In this thesis, ages have been derived for 4 young clusters by fitting the pre-main-sequence stars with semi-empirical models in colour-magnitude diagrams. Combining these ages with the (consistent) set presented in previous work, the first robust evidence of increased circumstellar disc lifetimes in low-mass, low-density regions is obtained. To obtain this result, the following steps were necessary: • Semi-empirical model isochrones have been constructed in a number of rizJHK photometric systems. These models overcome the issues typically seen in purely theoretical models in which the blue flux of low-mass stars is overestimated. These models are presented in a number of widely used filter sets for the first time, allowing for wider use with new clusters. Additionally the models constructed in previous filter sets have been refined using new observations. • To support the construction of these models, upper-main-sequence fitting is performed for 2 fiducial clusters, and it is demonstrated that the resulting age and distance measurements are consistent with other measures. • A new reduction process for data in the Blanco-DECam system is presented, and it is shown that the DECam photometric system is well characterised. • A photometric method for dereddening stars individually in regions of spatially variable extinction is presented, and applied to the young regions in this study. This method of photometric dereddening can be applied to large numbers of stars, greatly decreasing the time investment needed compared to spectroscopic methods. 2 The ages derived for the young clusters using the semi-empirical models are around a factor 2 older than typically assumed in the literature, which is in-line with that seen for the ages derived for other clusters using the same technique. By considering the disc fraction in these clusters as a function of age, it is shown that Taurus and Chamaeleon show a significant excess of discs compared to a set of massive, dense clusters of similar age. This is clear evidence that discs seem to survive longer in this low-mass, low-density region, giving crucial hints at different disc evolution in these regions. ρ-Oph is a low-mass region with a high stellar density, and so could be used to identify the dominant mechanism leading to these long-lived discs. However the presence of a similar disc excess in ρ-Oph is dependent on the assumed distance, which is currently poorly constrained, and so the dominant mechanism is still unclear.

To develop a complete picture of the early stages of star and planet formation, it is necessary to understand the molecular clouds from which they form. The physical properties of these clouds and the interactions between their gas and solid constituents are not well known. This thesis presents an observational study of the morphology and kinematics of carbon monoxide(CO) gas in molecular clouds. It forms part of a wider project to produce the first coupled gas-solid maps of a broad sample of starless and star forming clouds. Nine molecular clouds were observed with the James Clerk Maxwell Telescope, the Institut de Radio Astronomie Millimétrique 30 m Telescope and the Nobeyama 45 m Radio Telescope to produce large scale(14.6 × 14.6 arcmin), high resolution (15.3 arcsec) maps of CO emission. The COROLINE software was developed to derive maps from the observations which show how the CO gas is distributed; the temperature and density structures of the clouds; and how the morphology and kinematics of CO gas is influenced by embedded and nearby stars. A comprehensive analysis is made of Barnard 35A: a cloud heavily processed both internally by a young stellar object and externally by massive stars. The temperature, density and velocity structures of this cloud are explored in detail. Observational evidence is proposed for photoelectric heating through a correlation between the gas temperature and emission from PolyAromatic Hydrocarbons. A clump of enhanced emission ahead of a Herbig–Haro object is thought to be due to photodesorption of CO ice from the surface of dust grains. A chain of similar clumps, following an arc equidistant from the YSO, suggest that the outflow is precessing. This work concludes with the first coupled gas-solid maps. This thesis demonstrates the importance of understanding the often complex morphology and kinematics of a star forming region prior to considering the interactions between its gas and solid-phase species.

