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For more information, please contact the WRAP Team at: [email protected] warwick.ac.uk/lib-publications White dwarfs: perfect laboratories for understanding non-radial pulsations and revealing secrets on the single degenerate pathway towards Supernova type Ia by Odette Toloza Castillo Thesis Submitted to the University of Warwick for the degree of PhD Doctor of Philosophy Department of Physics March 2018 Contents Acknowledgments iv Declarations v Abstract vi Abbreviations viii Chapter 1 Introduction 1 1.1 White dwarfs . .1 1.1.1 Forming white dwarfs . .1 1.1.2 Fundamental properties of white dwarfs . .6 1.1.3 Pulsating white dwarfs . 15 1.2 White dwarfs in binary systems: Cataclysmic Variables . 19 1.2.1 Geometry of a binary . 21 1.2.2 Evolution of CVs: the initial stages . 24 1.2.3 Orbital period distribution . 27 1.2.4 CV sub-classes . 30 1.2.5 Dwarf Novae . 31 1.2.6 Shell burning on white dwarfs . 33 Chapter 2 Hubble Space Telescope 39 2.1 The Cosmic Origins Spectrograph . 39 2.2 Space Telescope Imaging Spectrograph . 44 2.3 Time-tagged data manipulation . 47 2.3.1 Ultraviolet light curves . 48 2.3.2 Spectra from selected time intervals: Peaks & troughs . 51 2.3.3 Radial velocity corrections for the eclipsing binary QS Vir . 52 2.3.4 Reducing the effects of airglow . 53 i 2.4 Side Projects: Analysis of light curves . 54 2.4.1 Spectroscopy from HST/COS of the southern nova-like BB Doradus in an intermediate state, Godon et al. 2016, APJ, 833, 146) . 54 2.4.2 The composition of a disrupted extrasolar planetesimal at SDSS J0845+2257, Wilson et al. 2015, MNRAS, 451, 3237 . 56 Chapter 3 Fitting HST spectroscopy of white dwarfs 60 3.1 White dwarf model atmospheres . 60 3.1.1 White dwarf atmospheric structure: atm and tlusty ........ 61 3.1.2 Synthetic emergent spectrum: syn and synspec ........... 61 3.2 Markov Chain Monte Carlo ensemble sampler . 62 3.2.1 Example: white dwarf radial velocity of QS Vir . 64 3.3 White dwarf Teff and log g from fits to ultraviolet spectroscopy: the need for constraints . 67 3.3.1 Distance as a prior . 68 3.3.2 Constraints from optical photometry . 71 3.4 Side projects . 72 3.4.1 The intermediate polar CC Sculptoris, Szkody et al. 2017, AJ, 153, 123 .................................. 73 3.4.2 The ultraviolet spectrum of the radio pulsar AR Sco Marsh et al. 2016, Nature, 537, 374 . 75 Chapter 4 G29-38: a pulsating white dwarf with metal pollution 78 4.1 Introduction . 78 4.1.1 Previous studies . 79 4.2 Flux variations due to the non-radial pulsations . 84 4.3 COS time-tagged spectroscopy . 85 4.4 Ultraviolet pulsation periods . 86 4.5 Spectral fits . 90 4.5.1 Grid of white dwarf models . 90 4.5.2 Average spectrum . 91 4.5.3 Five spectra . 94 4.6 Surface gravity of G29-38 from the lowest flux spectrum 1 . 97 4.7 Chemical abundances . 99 4.7.1 Average abundances on the white dwarf . 103 4.7.2 Abundances in a heated spot . 105 4.8 Conclusions . 107 ii Chapter 5 GW Librae: a cataclysmic variable with a pulsating white dwarf 109 5.1 Introduction . 109 5.2 Observations . 112 5.2.1 Ultraviolet spectroscopy . 112 5.2.2 Variability . 113 5.3 Spectral fitting . 116 5.3.1 2002, 2010, and 2011 observations . 116 5.3.2 2013 observation . 119 5.4 Discussion . 122 5.4.1 Possible scenarios explaining the change in flux of the white dwarf . 122 5.4.2 Nature of the second component . 129 5.5 Conclusions . 129 Chapter 6 Cataclysmic Variables with nuclear evolved donors 131 6.1 Introduction . 131 6.1.1 White dwarf masses in CVs . 133 6.1.2 Low C/N ultraviolet emission line flux ratio as a signature of super- soft X-rays source descendants . 134 6.2 HS0218+3229 & QZ Ser, two failed SNIa . 135 6.3 COS spectroscopy . 136 6.4 Spectral fits . 136 6.4.1 Results . 139 6.5 Stellar evolution of the evolved companion with MESA . 142 6.5.1 Physical inputs . 142 6.5.2 Procedure for the evolution of the donor star in the CV . 143 6.5.3 Evolutionary tracks for the donor . 145 6.5.4 Results . 146 6.6 Discussion . 153 6.6.1 Observable signatures of the He/H ratio . 