by Yip Ching Wa. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 142-147). / Text in English; abstract also in Chinese. / by Yip Ching Wa. / Acknowledgement --- p.i / Contents --- p.ii / List of Figures --- p.vii / List of Tables --- p.xi / Abstract --- p.xiii / Chapter Chapter 1. --- Introduction to stellar oscillations --- p.1 / Chapter 1.1 --- Motivations of study --- p.1 / Chapter 1.2 --- Historical background of stellar oscillation --- p.2 / Chapter 1.3 --- Compact stars --- p.4 / Chapter 1.4 --- The observational aspects --- p.5 / Chapter 1.5 --- Outline of the thesis --- p.6 / Chapter Chapter 2. --- Static Stars --- p.8 / Chapter 2.1 --- Newtonian stars --- p.8 / Chapter 2.2 --- Relativistic stars --- p.10 / Chapter Chapter 3. --- Mode classifications --- p.14 / Chapter 3.1 --- Newtonian vs relativistic oscillations --- p.14 / Chapter 3.2 --- Radial oscillations --- p.15 / Chapter 3.3 --- Nonradial oscillations --- p.16 / Chapter 3.3.1 --- Spheroidal mode --- p.17 / Chapter 3.3.1.1 --- f-mode (yundamental mode) --- p.17 / Chapter 3.3.1.2 --- p-mode (pressure mode) --- p.18 / Chapter 3.3.1.3 --- g-mode (gravity mode) --- p.18 / Chapter 3.3.1.4 --- w-mode (gravitational-wave mode) --- p.19 / Chapter 3.3.2 --- Toroidal mode --- p.20 / Chapter 3.3.2.1 --- t-mode : (torsional mode) --- p.21 / Chapter 3.3.3 --- Characteristic frequencies of local vibrations --- p.22 / Chapter 3.3.4 --- Stability --- p.23 / Chapter 3.3.4.1 --- Dynamical stability --- p.24 / Chapter 3.3.4.2 --- Secular stability --- p.24 / Chapter 3.3.4.3 --- Pulsational stability --- p.24 / Chapter 3.4 --- Summary --- p.24 / Chapter Chapter 4. --- Adiabatic radial oscillations of stars --- p.26 / Chapter 4.0.1 --- Newtonian case --- p.26 / Chapter 4.0.2 --- Relativistic case --- p.29 / Chapter 4.1 --- Results --- p.30 / Chapter 4.2 --- The stability criteria --- p.31 / Chapter 4.3 --- Summary --- p.39 / Chapter Chapter 5. --- Quasinormal modes of stars --- p.40 / Chapter 5.1 --- Introduction --- p.40 / Chapter 5.2 --- The Scattering Method --- p.42 / Chapter 5.3 --- WKB approximation --- p.43 / Chapter 5.4 --- Chandrasekhar and Detweiler's series --- p.44 / Chapter 5.4.1 --- Application of the series --- p.45 / Chapter 5.5 --- Leaver's series --- p.46 / Chapter 5.5.1 --- Application of the series --- p.48 / Chapter 5.6 --- Summary --- p.52 / Chapter Chapter 6. --- Relativistic nonradial oscillations --- p.53 / Chapter 6.1 --- Axial perturbation --- p.55 / Chapter 6.1.1 --- Perturbation equations --- p.55 / Chapter 6.1.2 --- Boundary conditions --- p.58 / Chapter 6.2 --- Polar perturbations --- p.58 / Chapter 6.2.1 --- Perturbation equations for r≤R --- p.58 / Chapter 6.2.2 --- Boundary condition at r→ 0 --- p.60 / Chapter 6.2.3 --- Perturbation equation for r ≥R --- p.63 / Chapter 6.2.4 --- Boundary condition at r→ ∞ --- p.64 / Chapter 6.3 --- Numerical integration of the perturbation equations --- p.64 / Chapter 6.4 --- The stability problem --- p.66 / Chapter 6.5 --- Summary --- p.66 / Chapter Chapter 7. --- Oscillations of simple model stars --- p.67 / Chapter 7.1 --- Motivations of study --- p.67 / Chapter 7.2 --- Equation of states --- p.68 / Chapter 7.2.1 --- Homogeneous stars --- p.68 / Chapter 7.2.2 --- Relativistic polytropic stars --- p.69 / Chapter 7.3 --- Static stars --- p.69 / Chapter 7.4 --- Oscillation spectra --- p.71 / Chapter 7.4.1 --- Homogeneous star --- p.72 / Chapter 7.4.1.1 --- Axial QNM --- p.72 / Chapter 7.4.1.2 --- Polar QNM --- p.76 / Chapter 7.4.2 --- Polytropic star --- p.78 / Chapter 7.4.2.1 --- Axial QNM --- p.78 / Chapter 7.4.2.2 --- Polar QNM --- p.80 / Chapter 7.4.3 --- Effects of specific ingredients of EOS --- p.82 / Chapter 7.5 --- A comparison of methods for evaluating outgoing-wave solutions --- p.85 / Chapter 7.6 --- Summary --- p.88 / Chapter Chapter 8. --- Oscillations of realistic neutron stars --- p.89 / Chapter 8.1 --- Motivations of study --- p.89 / Chapter 8.2 --- Equations of states --- p.90 / Chapter 8.3 --- Static stars --- p.94 / Chapter 8.4 --- Axial QNM --- p.96 / Chapter 8.5 --- Polar QNM --- p.97 / Chapter 8.6 --- Effects of specific ingredients of EOS --- p.99 / Chapter 8.6.1 --- Effects of neutral pion condensate --- p.100 / Chapter 8.7 --- Summary --- p.101 / Chapter Chapter 9. --- Oscillations of quark stars --- p.105 / Chapter 9.1 --- Motivations of study --- p.105 / Chapter 9.2 --- The equations of states --- p.106 / Chapter 9.2.1 --- Light-quark stars (LQS) --- p.106 / Chapter 9.2.2 --- Hybrid neutron stars with quark cores (HLQS) --- p.107 / Chapter 9.2.3 --- Hybrid neutron stars with