Browsing by Author "Warner, Brian"

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    A cataclysmic key program for SALT
    (2006) Warner, Brian
    The study of cataclysmic variable stars has a long history in South Africa. A brief overview of this history is followed by indications of where the new Southern African Large Telescope (SALT) is likely to make major contributions to this research field. There are about 60 staff and students in the SALT Consortium, who work on interacting binaries containing compact components. Most of these people work on cataclysmic variable stars, with the remainder researching neutron star and black hole binary pairs. This group will use a significant amount of SALT observing time. I look at some of the collective and individual topics that SALT will be able to address.
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    Accretion processes in cataclysmic variable stars: insights from optical transient surveys
    (2024) Motsoaledi, Mokhine; Woudt, Patrick; Buckley, David; Warner, Brian
    Cataclysmic variable (CV) stars are binary stars mostly characterised by accretion from a main sequence donor star to a white dwarf star. Multiple CV subclasses exist with varia tions in the nature of the CV, many of which have accretion discs surrounding the accretor. This study focuses on two sub-classes of CVs, namely AM Canum Venaticorum (AM CVn) stars with helium accretion from a (semi-)degenerate helium star or white dwarf donor star, and magnetic CVs, specifically polars, which have the strongest magnetic fields of the accreting white dwarfs in CVs and lack accretion discs. I make use of optical transient surveys to explore individual objects, as well as the global population of polars, with a focus on the Catalina Real-time Transient Survey (CRTS). Follow-up observations were carried out with the 1-m and 1.9-m telescopes and the 10-m Southern African Large Telescope (SALT) at the South African Astronomical Observatory (SAAO). Observations of 7 outbursting AM CVn stars with orbital periods ranging from
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    An analysis of beta cephei stars in NGC 3293 and the effects of stellar rotation
    (1994) Engelbrecht, Christian Albertus; Balona, Luis; Warner, Brian
    An intensive photometric study in the Johnson B band of ten β Cephei stars in the open cluster NGC 3293 is presented. High sampling rates of the stars' light curves allow the identification of many formerly unknown pulsation frequencies in these stars, by means of Fourier periodogram analysis. All of the stars are found to be multiperiodic, with up to five frequencies identified for individual stars. Physical parameters of the stars are determined from previous photometric results in the literature. These parameters are used to make a comparison of the observed frequencies with theoretical predictions, including the effects of rotation. The results imply a preference for first and second overtone quadrupole (l=2) pulsation in these stars, while a weak relationship between stellar mass and pulsation overtone is identified. One of the stars is identified as a member of an eclipsing binary system, only the second β Cephei star to be identified as such. Physical parameters of the system are determined. The effects of rapid rotation (specifically, the distortion of the stellar profile from a perfect sphere) on alternative methods of pulsation mode identification are investigated for a model β Cephei star. Ratios between light and velocity amplitudes show extreme differences between rotationally distorted and undistorted models, but these ratios show high sensitivity to other parameters as well. Thermally broadened spectral line profiles are calculated for the same model, for modes with l = 0 to 4 and equatorial rotation velocities up to 400 km/s. Neglect of rotational distortion is shown to lead to underestimation of true equatorial rotation velocities by as much as 253. This result confirms that observed B stars are probably rotating at break-up velocities. In summary, various aspects of the behaviour of the β Cephei stars in NGC 3293 in particular, and of β Cephei stars in general, including rapid rotators, are illuminated and augmented by this study.
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    Atomic processes in nova shells
    (1990) Smits, Derck Peter; Warner, Brian
    The unusual spectra of the spatially-resolved nova shells of DQ Her, T Aur and CP Pup are reviewed. Because calculations for these conditions have not been made, recombination models of HI, HeI and CNO from a neutral to a doubly ionized state were constructed. The models are described and the results presented for densities at various temperatures. A photoionization model is also described. The effects of non-uniform density distributions in nova shells have been investigated and the observational consequences discussed. A model of the CP Pup shell is described and the evolution of this plasma during the nova's constant luminosity phase traced. The properties of neon novae are summarized and compared with the features seen in the optical spectra of old classical novae. Enhanced neon abundances are used in the model of the CP Pup Shell to investigate its effect on the evolution of the shell. Finally, the results of some near-infrared observations made on the shells of CP Pup, RR Pic and T Pyx are reported.
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    Detecting optical transients and variables with MeerLICHT
    (2018) Paterson, Kerry; Woudt, Patrick; Warner, Brian; Groot, P. J.
