which have similar sizes and masses as present-day globular clusters, but are significantly younger. In this work, I present some of the strongest constraints from such studies coming from the gas content of young massive clusters and their star formation histories. These studies showed that: 1) young massive clusters are consistent with a single star formation burst, and 2) there is no significant cool gas reservoirs left within young massive clusters that can fuel future star-formation events. These results are in stark contrast with the predictions of nearly all the scenarios that have been proposed to explain the origin of abundance variations in globular cluster stars, which require that young massive clusters should host multiple star formation events.

High Mass X-ray Binaries (HMXBs) represent an important stage in the evolution of massive stars and are some of the brightest sources in the X-ray sky. In the first half of this thesis a detailed analysis of X-ray observations of two HMXBs, the Be/X-ray Binary (BeXRB) Swift J045106.8-694803 and the supergiant/X-ray Binary XTE J0421+560/CI Camelopardalis, is presented. Simulations of the X-ray spectrum of Swift J045106.8-694803 show that both the spectral and timing properties can be reproduced by a blackbody and power law pulsating ∼ pi out of phase with each other. The pulse profile of the blackbody is used to determine the angle between the rotation and magnetic axes of the neutron star and the angle between the rotation axis and line of sight. The apparently broad iron line of XTE J0421+560 is decomposed into three intrinsically narrow lines, FeI-Ka, FeIK b and FeXXIV-XXVKa. The light curve extracted in the energy range defined as the Fe-Ka line from the spectral fits shows marginal evidence for a lag when cross correlated with that of the continuum. The lag is interpreted as the light crossing time of the circumbinary torus and implies a radius of 10 AU. The second part of this thesis considers HMXBs as a population. I describe the search for XRBs in the Phoenix dwarf galaxy, a Local Group dwarf irregular galaxy which share many similarities with the Small Magellanic Cloud (SMC), which has an apparent overabundance of HMXBs. Finally, I discuss why the BeXRB population in the SMC is ideal for population studies and outline the work done to search for evidence for two different neutron star formation channels in their physical parameters

The application of the discrete Fourier transform to the determination of the frequency content of unevenly-sampled astronomical time series is discussed, and an interactive computer package which incorporates a variety of power-spectrum and time-domain techniques is described. A frequency analysis of the light curves of two hot, extreme helium stars, BD-9°4395 and HD160641, shows that their photometric variability is caused by non-radial pulsation. Spectroscopic evidence in support of non-uniform mass loss is presented for BD-9°4395. Spectroscopic and photometric observations of two early-type eclipsing binary systems, AL Sculptoris and DM Persei, have been analysed to yield their absolute dimensions. AL Scl is found to be a detached system in which both components rotate faster than synchronism. The origin of distortions in its light curve is unclear. DH Per is shown to be part of a triple system in which the third component is most probably a late-B star in a 98-day orbit with a semi-major axis of 0.9 A.U. The binary system is confirmed to be semi-detached and to have evolved through a phase of rapid mass transfer. DH Per joins a small group of massive, semi-detached systems whose characteristics differ significantly from the classical Algols, and which may result from case-A, mass-transfer processes. Spectroscopic and photometric observations of the F4V star HD123058 do not support the hypothesis that it is a binary system. Broad lines in its spectrum are attributed to a somewhat enhanced rotation rate, and the star is shown to be essentially unevolved. The derivation of the equation of condition in Sterne's rigorous method for the analysis of the spectroscopic elements of binary systems, and its modification for incorporating observed times of minimum light into the adjustment of the elements, are outlined. A computer code for the determination of orbital elements according to this scheme is described.

Most stars in the Galaxy are found in multiple systems of two or more stars orbiting together. Two stars orbiting around their centre of mass are called binary stars. In close binary stars, the evolution of one star affects its companion and evolutionary expansion of one star allows for mass exchange between the components. In most cases, the material from the less massive star forms an accretion disc around the heavier companion that has evolved into a compact stellar remnant, the final state of stellar evolution. We call these systems compact binary stars (CBs). The study of CBs is key to the development of two fundamental phenomena: accretion and evolution of binary stars. Statistical information on CBs can be deduced by extracting common properties and characteristic system parameter distributions from observed data. But, despite being fundamental for a wide range of astronomical phenomena, our comprehension of their formation and evolution is still poor, mainly because of the limited knowledge of crucial orbital parameters. This lack of reliable orbital parameters estimation is mainly due to observational handicaps, namely, the accretion disc outshines the system components. Astronomers have developed different techniques to overcome this, but are often very dependant of the signal to noise ratio of the data or are only able to obtain via target of opportunity programs (wait until the target is brighter). The focus of this work is to test and develop techniques, based on indirect imaging methods, that can overcome the main observational handicaps to estimate orbital parameters of CBs. We combine these techniques with the exploitation of more “exotic” emission lines that trace the irradiated face of the donor star, namely Ca II NIR triplet and the Bowen blend. We made use of empirical properties of Doppler tomography to estimate the values of the phase zero Á0 and the velocity of the irradiated face of the secondary star (Kem). We then used synthetic models accounting for an irradiated secondary to fit our measured Kem and perform a K-correction to derive the radial velocity of the secondary K2. To derive K1, we used the centre of symmetry technique, testing its validity among several emission lines and the stability of the results depending on the selected area. Having strong constraints for K1 and K2, we find estimates for the mass ratio q. Furthermore, we developed a variation from the Doppler tomography secondary emission method to constrain the value of the systemic velocity ƴ. We derive meaningful uncertainties of these parameters with the bootstrap technique. Using these techniques, we have successfully set dynamical constraints on the radial velocities of the binary components of CBs and derived fundamental orbital parameters, including the mass ratio, using basic properties of Doppler tomography.

Electron capture supernovae (EC-SNe) are the deaths of approximately 8-10M stars. In this thesis, the evolution of 8-12M stars is calculated using the MESA stellar evolution code. The aims of this thesis are to produce the most advanced and up-to-date progenitor models for electron capture supernovae (EC-SNe) and to study the behaviour of stars across the transition mass range between AGB stars and massive stars. These new stellar models will be the first of their kind since the 1980s and highlight new computational successes and persisting challenges in the field of stellar physics.