Browsing by Author "Whitelock, Patricia"

Now showing 1 - 5 of 5
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    Eta Carinae: A South African perspective
    (2005) Whitelock, Patricia
    Carinae is one of a kind: it is one of the most curious and enigmatic stars in our Galaxy. It is also an archetypal astronomical object that can be only fully characterized by studies across the entire electromagnetic spectrum - with something for everyone. At the time of writing (December 2005), NASA's Astrophysics Data System† lists 1131 papers which mention n Car in their abstracts, 427 of them written since 2000. I make no attempt here to review this huge corpus, but I hope to give an overall impression of why we might be interested in n Car and what we have learned about it to date, with a deliberate emphasis on observations made from South Africa.
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    Iras oxygen- and carbon-rich miras close to the galactic plane
    (1997) Leeuw, Lerothodi La Pula; Feast, Michael; Whitelock, Patricia
    Observations are presented for 101 I RAS sources that were expected to be Mira Variables and that lie close to the galactic plane: -7.0 3 < b < + 1° and 282.°5 < l < 285.°5. Over 1090 JHKL photometry modules were obtained for the sources. From the photometry the sample was shown to comprise 85 Miras, 12 of which had published periods. New periods were determined for all the Miras. Of the 85 Miras in the sample, 21 had previous published spectra and the rest were tentatively classified as oxygen or carbon-rich Miras using infrared two-colour diagrams. The oxygen and carbon-rich Miras, respectively, have mean periods and K half-amplitudes of 386 days and 0.36 mag and 442 days and 0.46 mag. Statistical tests showed that the probability that the period distribution functions of the oxygen and carbon-rich stars came from the same population was very small: 0.00055. The period as well as the K- Land K- [12μm] colours were shown to be functions of amplitude. For the carbon-rich Miras the [12μm]- [25μm] colour was shown to be correlated with amplitude. Bolometric magnitudes were determined by integrating under a spline curve fitted to the J H K L12, 25μm fluxes of the Miras. Distances to the stars have been calculated using absolute magnitude P- L relations from oxygen-rich and carbon-rich Miras in the LMC, where the LMC distance modulus of 18.57 was adopted. In addition, a search of Miras in the Galactic Bulge using APM-scanned !-plate data are presented. A total of 116 Mira candidates were identified, 43 of which were classified as strong candidates. The presented work will contribute to the study of Galactic structure.
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    Multiwavelength studies of classical novae
    (2018) Aydi, Elias; Mohamed, Shazrene; Whitelock, Patricia
    Classical novae are transient astronomical events resulting from an eruption on the surface of an accreting white dwarf in interacting binary stars, namely cataclysmic variables. These eruptions and their progenitors are ideal astronomical events/objects to study a variety of interesting topics, such as mass-transfer mechanisms, binary evolution, and thermonuclear and radiative emission processes. Novae share some general observational characteristics during their eruption phases, yet each nova is new and can show unique light-curve and spectral developments. Therefore, carrying out detailed multiwavelength studies of individual novae is important to provide an holistic picture of the eruption development with the ultimate aim of a general understanding of the physical mechanisms involved in nova eruptions, as well as the structure and evolutionary condition of their progenitors. This dissertation presents detailed multiwavelength studies of three nova events, namely V5852 Sgr, SMCN 2016-10, and V407 Lup, as a series of chapters (papers). Our data were collected using several ground-based and space telescopes including: high- and mediumresolution optical spectroscopy from SALT, SOAR, and FLOYDS, optical and near-infrared photometry from IRSF, OGLE and SMARTS, UV and X-ray data from Swift, Chandra, and XMM-Newton. In the second chapter, we present an optical and infrared study of the unusual nova V5852 Sgr. This nova is a moderately fast nova showing features of the Fe II spectroscopic type. The light-curve development is unique, showing a combination of several light-curve classes. Estimates of the distance and measurements of the line velocities suggest that the nova might be associated with the Sagittarius stream. If so, it would be the first nova to be discovered in a dwarf spheroidal galaxy. The third chapter presents a multiwavelength study of the very fast nova SMCN 2016-10a. The fast light-curve development suggests that the