    For a long time, the discovery of a transient in a one wavelength triggered observations in other wavelengths. Due to various constraints such as telescope access, scheduling and availability, and the need for human intervention, follow up observations of the transient event could take place long after the initial discovery. Due to the time-critical nature of some transients, with prompt emission that can fade rapidly, this delay in follow up observations can result in important information about the event being missed. Thus, the concept of MeerLICHT was born. MeerLICHT is an fully robotic, optical telescope whose main goal is the detection of transients in real-time. In collaboration with two legacy-style large survey projects (ThunderKAT and MeerTRAP) on MeerKAT (South Africa’s precursor to the Square Kilometre Array), MeerLICHT is the first fully dedicated telescope to follow another telescope in a different wavelength. Through linked-pointing, MeerLICHT and MeerKAT will provide simultaneous optical and radio data of the transient sky. This will provide invaluable information on emission at both wavelengths, including: the relation and evolution of emission from both wavelength windows, the discovery of optical counterparts of radio transients, the characterization of radio transients, and an early warning of radio transients using optical transients. This thesis contains the work on the development and implementation of the data processing pipeline for MeerLICHT. This data processing pipeline was developed for the automatic processing of data from MeerLICHT/BlackGEM for transient detection in real time.
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    The detection of gravitational microlensing anomalies
    (1999) Vermaak, Pierre le Roux; Menzies, John; Warner, Brian
    The detection and measurability of microlensing anomalies including planet detection, resolved source, blending and parallax effects are investigated by way of an extensive computer simulation as a function of the observation strategy. The most efficient strategy has a planet detection probability exceeding 753 for the most favourable binary geometry, while results for typical event follow up and survey observations agree well with previous work. A complex dependence of planet detection on the resolved source radius is discovered leading to a possible increase in the probability of detecting planets with mass ratios smaller than q = 10⁻³
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    Exploring the super-orbital periodic behaviour of X-Ray binaries
    (2012) Kotze, Marissa Mimi; Charles, PA; Warner, Brian
    Quasi-periodic super-orbital modulations are an established feature of several luminous X-ray binaries. The mechanisms that have been proposed to be responsible for such modulations include irradiation-driven warping/tilting and tidally-induced precession of the accretion disc, jet precession and the modulation of the mass-transfer rate due to an accretion disc instability or the effects of a third body.
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    The gravity oscillations of white dwarf stars
    (1974) Brickhill, Anthony John; Warner, Brian
    The numerical investigation of the oscillations of white dwarfs which is discussed below was begun at the suggestion of Professor Warner soon after the discovery of periodic variations in several cataclysmic variables. Its completion within a reasonable time was greatly helped by the availability of a computer program for the solution of the equations governing the structure of white dwarf stars provided by Dr W. B. Hubbard. Although the program could not be used in the original form for the present investigation, where the accuracy of the outer layers is critical, it provided an invaluable starting point and the coding of the equation of state of the core was used in an almost unaltered form.
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    An investigation of eclipsing polars
    (2005) Wiehahn, Michelle; Potter, Stephen; Warner, Brian
    Includes bibliographical references (leaves 47-54).
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    Investigation of the mass loss, circumstellar environment and evolution of the hydrogen-deficient stars and V838 Mon
    (2006) Crause, Lisa; Kilkenny, Dave; Lawson, Warrick; Warner, Brian
    Includes bibliographical references (p. 167-175)
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    Long-term properties of X-ray binaries in the magellanic clouds
    (2013) Rajoelimanana, Andry Fitiavana; Charles, PA; Warner, Brian
    Long-term variability in all types of X-ray binaries is a well established characteristic, but due to observation limitations and lack of long-term monitoring capability these variations have not been studied before in a systematic way. In this thesis, we exploit the ~ 16 yr optical light curves from the MACHO and OGLE databases, and combine these with archival XMM-Newton X-ray observations to study the long-term properties of Be/X-ray binaries and Supersoft X-ray Sources in the Magellanic Clouds.
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    Magnetohydrodynamical phenomena in gaseous disks
    (1979) Koen, Marthinus Christoffel; Warner, Brian
    The purpose of this thesis is a discussion of the generation and influence of magnetic fields in gaseous disks. It builds mainly on the work of Vainstein and Ruzmaiken (1972) on the generation of galactic magnetic fields. Emphasis here is on application to accretion disks in the cataclysmic variables (CVs). These include novae, dwarf novae and nova-like variables. Disks are maintained by mass transferred from a Roche-lobe filling secondary star to a white dwarf. More information can be found in Warner (1976) and Robinson (1976). As is pointed out in the final chapter, the basic theory should have wider applicability.
