Browsing by Author "Feast, Michael "

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    Asymptotic giant branch variables in NGC 6822
    (2010) Nsengiyumva, Francois; Whitelock, Patricia A; Menzies, John; Feast, Michael
    NGC 6822, a Local Group dwarf irregular galaxy, was observed in the nearinfrared JHKS bands using the Japanese-SAAO Infrared Survey Facility (IRSF) telescope over a period of about four years. An extensive search for large amplitude asymptotic giant branch variables over the observed 7.5' x 21.1' area of NGC 6822 found 162 probable variables. In particular, 30 Miras and 9 semiregulars have been identified and characterised.
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    Galactic and extragalactic distance scales: Some South African contributions
    (2005) Feast, Michael
    The work of South African astronomers in establishing and refining methods of measuring the distances of stars and galaxies is reviewed.
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    Gravitational microlensing and the search for extrasolar planets
    (2013) Mengistie, Getachew Mekonnen; Feast, Michael; Menzies, John
    This project is aimed at tackling binary gravitational microlensing events. Since the early conception of gravitational lensing before Einstein’s General Theory of Relativity, physicists and astronomers of the early 18th century speculated that light, if treated as particle, can be affected by gravitational field. Gravitational Microlensing is defined as a phenomenon occurring when light coming from a distant (source) star is bent by the gravitational field of an intervening mass (lens) creating multiple image of the source which cannot be resolved. In this project, We applied a model for binary lens microlensing events. We analysed data obtained from different observatories: PLAXET, which includes SAAO, Sutherland, Canopus and Perth observatories, OGLE, Danish Telescope, LaSilla. For the observed microlensing target OGLE-2011-BLG-265, analysis of the data using the program supplied by Keith Horne, using different minimization schemes and algorithms, I found best fit model to be a binary lens with mass ratio q = 0.0042 and separation of components is found to be u = 1.034 where in this case the separation falls under an intermediate binary lens topology. Finally, the deviation from the point-source point-lens microlensing events on the light curve and small mass ratio suggested a planetary system. I also calculated the time, tp, that the source needs to cross the diameter of the planet Einstein Ring radius, which is related to the mass ratio, and is found to be 3.30 days.
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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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    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.