No description available.

No description available.

No description available.

This thesis explores the photometric variability in isolated magnetic white dwarfs (MWDs) to search for spin periods. Approximately 40% of MWDs exhibit photometric modulations as the star rotates due to the effects from a strong magnetic field or star spots. A sample of 77 MWDs is studied to discover periods on timescales of minutes to one week. Well-defined periods are determined in 12 MWDs, with periods of roughly an hour to a few days, and variability with poorly constrained periods is found in a further 13 stars. MWD spin periods can provide important constraints for their post main-sequence evolution and formation, and in particular, potential information on the influence magnetism plays on the mass and angular momentum loss of the evolving star. A correlation has emerged between the spin period and magnetic field strength and temperature, suggesting hotter MWDs spin faster and have higher field strengths, characteristics possibly associated with MWDs that might have formed in binary mergers. A similar investigation is carried out on longer timescales (months – years) for ten single MWDs, which are stable on short timescales but were previously found to display modulations between observing seasons. However, no significant variability is detected in the sample, although G240-72 may display variations over months. Finally, the spin period evolution over ∼ 20 years is studied in the hot, massive, highly magnetic, rapidly rotating MWD, RE J0317-853. A rate of period change is measured as Ṗ = (9.6 ± 1.4) × 10[superscript −14] s/s which is most likely due to the orbital motion of the wide binary pair of RE J0317-853 and LB9802. Spin-down from magnetic dipole radiation is ruled out as a possible mechanism. Periodic variations in the expected arrival times of maximum flux tentatively suggest a low-mass planetary companion may be orbiting RE J0317-853.

An investigation into the physical properties and evolutionary status of DAO white dwarfs has been conducted, using optical and far-UV observations. Out of the 22 objects for which optical data were obtained, the spectrum of only one is best fitted by a stratified H+He composition model; this may be an object transferring between the He- and H-rich cooling sequences.;Homogeneous models are preferred for the remainder, but with 90% confidence in less than half the objects. The DAOs fall into two groups: The low Teff, high log g objects have temperatures and gravities that are similar to the majority of the DAs and mostly consist of DAO+dM binaries. The high T eff, low log g objects, which make up approximately two thirds of the sample, have relatively high luminosities for white dwarfs, and the most likely mechanism that is countering the gravitational settling for these objects is mass loss. When this ceases these objects will become low-mass DAs.;A search for evidence of binarity in the far-UV FUSE observations of 22 objects yielded positive results for only 5, suggesting that the majority of DAOs are descended from isolated stars. As seen in the DAs, the Teff derived from Balmer and Lyman lines for objects hotter than ∼50,000 K were found to be different, with the latter yielding the higher temperature. For 7 objects, the Lyman lines are so shallow that a model with Teff > 120,000 K is required to reproduce them. An investigation of the strength of photospheric absorption lines showed that the low log g objects have high heavy element abundances compared to DAs, whereas the DAO+dM binaries have slightly elevated lighter element abundances, with the heavier elements abundances similar to those seen in the DAs. This is qualitatively consistent with mass loss and accretion from a companions wind respectively, as the explanation for the helium that is observed in the spectra of the stars.

A thesis describing and analysing the controversy between Subrahmanyan Chandrasekhar and Arthur Stanley Eddington over the limiting mass of white dwarf stars. The aim of the thesis is to discover why the controversy occurred and to analyse the reasons behind Eddington's rejection of relativistic degeneracy and the limiting mass. The ultimate reason behind Eddington's attack on relativistic degeneracy was found to be Eddington's severe objection to singularities which was apparent long before Chandrasekhar's discovery of the limiting mass and occurred in three separate areas of research undertaken by Eddington during this period: astrophysics, cosmology, general relativity and Dirac's relativistic equation of the electron which led to Eddington's fundamental theory. The thesis will focus on the problem of the limiting mass of white dwarfs between 1929 and 1935 but will use the problem to analyse Eddington's view of singularities within the three different research areas spanning two decades from 1916 to 1936. The Chandrasekhar-Eddington controversy is set within Eddington's earlier controversies with James Jeans and Edward Arthur Milne who together with Eddington founded theoretical astrophysics during the 1920s. The thesis will examine the problem of white dwarfs within the context of the earlier controversies on stellar structure. As well as the technical analysis of the controversy, the thesis will also analyse the social dynamics and interactions within the astronomical community and their impact on the controversies. The aim of this thesis is to create a more complete picture of the Chandrasekhar-Eddington controversy by analysing Eddington's arguments for rejecting relativistic degeneracy, the limiting mass of white dwarfs and singularities not just within the context of astrophysics, but also cosmology, general relativity and quantum mechanics and to provide some new explanations as to why Eddington opposed relativistic degeneracy.