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    Nonlinear behaviour of pulsating white dwarfs
    (1999) Vuille, Francois; O'Donoghue, Darragh; Warner, Brian
    Using a phenomenological approach, I have investigated the nonlinear properties of the pulsations of two white dwarfs, namely DA G29-38 and DB GD358. The data at my disposal comprised numerous single- and multi-sites time series photometric campaigns, including light curves from four Whole Earth Telescope runs conducted respectively in 1988 and 1992 on G29-38, and in 1990 and 1994 on GD358. Thanks to their length and quality, several of these individual data sets have the beating between the excited eigenmodes resolved. Amplitude changes are nevertheless visible between these various amplitude spectra, suggesting the presence of intrinsic nonlinear processes. However, I showed that only when the spectral changes are drastic have time-dependent nonlinear phenomena to be invoked; no matter how long the data sets, mild seasonal amplitude variations can often be accounted for by beating between the eigen-modes and high order cross-frequencies when harmonic distortion is strong enough. From third order of perturbation, cross-frequencies naturally appear in the direct vicinity of the normal modes, which not only alter the simple eigenmultiplet structures thus rendering the mode identification more difficult, but also generate long and complex beating processes. In GD358, for instance, 153 such third order combination frequencies are expected to appear in the frequency range of each eigentriplet; a light curve spanning at least 9 months is thus necessary to resolve the period structure of this star. Drastic spectral changes were nevertheless recorded in both G29s38 and GD358 which could not be accounted for by such high order beating processes. I could securely conclude that the pulsations of both these stars experience intrinsic amplitude variations on time-scales ranging from days to years, suggesting that different nonlinear processes dominate the evolution of the pulsations at different time.
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    On the model for U Geminorum
    (1973) Harwood, John Malcolm; Warner, Brian
    The term cataclysmic variable comprises several types of variable star - novae, recurrent novae, dwarf novae; and to these we add the nova-like variables, stars which exhibit many of the characteristics of the novae but which have not been observed to erupt. U Geminorum, discovered by Hinds in 1855, is the nomotype of the dwarf novae. These stars are characterised by large, abrupt, temporary increases in brightness that occur erratically at intervals of ten days to a year or more. The dwarf novae are faint objects, few reaching 11th magnitude at maximum, whereas at minimum light most are fainter than 16th magnitude. Thus they are spectroscopically inaccessible to all but the largest telescopes. Conventional photometric techniques applied to these objects gave no information indicating their physical nature. The time-scale of the observations was so long that only the gross variations in brightness were detected. Much of the observing was undertaken by amateurs (Fig. 1), and from this work several of the basic features of this group have emerged.
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    Photographic photometry of Magellanic Cloud cepheids
    (1980) Martin, William Longmate; Warner, Brian
    Photographic (B,V) light curves of 213 LMC and 180 SMC cepheids (and W Virginis stars) have been obtained and the relevant light-curve parameters derived. Comparison of this work with stars in common with photoelectric data indicate that the and ( - ) magnitudes differ from their photoelectric counterparts by ±0ᵐ.09 and ±0ᵐ.12 respectively for the fainter cepheids (log P <0.8) and by ±0ᵐ.06 and ±0ᵐ.07 respectively for the brighter cepheids (log P >0.8).
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    Polarimetry of magnetic cataclysmic variables
    (1985) Cropper, Mark Scott; Warner, Brian
    The design and construction of an astronomical polarimeter is described and an evaluation made of its performance. Extensive observations of cataclysmic variables with emphasis on the AM Her and DQ Her classes are then presented. After consideration of the basic principles involved in the development of an efficient and accurate polarimeter, a design using two super-achromatic retarders (a 1/4 wave and a 1/2 wave) rotating above a fixed analyser was adopted. This permitted simultaneous linear and circular polarisation measurements, or, by rearranging the order of the retarders in the beam, linear polarisation measurements alone, or circular polarisation measurements alone, with enhanced efficiency. The polarimeter was found to have extremely low instrumental polarisations and, because of the superachromatic retarders used, the efficiency correction factors were very close to 1 at all wavelengths. The polarisations are calculated at the telescope and the light curve at a higher time resolution may also be recorded if this is required. Extensive sets of observations using the polarimeter were obtained for six of the ten AM Her variables (or "polars"). EF Eri and El405-451 were observed most. Evidence was found in El405-451 for movement of the apparent location of the accretion region on the primary star and the inclination and magnetic dipole off set from the rotation axis was determined. This allowed a comparison to be made between the competing models for the cyclotron emission, showing that those which take into account the temperature structure of the accretion region provide the best results. Observations and an analysis of the polarisation data from H0139-68, E2003+225, VV Puppis and PG1550+191 are also presented in some detail. A final chapter presents results from observations made to detect a modulation in the polarisation at the rotation period of the primary in the DQ Her variables. Upper limits are set for 4 members of the class and the implications of the results are discussed.
