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1.4 the Magellanic Clouds The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University THE STRUCTURE OF THE LARGE MAGELLANIC CLOUD By Oyirwoth Patrick Abedigamba Town A dissertation submitted in partial fulfillmentCape of the requirements for the degree M.Sc. in the Departmentof of Astronomy, as part of the National Astrophysics and Space Science Programme UNIVERSITY OF CAPE TOWN AUGUST 2010 University Supervisors: Honorary. Prof. Michael Feast1,2 Prof. Patricia Whitelock2,1 1Astronomy Department, University of Cape Town 2South African Astronomical Observatory Key words Large Magellanic Cloud, RR Lyrae stars, Optical Gravitational Lensing Experiment, Fourier parameters, metallicity [Fe/H]. Town Cape of University i Abstract 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 withTown estimates of metallicity gradients of other LMC populations. Cape of University ii Declaration I declare that The structure of the Large Magellanic Cloud is my own work and with the advice of my supervisors, that it has not been submitted for any degree or examination in any other university, and that all the sources I have used or quoted have been indicated and acknowledged by complete references. Copyright and Intellectual property right Copyright in text of this work rests with the author and any more additional copies made should be done so with permission from the author. The ownership of any intellectual property rights which may be described in this work is vested in the University of Cape Town (UCT), subject to any prior agreement to the contrary and may not be made available for use by third partiesTown without the written permission of UCT, which will prescribe the terms and conditions of any such agreement. Cape of Oyirwoth Patrick Abedigamba University iii Acknowledgments I wish to thank the National Astrophysics and Space Science Program (NASSP) for giving me an opportunity to gain knowledge in this field of study. I wish to thank my supervisors Prof. Micheal Feast and Prof. Patricia Whitelock for patiently guiding and supporting me throughout this work. In addition, I would like to extend my sincere thanks to Dr. Maria-Rosa L. Cioni (University of Hertfordshire) for the wonderful in- ternet discussions given to me pertaining this work whenever I needed some assistance, Dr. A Layden for providing us with his private communication data, and not forgetting Dr. Bruno Lertate in our research group at the South African Astronomical Observa- tory (S.A.A.O) for the wonderful contributions towards this project. In a special way I would like to say many thanks to the librarians Davis Shireen and Ntsham Nomgcobo of S.A.A.O. Town I also extend my gratitude and appreciation to the University of Cape Town (UCT) and the South African Astronomical Observatory (SAAO) for all the moral and financial support. In a special way I would like to appreciateCape the help offered by the Information Technology (IT) departments of UCT, SAAOof and NASSP who have been there for me whenever I was in trouble with my computer. I would like also to thank Dr. Edward Jurua for his educational & moral support and all my friends for all the help rendered most especially Zara. University I also extend my appreciation to my family members who have been there for me in terms of encouragement and support for all this period when I was away from them. Above all, I thank the Almighty God for guiding and protecting me during this duration of time. Let his name be Glorified!!. This dissertation makes use of data from the Optical Gravitational Lensing Experiment iv Town III (OGLE III), which is a joint project of Warsaw University, Princeton University and the Carnegie institution, and is a Polish Astronomical project at Las Campanas Observatory Chile. Cape of University v Contents Keywords..................................... i Abstract...................................... ii Declaration.................................... iii Acknowledgments................................. iv 1 IntroductionTown 2 1.1Problemstatement............................. 2 1.2Specificaimandobjectiveofthestudy.................. 