M-type red dwarfs are among the most active stars. Their light curves display random variability of rapid increase and gradual decrease in emission. It is believed that these large energy events, or flares, are the manifestation of the permanently reforming magnetic field of the stellar atmosphere. Stellar coronal flares are observed in the radio, optical, ultraviolet and X-rays. With the new generation of X-ray telescopes, XMM-Newton and Chandra, it has become possible to study these flares in much greater detail than ever before. This thesis focuses on three core issues about flares: (i) how their X-ray emission is correlated with the ultraviolet, (ii) using an oscillation to determine the loop length and the magnetic field strength of a particular flare, and (iii) investigating the change of density sensitive lines during flares using high-resolution X-ray spectra, (i) It is known that flare emission in different wavebands often correlate in time. However, here is the first time where data is presented which shows a correlation between emission from two different wavebands (soft X-rays and ultraviolet) over various sized flares and from five stars, which supports that the flare process is governed by common physical parameters scaling over a large range. (ii) As it is impossible to spatially resolve any but a very few giant stars, the only information on spatial dimensions as well as the magnetic field strength of stellar coronae has to come from indirect measurements. Using wavelet analysis, I isolated the first stellar X-ray flare oscillation. Interpreting it as a standing coronal flare loop oscillation, I derived a flare loop length as well as the magnetic field strength for this X-ray flare. (iii) The high-resolution soft X-ray spectra of Chandra and XMM-Newton allow us to determine temperatures, densities and abundances of the stellar coronae. De spite a low signal-to-noise ratio because of the relatively short duration of a flare, we find that, if adding up the photons of several flares, certain density sensitive spectral lines change significantly between quiescent and flaring states. This project led on to investigate the flaring spectrum further, and it is found that the plasma is no longer in collisional ionisation equilibrium, but that it is dominated by recombinations.

In this thesis, a study of the high ionisation-stage metal absorption features in the spectra of hot DA white dwarfs is presented. Metals are present in the photospheres of such stars due to radiative levitation (Chayer et al. 1994, 1995; Chayer Fontaine & Wesemael 1995). However, studies of the patterns between metal abundance and Teff show that, though the broad patterns predicted are seen, individual abundance measurements often do not reflect the predictions of radiative levitation theory (e.g. Barstow et al. 2003b). In this thesis, an analysis of the nitrogen abundance in three stars is performed, where a highly abundant layer of nitrogen was thought to reside at the top of the photospheres of the stars. The nitrogen abundance and distribution in these DAs is found to be homogeneous and of an abundance in keeping with stars of higher Teff. The accretion of metals from circumstellar discs has been shown to be the source of photospheric metals in DAs with Teff < 25,000 K (e.g. Zuckerman et al. 2003), where gravitational diffusion dominates (Koester & Wilken, 2006). In some cases, gaseous components are seen at such discs (e.g. SDSS 122859.93+104032.9; Gänsicke et al. 2006). A survey is made of a sample of hot (19,000 K < Teff < 51,000 K) DAs, where similar accretion may explain the inability of radiative levitation theory alone to account for the detected photospheric metal abundances. No circumstellar gas discs are found, though accretion from as yet undetected circumstellar sources remains an attractive explanation of the photospheric abundances of the stars. Circumstellar absorption is seen in the UV spectra of some hot DA stars (Holberg et al. 1998; Bannister et al. 2003). Sources suggested for this material include circumstellar discs, the ionisation of the ISM, stellar mass loss and planetary nebulae. A re-analysis of this absorption is presented, using a technique that for the first time allows proper modelling of the circumstellar absorption features, and provides column densities for all components. The ionisation of circumstellar discs or planetesimals, the ionisation of the ISM and the ionisation of mass lost by binary companions are put forward as the origin for this circumstellar material.

No description available.

The aim of this thesis is to investigate uncertainties in the input physics of stellar models that are relevant for the evolution of stars and the related nucleosynthesis, in particular the s-process. Nuclear reaction rates and mixing prescriptions in particular can modify signi�cantly the yields of heavy elements in stellar models. The s-process, which is a slow neutron-capture process that can occur in massive stars and asymptotic giant branch (AGB) stars, is an important driver for uncertainty studies because the output yields of heavy nuclides in these astrophysical sites are sensitive to the interior conditions and the input physics. In this work, two uncertainties are considered. The �rst is the 12C + 12C nuclear reaction rate which, despite considerable experimental e�orts, remains uncertain at temperatures relevant for hydrostatic carbon burning in massive stars. We show that changes to this reaction rate a�ect the stellar structure and nucleosynthesis of massive stars and, consequently, the �nal yields. A comparison of these yields with the Solar system abundances enabled us to constrain the 12C + 12C reaction rate in the relevant temperature range. The second of these uncertainties is the treatment of convective-radiative interfaces in 1D stellar models, which are particularly important for modelling thermal pulses in AGB stars. The s-process during thermal pulses is sensitive to the treatment of mixing across convective-radiative interfaces. A possible link between full 3D hydrodynamics models of convective-radiative interfaces and 1D stellar models was investigated by considering a di�usion approximation. A technique for calculating di�usion coe�cients from the output of hydrodynamics models was developed and an exploration of the di�usive approach for convective-boundary mixing is presented, along with the successes and limitations of this approach.