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    Pulsations in the white dwarf primary of a cataclysmic variable star
    (1999) Van Zyl, Liza Marie; Warner, Brian
    The field of Asteroseismology provides powerful techniques for the study of pulsating white dwarf stars. If these techniques could be applied to the white dwarf primaries of Cataclysmic Variable stars, the study of accretion in interacting binary systems would benefit enormously. Our understanding of normal modes in white dwarf pulsators would benefit likewise from the study of how they respond to accretion. This dissertation presents the analysis of preliminary observations of the Dwarf Nova GW Librae , the first Cataclysmic Variable star found to have a DAY primary. A brief introduction to Cataclysmic Variables and the pulsating white dwarfs is given, and the significance of GW Librae to these fields of study is discussed. In this dissertation I present the analysis of 7 weeks of high speed CCD observations obtained on GW Librae during 1997 and 1998. The power in the pulsation spectrum concentrates principally in regions near 376s and 650s. GW Librae has a pulsation spectrum that shows dramatic changes on a monthly basis, typical of the pulsating DA white dwarfs.
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    Rapid oscillations in cataclysmic variable stars
    (2004) Pretorius, Magaretha L; Warner, Brian; Woudt, Patrick Alan
    Rapid quasi-coherent oscillations were detected in the optical light curves of 24 cataclysmic variable stars (CVS). In 12 of these systems (TU Men, WW Cet, HX Peg, BP CrA, BR Lup, HP Nor, AG Hya, TW Vir, PU CMa, V426 Ooph, V1193 Ori, and CR Boo) for the first time. The results contribute to the observational record of the phenomenology of dwarf nova oscillations (DNOs), quasi-periodic oscillations (QPOs), and longer period dwarf nova oscillations (lpDNOs), strengthen the correlation, valid over nearly six orders of magnitude in frequency, between the ratio of time scales of different classes of oscillations in white dwarf, neutron star, and black hole binaries, and show that the recently recognized lpDNOs occur fairly commonly in high mass transfer rate CVs.
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    Rapid oscillations in cataclysmic variables
    (1986) Allen, Sue; Warner, Brian
    Coherent and quasi-periodic rapid oscillations have been seen in the optical and X-ray light curves of more than 30 cataclysmic variables to date. The observational characteristics of these oscillations are reviewed, and current models discussed. To put the observations in context, a detailed review is given of the canonical model of cataclysmic variables and the variations that give rise to its various sub-classes. Observations of two systems of particular interest are presented, and their contributions to the modelling of rapid oscillations are discussed. High-speed photometry of the very bright nova-like variable CPD-48°1577 (IX Velorum) is shown to reveal low-amplitude oscillations with periods between 24 and 31 seconds. The oscillations are present in more than half of the runs, making this variable an attractive target for simultaneous observations in different wavelength bands. The dwarf nova Z Cha is one of four cataclysmic variables which show rapid oscillations as well as primary eclipses. Photometry of z Cha, archived at the University of Cape Town over a period of ten years, is searched for rapid oscillations. Two runs show oscillations, with periods of 24 and 28 seconds. They exhibit eclipse-related phase changes which show qualitative repetition within a run, but significant long-term variation. This behaviour has not been seen in any other cataclysmic variable, and makes z Cha a potentially powerful discriminating tool for models of oscillation and superoutburst.
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    Spectroscopic studies of magnetic cataclysmic variables
    (1999) Van der Heyden, Kurt Josias; Buckley, D A H; Warner, Brian
    In this thesis I present the results of optical spectroscopic analyses of four Polars and the spectra of five new systems. The results of four candidate magnetic cataclysmic variables, which were selected from the ROSAT Bright Survey, are also presented. Time resolved spectra are used for the dual purpose of conducting Doppler tomographical studies and searching for any evidence of cyclotron beaming. The first five chapters are introductory and background sections, including a review of all individual objects with magnetic field strengths determined by cyclotron spectroscopy. In Chapter 6 I discuss the observations, data reduction and analysis techniques while the results of individual objects are presented in Chapter 7. Results are presented for the two Polars HU Aqr and QS Tel, which were selected as test objects to confirm the techniques of Doppler tomography and cyclotron spectroscopy, respectively. Doppler maps, similar to those previously obtained, are produced for HU Aqr. Two cyclotron humps are also detected in the HU Aqr spectra corresponding to a magnetic field of 39 MG, again similar to values quoted in the literature. A complex cyclotron spectrum for QS Tel indicates that the second magnetic pole is more dominant than in previous studies. Spectra are presented for the new Polars RX J1313-32, RX J1141-64, RX J1610+03, RX J0153-59, RX J0501-03, RX J0512-72 and.RX J0515+01. More extensive observations were made for RX Jl313-32 and RX Jll41-64, with Doppler maps derived for RX J1313-32 while the RX J1141-64 spectra show a rise in the spectral slope, indicative of cyclotron radiation. Four ROSAT Bright Survey sources, RBS 0324, RBS 0372, RBS 0490 & RBS 1969, were selected to search for any evidence of them being magnetic cataclysmic variables. More detailed spectroscopy and photometry was obtained for RBS 0324, the only RBS source which shows evidence of being a magnetic system. The other RBS systems are all probably dwarf novae in quiescence.