The purpose of this thesis was to create an automated classifier for periodic stellar objects in the Wide-Angle Search for Planets Survey (SuperWASP) and to use the classified stars to investigate three phenomena: differentiation of Beta Lyrae and W UMa eclipsing binary stars using eclipse-depth ratio; identification of RR Lyrae stars exhibiting the Blazhko effect; and, the presence of the Oosterhoff dichotomy in the Milky Way. During this work, period/amplitude ranges and distribution maps were created for the classified stars in stellar classes Algol, Beta Lyrae, W UMa, Delta Scuti and RR Lyrae (RRAB) and comparison made with published equivalents. SuperWASP objects known in the General Catalogue of Variable Stars (GCVS) were also assessed to identify differences. The automated system contained three neural networks (NN s) that processed parameters defining the' shape of the phase-folded light-curve and they were trained with representative sets of eclipsing binary, pulsating and sinusoidal-like stars. The system, installed at Leicester University processed 4.3 million object/periods from the SuperWASP database, of which 1.1 million were given prospective classifications. From these, approximately 64 thousand objects consisting of eclipsing binary and pulsating stars were assessed manually to confirm the given period/classifications and roughly half were classified correctly. The reasons for the misclassifications were identified and recommendations made on improving the results. The manually confirmed objects consisted of 12,882 Algols, 5,226 Beta Lyrae, 2,875 W UMa, 1,979 Delta Scutis and 8,322 RR Lyraes (RRAB), where significant numbers of each were unknowrr in SIMBAD or the GCVS. Many objects had periods and/or amplitudes outside published ranges with the surprising result that the majority of Beta Lyraes had periods shorter than published. A separation range for eclipse-depth ratio was identified but a cross-over point existed where differentiation was not possible. A number of new RRAB Blazhko stars were identified and the Research Dissertation September 2012 Page 1 of2 In Confidence Stanley G. Payne M387S063 amplitude range between peaks calculated. The presence of the Oosterhoff dichotomy in the Milky Way galaxy was supported, but the causative factors could not be confmned. Comparison of the SuperW ASP periods with the GCVS resulted in 649 variable stars being added to the GCVS catalogue where the period was unknown in the GCVS and also revision of the variability period of 194 GCVS variable stars was suggested. For comparison of classification, sub-classes were suggested for 333 unconfirmed objects in the GCVS (e.g. CEP:, EA:, RR etc.) and re-classification was suggested for 197 GCVS objects with suspected incorrect classes.

No description available.

This thesis presents numerical simulations of the ablation of galaxies and stellar disks. A sub-grid turbulence model is used to improve the accuracy of the results. Firstly simulations of galactic ram pressure stripping are presented. A spherically symmetric galactic halo is subjected to a wind of a number of different Mach numbers, both with and without the turbulence model. The initial, instantaneous stripping is unaffected by the use of the turbulence model. The turbulence model leads to significantly greater levels of long term ablation, which is more consistent with what simple analytic calculations indicate, showing that the continual Kelvin-Helmholtz stripping plays a significant role in the ram pressure stripping and this is hidden by the finite resolution of simulations without turbulence models. Secondly the turbulence model is applied to galactic disk simulations. In high Mach number (∼ 15) rotating disks the turbulence generated from the internal stresses in the disk causes it to become unstable and erroneously expand in the z direction. This is not the case for lower Mach number disks, such as dwarf disk galaxies. Further developments are needed in turbulence modelling if disk galaxies are to be simulated in this way. Finally inviscid simulations of the interaction of a supernova remnant on a stellar disk are presented. The supernova remnant is simulated hydrodynamically as opposed to using analytical approximations which represents an improvement on previous models. A number of inclination angles of the disk are considered which is not possible with previous two dimensional simulations. Mass loss rates are calculated and compared to other competing processes. Similarly to disk galaxies, the inclination angle only has a large effect on the evolution when the disk is close to edge on. Edge on disks develop an asymmetry when the ablation period is less than the rotation period of the disk. Contamination of the disk with supernova material is also investigate and is found to be low (about one part in 200, 000 in the highest case), consistent with previous results.

Studying the interplay of accretion and star formation is crucial to our understanding of galaxy evolution. The new generation of radio surveys are able to play a key role in this area, since both processes produce radio emission. As we probe low radio-luminosities, the two main populations are star-forming galaxies and radio-quiet quasars (RQQs). How they contribute to the total radio emission is under debate, with previous authors arguing that star formation dominates. In this thesis I investigate the relative levels of radio emission due to accretion and star formation in RQQs. Firstly, I select a sample of 74 quasars from the VISTA Deep Extragalactic Observations (VIDEO) survey, whose depth allows me to study very low accretion rates and/or lower-mass black holes. By comparing radio emission from the quasars with that from a control sample of galaxies, and calculating two independent estimates of star-formation rate, I show that this emission is predominantly related to the accretion process. A second sample of 72 RQQs is obtained from the Spitzer-Herschel Active Galaxy Survey, spanning a factor of ~100 in optical luminosity over a narrow redshift range at z~1. This enables evolutionary effects to be decoupled when comparisons are made with the VIDEO sample. I reduce radio data from the Karl G. Jansky Very Large Array (JVLA) for these objects, and find further support that accretion makes a significant contribution to the radio emission in RQQs. In addition, the levels of accretion and star formation appear to be weakly correlated with each other, and with optical luminosity. These findings offer an insight into how the mechanisms behind galaxy evolution may interact differently in RQQs compared to their radio-loud counterparts. They also have important implications for modelling radio populations below 1mJy, which is necessary for the development of the Square Kilometre Array.