system hosts a massive white dwarf (& 1.25 M), in good agreement with the high temperature of the super-soft X-ray emission and the turn-on/turn-off time of the super-soft state. At the distance of the Small Magellanic Cloud our measurements suggest that SMCN 2016-10a is the brightest nova in the Small Magellanic Cloud and one of the brightest novae on record, with an absolute maximum magnitude of ∼ −10.5 in the V -band. Chapter four is a multiwavelength study of nova V407 Lup. With a light-curve decline time t2 ≤ 2.9 d, this is one of the fastest known examples and the white dwarf is possibly more massive than 1.25 M. Our set of optical, UV, and X-ray data suggest that this system is an intermediate-polar cataclysmic variable based essentially on the presence of two periodicities (3.57 h and 565 s) in the light-curves. These periodicities are attributed to the orbital period of the binary and rotational period of the white dwarf. The late optical spectra (taken from day 165 post-eruption) show narrow and moving lines of He II and O VI, possibly associated with accretion regions within the binary system. This, along with the X-ray light-curves and spectra, suggest that the accretion probably resumed around 168 days post-eruption. These studies are modest, yet essential steps in the quest for a better understanding of nova eruptions. They also demonstrate the importance of multiwavelength follow-up of novae for constraining the physical parameters of the eruption, the ejecta, and the properties of the progenitor. In the era of large all-sky surveys, such as the All-Sky Automated Survey for Supernovae and eventually the Large Synoptic Survey Telescope which have been and are ex- pected to find a large number of optical transients and classical novae, similar multiwavelength follow-up will play a crucial role for initially identifying these transient events and further understanding their physical behaviour.
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    Reverberation mapping of Active Galactic Nuclei
    (2023) Hlabathe, Michael; Whitelock, Patricia
    Recent times have seen an increase in observational campaigns aimed at contributing to the overall sample of active galactic nuclei (AGNs) with central super-massive black hole (SMBH) mass estimates using reverberation mapping (RM) technique. RM makes use of the time variability of the broad emission lines from the nearby broad-line region (BLR) to measure their time response or delay (τ) relative to the incident ionizing continuum from the accretion disc. This delay is a consequence of light-travel time effects and directly enables the size of the BLR to be determined as RBLR = cτ (c is speed of light). Assuming virialized motions of the gas clouds in the BLR, and combining the velocity width (∆V ) of a Doppler broadened emission line with RBLR leads to the estimation of the black hole mass M. RM can also be used to measure the accretion-disc size in which the X-ray variability source is assumed to drive UV/optical variations in the accretion disc leading to wavelength-dependent delays that scale as τ ∝ λ 4/3 . Here, both aspects of RM are explored using photometric and spectroscopic observations of 3C 120, PG 2304+043 and SDSS J132447.65+032432.6, with observations emanating from the Southern African Large Telescope (SALT) and Las Cumbres Observatory (LCO) robotic telescopes. The broad emission-line and inter-band continuum delays in these AGNs are measured using the Interpolated Cross-Correlation Function (ICCF) and Continuum REverberation AGN Markov Chain Monte Carlo (CREAM).
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    The structure of the large magellanic cloud
    (2010) Abedigamba_OP, Oyirwoth; Feast, Michael; Whitelock, Patricia
    This work gives an account of the study of the metallicity [Fe/H] distribution (gradient) in the oldest population in the Large Magellanic Cloud (LMC), by making use of the available RR Lyrae data from the Optical Gravitational Lensing Experiment III (OGLE III). RR Lyrae stars are amongst the oldest objects in the universe and they have a range in element (metal) abundances. Measuring the distribution of metallicities of RR Lyrae stars in a galaxy gives one clues to the origin of galaxies. It is known that the pulsation periods of RR Lyraes is broadly correlated with their metallicity. This fact has been used for investigating the metallicity distribution of RR Lyrae stars in the LMC. I have found an indication that the proportion of metal poor RR Lyrae stars increases with distance from the centre of the LMC. In addition, an attempt was made to improve the metallicity-period relation by introducing the Fourier parameters, but this was unsuccessful. Lastly, a comparison is made with estimates of metallicity gradients of other LMC populations.