strange quark cores (HSSI and HSSII) --- p.107 / Chapter 9.3 --- Axial QNM --- p.111 / Chapter 9.4 --- Polar QNM --- p.114 / Chapter 9.5 --- Effects of specific ingredients of EOS --- p.117 / Chapter 9.6 --- Properties of wII modes --- p.118 / Chapter 9.7 --- Summary --- p.121 / Chapter Chapter 10. --- Conclusion --- p.123 / Chapter 10.1 --- Summary of results --- p.123 / Chapter 10.2 --- Outlook of the problem --- p.124 / Appendix A. Unit conventions --- p.126 / Appendix B. Proof of the regularity of the singular point Vrw(r = 0) --- p.127 / Appendix C. Derivation of transformation between Ψrwand Ψz --- p.129 / Appendix D. Newtonian Cowling Approximation --- p.132 / Chapter D.1 --- Cowling Approximation --- p.132 / Chapter D.2 --- Local Analysis --- p.134 / Chapter D.3 --- Existence of p and g-modes --- p.135 / Appendix E. Relativistic Cowling Approximation --- p.138 / Bibliography --- p.143

The population of all magnetic chemically peculiar stars (Ap stars) within 100 parsecs of the sun has been identified and investigated, determining fundamental parameters, rotational properties, and magnetic field characteristics. Using the HIPPARCOS Catalogue all intermediate mass stars within 100 pc of the sun have been identified. From published catalogues and other literature sources, we have identified 57 bona fide magnetic Ap/Bp stars in this distance-selected sample. Effective temperature, luminosity, radius, and mass were determined for each of the sample stars using published photometry and photometric calibrations, energy distributions, and HIPPARCOS parallaxes. Using the MuSiCoS spectropolarimeter at the Pic du Midi Observatory and the Least Squares Deconvolution procedure, Stokes I and V profiles were obtained for 26 of the 57 Ap sample stars. These observations were used in combination with previously published data to refine rotation periods, to determine projected rotational velocities, and to determine magnetic field strengths and geometries. Using the mass statistics of the sample Ap and non-Ap stars, the mass incidence distribution of magnetic intermediate mass stars in the solar neighbourhood has been derived. The Ap stars make up 1.7 to 2.8% (57 Ap stars out of 3904 intermediate mass stars) of all intermediate mass stars within 100 parsecs of the sun, and appear uniformly distributed across the main sequence. Statistical distributions of various properties, including distributions of the surface dipole field strength, rotational axis inclination, magnetic obliquity angle, and rotational period were analyzed. The rotation period distribution peaks at $2.1\pm1.1$ days, and displays an extended tail to very long periods. The majority of Ap stars in the sample have rotational velocities below 70 km/s, although some exhibit v sin i as large as 169 km/s. The histogram of obliquity angles shows a bimodal distribution, with a strong preference for beta angles near 0 and 90 degrees. The derived distribution of inclination angles agrees with that found by Abt (2001) for randomly-oriented rotation axes. The distribution of surface dipole field strengths exhibits a plateau at 2.5+/-0.5 kG, dropping off to higher and lower field strengths. The results obtained agree with those obtained by Auriere et al. (2007), who reported a lower surface field limit at 300 G, proposing a lower limit critical field strength necessary for stable magnetic configurations to exist. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2007-09-20 09:23:09.29

Time series of high resolution CFHT spectra of four δ Scuti stars are examined for consistency with the presence of nonradial pulsations (nrp). Each series exhibits a progression of subfeatures moving from blue to red through the absorption lines. We have reproduced the profile variations using a geometrical model which imposes sectorial modes on the surface of the star. Modeling of the low-degree modes is guided by radial-velocity variations and the known photometric variations. Synthetic spectra generated with the appropriate Teff and log g for each star are used as input for the model. In this way, the entire wavelength region covered by the observations can be reproduced and the effects of blending on the nrp profiles are included explicitly. The extension from a single-line model to one generated over a wide spectral region provides a much more sensitive comparison with the observations. In general, we find that the data can be reproduced by the combination of a high-degree mode (ℓ > 8) and a low-degree mode [ℓ < 2). The intrinsic line widths and υ sinί together set a limit on the resolution of the stellar surface and by including this resolution in our treatment we can estimate the velocity amplitude of the oscillations (~ 5 km/s). We find that low values of k (≤ 0.1) expected for p-mode oscillations are consistent with the observations. A possible relationship between the periods of the high- and low-degree modes is noted, the question of uniqueness is addressed and comparisons are made with models invoking starspots. For at least one of the stars (қ² Boo), it is impossible to fit the observations by a starspot model without assuming unrealistic values of radius or equatorial velocity. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The recent space-based exoplanet transit searches, CoRoT and Kepler, have revolutionised the field of stellar variability. In this thesis I exploit the public Kepler data to characterise stellar variability, and study rotation periods. For the study of stellar variability it is a complicated but necessary process to remove instrumental systematics while maintaining intrinsic stellar signal. I was involved in the development of a new correction method for systematics, denoted ARC (Astrophysically Robust Correction). This method relies on the removal of a set of basis functions that are determined to be present in small amounts across many light curves. Using the first month of Kepler data, corrected with the ARC method, I studied the variability properties of main sequence stars as a function of fundamental stellar parameters. I find that the fraction of stars with variability greater than that of the Sun is 60%, and confirm the trend of increasing variability with decreasing effective temperatures. I show tentative evidence that the more active stars have lower proper motions and may be located closer to the galactic plane. I also investigate the frequency content of the variability, showing that there exist significant differences in the nature of variability between spectral types, with a trend towards longer periods at later spectral types. In order to exploit the full potential of the Kepler data for stellar rotation period measurement, I developed a novel method of period detection for use on star spot modulated light curves. Standard approaches to period detection are based on Fourier decomposition or least-squares fitting of sinusoidal models. However, typical stellar light curves are neither sinusoidal nor strictly periodic. Therefore, I developed an algorithm for period detection based on the autocorrelation function (ACF) of the light curve. Because the ACF measures only the degree of self-similarity of the light curve at a given time lag, the period remains detectable even when the amplitude and phase of the photometric modulation evolve significantly. I applied the ACF method for the sample of M-dwarfs observed during the first 10 months of the Kepler mission, and detected rotation periods in 1570, ranging from 0.37-69.7 days. The rotation period distribution is clearly bimodal, with peaks at ~19 and ~33 days, hinting at two distinct waves of star formation. These two peaks form two distinct sequences in period-temperature space, with the period decreasing with increasing temperature. In a natural continuation to this work I applied measured periods for 1000 stars in each of the F, G and K-dwarf sets observed by Kepler, and combined these with the M-dwarf results. The trend of increasing rotation period with increasing mass is clear throughout, as the observations fall along a wide by distinct sequence. Comparison to the rotational isochrones of Barnes (2007) show an overall agreement, although the dataset, which I believe is the largest set of rotation period measurements for main sequence stars, shows addition detail, not captured by the gyrochronology relations. This includes a dip in the rotation period distribution at ~0.6 M⊙ and a steep increase in period for the M-dwarfs. I also applied the ACF method to the Kepler exoplanet candidate host stars and used the results to search for evidence of tidal interaction between the star and planet. I show that for the majority of exoplanet host stars, spin-orbit interaction will not have affected the stellar rotation period, permitting the application of gyrochronology for age determination. A comparison of the host stars with a sample of field stars selected to match their temperature and magnitude distribution also indicates no significant difference in the period or amplitude distributions of the two sets. The only notable variation is the lack of planets around the very fast rotators across all spectral types.