New observations of multiple molecular gas tracers in nearby early- and late-types galaxies are presented and are used to study the physical conditions of the gas within different morphological structures. The CO Tully-Fisher relation is also constructed for a sample of star-forming galaxies at z = 0:05 - 0:3, probing their mass and size evolution. First, using single-dish observations of multiple locations within the nearby spiral galaxy NGC 6946, extensive CO ladders are generated. The molecular line ratios reveal a large variety of physical conditions across the molecular gas complexes, depending primarily on the presence of current or recent star formation, itself compared with that in the centre of the galaxy and other galaxies. Second, interferometric observations of CO and high density molecular tracers in the nearby edge-on early-type galaxies NGC 4710 and NGC 5866 are presented. The gas kinematics reveals that the galaxies are barred, with most of the gas contained within a nuclear disc and a distinct inner ring. Using the molecular line ratios to probe the physical conditions of the gas, the nuclear discs appear to have a more diffuse and hotter molecular medium than the inner rings, with more embedded dense clumps. This suggests that the conditions in the nuclear discs are similar to those in photo-dissociation regions, with intense UV radiation from young stars and few cosmic rays. Indeed, the observed molecular line ratios are also intermediate between those of spiral galaxies and starbursts, with even milder star formation in the inner rings. Third, homogeneously measuring the line widths in the CO spectra of star-forming disc galaxies at z = 0:05 - 0:3, their Ks-band CO Tully-Fisher relation is constructed. A comparison to local star-forming galaxy TFRs from the literature provides mild evidence that our sample galaxies are ≈ 0:89 mag brighter than local ones at a given rotational velocity, a result entirely consistent with our stellar mass TFR, suggesting that our sample galaxies are more massive than local ones by ≈ 0:35 dex. While they deserve further scrutiny, we suspect that these results are due to our sample galaxies being more heavily star-forming (and thus brighter at a given mass) than the comparison sample galaxies.

Stellar evolution models underpin much of astrophysics, yet they are severely under-constrained on the pre-main sequence. The masses, radii, temperatures and luminosities of detached double-lined eclipsing binaries (EBs) can be determined in a model-independent manner from the light and radial velocity curves of the system. When these reach a precision of a few percent or less, they provide one of the most powerful observational tests of stellar evolution theory. Furthermore, young binary star systems display significant photometric and spectroscopic variability over a range of timescales and wavelengths. EBs offer powerful test-beds for characterising such variability, as the stellar parameters and orbital geometry are known to high precision. This thesis presents the characterisation of EBs discovered by the CoRoT and Spitzer space missions in the ~3 Myr old NGC 2264 star forming region. A sample of EBs in a young open cluster such as this, constitutes a particularly strong test of stellar evolution models because the EBs share the same age and composition, yet span a wide range of masses. We present the discovery of CoRoT 223992193, a detached, double-lined eclipsing binary, which comprises two pre-main sequence M-dwarfs and shows evidence of a circumbinary disk. We develop innovative approaches, based on Gaussian process regression, to determine the fundamental stellar parameters. At the time of discovery this was only the ninth pre-main sequence eclipsing binary (PMS EB) with component masses below 1.5 M? and provides a useful test of stellar evolution models. CoRoT 223992193 is the first low-mass PMS EB to show evidence of a circumbinary disk and displays significant photometric and spectroscopic variability. The two stars are found to possess slightly different rotation periods: the primary is consistent with synchronisation while the secondary rotates slightly supersynchronously. The photometric and spectroscopic variations are consistent with the picture of two active stars possibly undergoing non-steady, low-level accretion; the system's very high inclination provides a new view of such variability. The techniques developed in characterising CoRoT223992193 are applied to the NGC2264 EB sample. These work well for near equal-mass systems and two main sequence EBs are solved. However, primarily due to the one-dimensional nature of the radial velocity determination, the majority of the sample remains unsolved; more powerful methods are presented and are suggested for further work. Four low-mass EBs show evidence for youth, with three of these displaying low-to-extreme brightness (and hence mass) ratios. No such PMS systems have been reported in the literature. Once complete the full NGC 2264 sample will form one of the most stringent tests of PMS stellar evolution models to date. Finally, we present a visual multiple containing three EBs spanning B-M spectral types. Two of these systems show evidence for youth and cluster membership; the third is more ambiguous. Further observations are required to confirm the nature of this multiple, but if all three EBs are constituent members, it would be the first triply-eclipsing multiple system discovered, and will provide a powerful test of star formation and stellar evolution models in dense cluster environments.

