Browsing by Author "McBride, Vanessa A"

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    Cluster model investigation of nuclear spectra and electric quadrupole transitions
    (2003) McBride, Vanessa A; Perez, Sandro M
    We review a cluster model for even-even nuclei and derive a new relation between the excitation energies and the electric quadrupole transition strengths of these nuclei. A novel method for determining the likely clustercore decompositions of a given nucleus is described, and is tested by comparing theoretical cluster model spectra with their experimental counterparts for a representative set of Yb isotopes. A further method, based on least squares fits to spectra, is also examined.
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    How circumstellar discs affect mass accretion in Be X-ray binaries
    (2014) Monageng, Itumeleng Matuba; McBride, Vanessa A
    Long-term optical spectroscopic monitoring of Galactic Be X-ray binaries (BeXBs) is performed using the Liverpool Telescope and the Southern African Large Telescope on northern and southern objects, respectively. Be disc size variations are presented to investigate their influence on mass accretion producing X-ray activity. Be disc variability is traced observationally through Balmer emission lines, the strongest and best studied being the Hα line. The peak separations of the double peaked Hα emission line are measured, and along with the mass of the Be star and the inclination of the disc obtained from literature, are used to determine the Be disc radius. For single peaked Hα profiles the peak separation cannot be obtained directly; however, using the empirically determined relationship between the equivalent width and the peak separation of the double peaked profiles, an estimate of the peak separation for the single peaked profiles is obtained. The work is done in the context of the viscous decretion disc model presented by Okazaki & Negueruela (2001), which predicts that the circumstellar discs around Be stars in binary systems are truncated by resonant torques from the neutron star in its orbit. The calculated disc radii are compared to the expected resonance radii from the viscous decretion disc model to determine how different truncation radii affect mass accretion producing X-ray outbursts. Type I outbursts are seen to occur when the disc is truncated close to/beyond the mean critical Roche lobe radius at periastron passage of the neutron star, in agreement with model predictions. Type II outbursts, however, do not show any correlation (or anticorrelation) with the disc size, as they are seen to occur both at relatively small and large sizes of the disc. Additional information on the Hα emission line profile variations, such as the line-shape variations of high-resolution spectra, is required to investigate the origin of type II outbursts in order to make reliable predictions of them. Spectroscopic follow-up of the gamma-ray binary system LSI +61 303 was also performed with the Liverpool Telescope and the Hα line profile variations are presented. In addition to data from the Liverpool Telescope monitoring campaign, published equivalent width measurements are used and timing analysis of the combined measurements is performed. The orbital period and super-orbital period are found, which are similar to those found at other wavelengths. The periodicities in the system can therefore be associated with circumstellar disc variations.
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    In search of binary Wolf-Rayet central stars of planetary nebulae
    (2013) Manick, Rajeev; Miszalski, Brent; McBride, Vanessa A
    Planetary Nebulae (PNe) are circumstellar gas envelopes ejected during Asymptotic Giant Branch (AGB) phase of the stars between 1M࿉ to 8M࿉. In around 3000 galactic PNe, 10 to 20 % are known to host a Wolf-Rayet type central star ([WR] CSPN). These [WR] CSPNe are known to be very hydrogen deficient (H-deficient), the mechanism behind which is not clearly understood. A binary central star (CS) or a merger scenario is very likely to be responsible for the H-deficiency. As an attempt to investigate the former, a radial velocity (RV) monitoring program of 7 [WR] CSPNe was conducted, which is the first systematic study of its kind. Spectroscopic observations were made at the SAAO 1.9m Telescope over 3 separate weeks in May 2012, June 2012 and July 2013. The spectra were reduced and analyzed with a cross-correlation method to determine RV shifts in the stellar and nebular lines. Together with a review of the major mechanisms involved in the formation and evolution [WR] CSPNe, the main results obtained in this work are discussed. An offset is found between the mean stellar and nebular lines, which might be due to the fact that most of the stellar emission lines are formed in the wind and are likely to be blue-shifted as in several Of-type central stars. No significant amplitude or periodic variations are seen in the stellar lines, as we would expect from a close binary system. However, small (~ 10-30 km s-¹) stellar RV variations are seen and these are most likely due to wind variability. A Χ² probability test reveals that the objects are only 30 - 40 % variable and this variability is not high enough to be attributed to a binary CS. The main conclusions drawn are that these [WR] CSPNe might contain wide binaries with longer periods that would produce RV variations below our detection limit. Another feasible explanation might be that [WR] CSPNe are actually the result of post-CE mergers. We absolutely need RV monitoring using high resolution spectra (e.g. High Resolution Spectrograph on the South African Large Telescope) in the future to confirm the presented results.