The stellar halo of the Milky Way is populated by mostly old and metal-poor stars. As dynamical timescales are of order ~Gyrs at these large distances, accreted stellar substructures, such as dwarf galaxies or globular clusters, survive here as coherent entities longer than anywhere else in the Galaxy. These substructures represent our “fossil record” which can be used to reconstruct the Galaxy’s complex past. In this work, we seek to identify the structures found in the far reaches of the stellar halo as a step towards a correct interpretation of this fossil record. The advent of all-sky surveys in the Gaia era has ignited a prosperous period for this field of Galactic archaeology, but exploring the distant Milky Way (>10 kpc) with Gaia is difficult. Parallax measurements are much less accurate beyond the Solar neighborhood, though Gaia’s proper motions remain useful out to large radii. To push Gaia into the distant Galaxy, we combined these astrometric data with u-band photometry from the Canada-France Imaging Survey (CFIS). We exploited CFIS’ excellent photometric quality and depth (which extends 3 magnitudes deeper than that of the Sloan Digital Sky Survey) to use blue horizontal branch stars (BHBs) as a tracer population with well-measured distances. We first examined the distribution of BHBs using the OPTICS (Ordering Points To Identify the Clustering Structure) clustering algorithm to visualize the hierarchical nature of outer halo substructure. We then identified several well-known satellites, including a group of stars in the vicinity of a distant globular cluster (NGC 5466). Analysis of their kinematics suggested a few of these BHBs outside the cluster’s tidal radius were co-moving with NGC 5466, implying they may be tidal debris from this system. Interestingly, a stream had previously been detected extending from this globular cluster. However, its properties had not been studied in the decade since its discovery, and previous dynamical models were unable to reproduce many of the reported features. As one of the (allegedly) longest globular cluster streams on the sky - and given its distance and utility to constrain the Milky Way’s mass at large Galactic radius - we sought to explore this structure further. We subsequently used red giant branch stars (RGBs) identified in CFIS to try to better quantify the characteristics of the putative stream. We were able to filter these data and obtain a sample of stars that are fully consistent with stream membership and which span approximately 31 degrees of sky. Combined with the BHBs, we used these populations to trace the path of the stream, its distance and distance gradient across the stream’s longitude, and additionally estimated a lower limit to the stream’s luminosity. Our measurements suggest that the stream is at least 11% of the luminosity of the cluster. We then compared our observational data to dynamical models, which showed generally good agreement with the observed stream. This success reflects the updated properties of data measured in this work, and the inclusion of new data (especially proper motions). Our model suggests that the pericenter and apocenter of NGC 5466's orbit are 6.4 and 43 kpc, respectively, resulting in a very eccentric orbit (ε = 0.74). We also find evidence that the cluster experienced a recent interaction (within the past ~100 Myrs) with the Galactic disk, suggesting that the primary source of mass loss in this system may be caused by disk-shocking. The NGC 5466 stellar stream also exhibits an interesting heliocentric gradient in the leading arm, which our simplistic spherical halo model does not fully reproduce. Dynamical experiments with various halo shapes fit to this stream will prove interesting for future work. For local cosmology in particular, long, thin, dynamically cold stellar streams are ideal systems for constraining properties of the Milky Way’s dark matter halo, and streams at large radius are especially useful for measuring the Galaxy's mass interior to the stream. In this respect, we anticipate that NGC 5466 will be exceptionally useful as a probe of the shape, mass, and dark substructure of the Milky Way's distant dark matter halo. / Graduate

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This thesis presents the results of calculating model atmospheres for the accretion column of a magnetic white dwarf. A basic stellar atmosphere calculation is refined to model the specific conditions at the base of an accretion column. Calculated spectra for a variety of different input conditions are shown. The calculated spectra are fitted with black body spectra in order to ascertain the errors associated with black body fitting of observed spectra. Simulated lightcurves are calculated using these model atmosphere spectra. The resultant lightcurves are folded through the EXOSAT (European X-ray Observatory Satellite) detectors and used to fit lightcurves from the magnetic polar system AM Herculis. Following the assumption that a thin accretion disc around a supermassive black hole is the central power source for active galactic nuclei (AGN's) a large grid of model atmospheres is calculated. This grid is then used to calculate the spectrum from such a disc.

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No description available.

No description available.