Timing variations in pulsars, low frequency ubiquitous structure known as timing noise and sudden increases in the rotational frequency which we call glitches, provide a means to study neutron stars. Since the first observations, many models have been proposed, yet no definitive explanation has arisen. In this thesis, we aim to improve this situation by developing models of timing noise. We focus chiefly on precession models which explain periodic modulation seen in radio pulsar data. Developing models and testing them provides an opportunity to infer the elemental properties of neutron stars: evidence for long period precession has implications for the superfluid component predicted by models used to explain glitches. However, often more than one model can qualitatively explain the data, therefore we need a method to decide which model best fits the data. This is precisely the case for PSR B1828-11 which has been used as evidence for both precession and so-called magnetospheric switching. We address this confusion by applying the tools of probability theory to develop a Bayesian model comparison and find that the evidence is in favour of precession. In the second part of this thesis, we will discuss the implications of timing variations for the detection of continuous gravitational waves from neutron stars. To search for these signals, matched filtering methods are used which require a template, a guess for what the signal ‘looks like’. Timing variations, as seen in the electromagnetic signal, may also exist in the gravitational wave signal. If detected, these could provide an invaluable source of information about neutron stars. However, if not included in the template, they may mean that the gravitational wave signal is not detected in the first place. We investigate this issue for both timing noise and glitches, using electromagnetic observations to predict for what types of gravitational wave searches this may be an issue. We find that while timing noise is unlikely to be an issue for current gravitational wave searches, glitches may cause a significant problem in all-sky searches for gravitational waves from neutron stars.

This thesis studies the implications of cosmic condensates, specifically the dynamics of superfluid vortices and superconducting fluxtubes, on astrophysical observables. Firstly, several mutual friction forces, arising from the interactions of vortices and their surroundings, are examined. We separately address mesoscopic mechanisms acting in the neutron star core and crust and analyse the strength of the resulting macroscopic mutual friction for realistic equations of state. It is obtained that the coupling strengths vary significantly within both layers and the dissipation changes drastically across the crust-core boundary. In analogy with helium experiments, the interface should therefore have important implications for the stars’ rotational properties. This is followed by an analysis of mechanisms affecting the superconducting flux-tubes. Their motion governs the dynamics of the interior magnetic field and characteristic evolution timescales are presented for a realistic equation of state. While these results are only preliminary and a more detailed analysis of additional processes is needed, they point towards deficiencies in earlier work on this subject. Subsequently, one of the flux-tube mechanisms is investigated in more detail and the analogy with normal magneto-hydrodynamics is employed to derive a superconducting induction equation. While this equation differs significantly from the normal resistive equivalent, several key notions of standard magnetohydrodynamics are retained. From the field evolution equation we further deduce that the canonical fluxtube dissipation is not strong enough to explain field evolution timescales invoked from observations. To reconcile these, entirely different fluxtube coupling mechanisms are required. Finally, the possibility of using laboratory condensates to study aspects of neutron star physics, only poorly understood, is examined. Specifically helium, ultra-cold gases and superconductors are prime candidates to mimic the behaviour of neutron stars on smaller scales. By looking at typical characteristics such as the two-fluid nature, super-fluid turbulence and pinning, we find that terrestrial quantum states could provide a promising new angle to fill the missing pieces of neutron star astrophysics.