3 1.3Outlineofthedissertation.........................Cape 3 1.4TheMagellanicClouds........................... 4 1.4.1 TheLargeMagellanicCloud....................of 5 1.4.2 MagellanicCloudsasgalaxies................... 6 1.5PopulationsintheLMC.......................... 7 1.5.1 Theold,intermediateandyoung-agegenerations........ 7 1.5.2 ThedistanceoftheLargeMagellanicCloud........... 7 1.6StellarPulsation..............................University 7 1.6.1 Drivingpulsation(KappaandGammaMechanisms)...... 7 1.6.2 Hertzsprung-Russelldiagram(HR)................ 9 1.6.3 Pulsationperiods.......................... 10 1.7RRLyraevariables............................. 11 2 Iron abundance from light curve shapes 15 2.1Obtaining[Fe/H]fromlightcurveshapes................. 15 vi 2.1.1 Method used in analysing the basic relation connecting [Fe/H], period and ϕ31 ........................... 16 2.1.2 Conclusion on [Fe/H], period and ϕ31 analysis.......... 18 2.2 Metallicity, Period and Amplitude (AV )relation............. 18 2.2.1 Method in analysing the basic relation connecting [Fe/H], period and AV ............................... 19 2.2.2 Conclusion on [Fe/H], period and AV relation.......... 19 2.3Generalconclusion............................. 19 2.4OpticalGravitationalLensingExperimentIII(OGLEIII)....... 22 2.4.1 Descriptionofthesurvey...................... 22 2.5MethodsforanalysingtheOGLEIIIdata................ 23 2.5.1 Analysis of fundamental mode RR Lyrae stars (RRab) ..... 23 2.6 Part A-samples with the bright stars removed (16864)Town......... 24 3 Iron abundance and galactocentric distance-selection according to model 26 3.1Calculationofgalactocentricdistance................... 26 3.1.1 Angularcoordinate.........................Cape 26 3.1.2 Galactocentricdistance.......................of 27 3.1.3 Iron abundance [Fe/H] calculation ................. 28 3.1.4 WeightedLeastSquareStraightLineFitting(WLSSLF).... 29 3.1.5 Analysis and results for iron abundance & galactocentric distance 30 3.1.6 Conclusionon[Fe/H]-periodrelation............... 33 3.1.7 DiscussionUniversity and interpretation of the analysis on [Fe/H] and RGC 34 3.2 [Fe/H] for all RR Lyrae stars from Borissova and Gratton surveys . 36 3.2.1 Comparision of the metallicity gradients . ........... 36 3.3Checkingforblending............................ 37 3.3.1 Amplitude-periodrelation..................... 37 3.3.2 RatioofRRabtoRRcanalysis.................. 39 4 Further test on metallicity gradient 41 4.1Selectionaccordingtospacedensity.................... 41 vii 4.2 Part A-samples when the bright stars removed (16864) ......... 44 4.3 Three groupings-part A .......................... 44 4.4 Conclusion on three groupings-part A .................. 49 4.5 Five groupings-part A ........................... 50 4.6 Conclusion on five groupings-part A ................... 53 4.7 Part B-samples containing the bright stars (17692) ........... 54 4.8 Three groupings-part B .......................... 54 4.9 Conclusion on three groupings-part B .................. 57 4.10 Five groupings-part B ........................... 58 4.11 Conclusion on five groupings-part B ................... 60 4.12Mainconclusion............................... 60 4.13Generalconclusionontheselectionaccordingtospacedensity..... 61 5 Conclusions and recommendationsTown 62 5.1Generalconclusion............................. 62 5.2Futureworkandrecommendation..................... 63 Cape of University 1 Chapter 1 Introduction The aim of this study is an in-depth investigation of the structure of the Large Mag- ellanic Cloud (LMC). The LMC is a nearby satellite galaxy that is interacting gravi- tationally with our Milky Way. Given its close proximity, theTown LMC offers us a unique opportunity to study in detail the dynamics and composition of another galaxy. An interesting theme in all Magellanic Cloud research is their evolution and present structure. Important topics like chemical composition,Cape evolution and kinematics are the current research interests in the area ofof the Magellanic Clouds. This dissertation focusses on the investigation of a possible metallicity gradient in the LMC. 1.1 Problem statement RR Lyrae stars are representative of the oldest populations
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