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    Post-common-envelope binary central stars of Planetary Nebulae in the OGLE-IV survey
    (2016) Hlabathe, Michael; Miszalski, Brent; McBride, Vanessa A
    Planetary Nebulae (PNe) are defined as the ionized shells of circumstellar gas ejected through an intense stellar wind at the end of the star's life. PNe come in different shapes, from spherical to highly complex, non-spherical shapes. Mass loss in AGB stars is presumed as the shaping mechanism but how it results in different PNe morphologies is still unclear. Binary central stars that have undergone common envelope evolution are thought to be a possible solution to this longstanding problem. Using photometry from the OGLEIV survey, we present the newly identified close binary central stars of Planetary Nebulae (CSPNe), six in total. Of the six PNe with close binary CSPNe, one looks spherical which presents a very interesting argument in terms of our understanding of PNe evolution. The orbital distribution is derived and compared against current orbital distribution for binary CSPNe, with most binaries from the distribution exhibiting orbital periods less than a day. A binary fraction estimate of 6% is presented and possible cases are discussed that might have influenced our estimation to be different from the expected 10-15%.
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    Transforming Doppler tomography to unravel the accretion dynamics in cataclysmic variables
    (2017) Kotze, Enrico Juan; McBride, Vanessa A; Potter, Stephen B
    Doppler tomography has revolutionised the interpretation of phase-resolved spectra of interacting binaries. The standard technique extracts emission and kinematic information contained in such spectra and projects it onto a two-dimensional velocity coordinate frame. This thesis describes my investigation of constructing Doppler tomograms in 'inside-out' coordinates by reversing the velocity axis. The aim was to determine if the emission distribution in the inside-out tomogram is more intuitive to interpret than the standard tomogram, and if it reveals indiscernible or enhances less discernible details. The inside-out tomogram is constructed by projecting the spectra onto the inside-out coordinate frame with zero velocity transposed to the outer circumference and the maximum velocities to the centre of the tomogram. In addition, this thesis describes a new flux modulation mapping technique applied to the standard and inside-out Doppler tomography of magnetic cataclysmic variables. I developed this technique with the aim to obtain more information from their observed spectra and present it in a useful format. It extracts the flux modulation from consecutive half-phase tomograms and constructs maps of the amplitude and phasing characteristics of the modulation in these systems. My investigation, involving the spectra of non-magnetic and magnetic cataclysmic variables, shows that the inside-out projection redistributes the relative contrast levels in and amongst the emission components. The inside-out projection exposes low-velocity emission details which are overly compacted and enhances high-velocity emission details which are overly tenuous in the standard projection. In addition, the flux modulation mapping technique gives a significant improvement in reproducing the input spectra adding more confidence in the interpretation of the modulation maps. Notable results were obtained for the polars where the blob-like low-velocity emission in their standard tomograms is more exposed in their inside-out tomograms, making it easier to distinguish between the ballistic and magnetic accretion flows that are evident in their trailed spectra. Also, for all the polars investigated the high-velocity magnetic accretion flows not seen in their standard tomograms are revealed in their inside-out tomograms. This extra information is extremely valuable to form a more complete picture of the emission components, broadening our knowledge of the accretion dynamics in these systems.