153 6.6.2 Side project: TYC6760-497-1, the first pre-supersoft X-rays source, (Parsons et al., 2015, MNRAS, 452, 1754) . 155 6.7 Conclusions . 157 Chapter 7 Concluding summary 159 iii Acknowledgments I come from the place with the clearest skies where the view of the beautiful band of the Milky Way stretching above us, inspired me to pursue the pathway that today comes to the end of a chapter. If you are reading, let me tell you that the greatest achievement is not to get to the goal, but all the experiences learnt during the journey. Along my universal evolution, I am very grateful to everyone who somehow shared my successes and failures, from the Big Bang (my pillars, dad & mum, and my best partners, my siblings, who continuously prodded and encouraged me), trough the reionization era (Lissete, Nadia, Lili & Nicolle who have taught me how meaningful friendship is) until the present day in Warwick. Here, I need to do a special mention to whom has been the key in this journey, my supervisor Boris who was always willing to support and share valuable advices, and undoubtedly can say that I could not have ask for better supervisor. A further big thank you goes to Danny and Stuart, who donate part of their precious time to read my work, who provided the final bricks to give good shape to this thesis. Finally, a huge thanks to the ones that join later: the climbing Tim and Mediloza School who always shared a good laugh and endless conversations, and to all my fellows and friends that I have been honoured to meet. While unexplored worlds are beyond that darkness in the sky, what I can explore is the beauty of this world and its people that make that every single day I can learn wonderful things. iv Declarations This thesis is submitted to the University of Warwick in support of my application for the degree of Doctor of Philosophy. The analysis and work presented within this thesis were completed mainly by the author, except the models of the g-mode splitting due to fast rotation presented in Chapter 5 that were made by Dean Townsley, the ULTRACAM data presented in Chapter 4 was reduced and analysed by Boris Gänsicke, and the Xshooter spectrum of KT Eri shown in Figure 7.2 was reduced by N. Gentile Fusillo. The work presented within Chapter 5 is based on a paper published by the journal Monthly Notices of the Royal Astronomical Society (MNRAS): GW Librae: a unique laboratory for pulsations in an accreting white dwarf. The data within this thesis is mainly spectroscopy taken with the HST. The HST data of WD1919+145 used to explain the method in Chapter 3 is from program ID 14077 (PI: Gänsicke). Further HST data of QS Virginis was used in order to explain Markov Chain Monte Carlo technique in Chapters 2 and 3, was kindly provided by the PI, Jeremy Drake (ID:13754). G29-38 HST data used in Chapter 4 is publicly available (PI: Jura, ID:12290) and the ULTRACAM data of G29-30 was obtained by J. Farihi and S. Parsons. GW Librae (Chapter 5), QZ Serpenties (Chapter 6), and HS0218+3229 (Chapter 6) spectroscopic data was taken with HST under the program ID:12870 (PI: Gänsicke). The codes used in this work are mainly public: EMCEE (Foreman-Mackey et al., 2013), Period04 (Lenz & Breger, 2005), MESA (Paxton et al., 2011, 2013, 2015), and TLUSTY/SYNSPEC (Hubeny & Lanz, 1995), except the white dwarf atmosphere code ATM/SYN written by prof. Detlev (Koester, 2010), that was kindly provided by himself. v Abstract White dwarfs are elderly stars that represent the endpoint of stars with masses lower than '9 M , which comprise 95%–98% of all stars in our Galaxy, including our Sun. Hence, the motivation of their study is to reveal important insight about the future of our Solar system. In this thesis I present three main projects that are linked because the analysed systems host white dwarfs. I begin by introducing in Chapter 1 the evolution and properties of single white dwarfs and in cataclysmic variables. Most of the light emitted by the white dwarf is detected in the ultraviolet, therefore the spectrographs mounted in the Hubble Space Telescope are ideal for the white dwarf science. In this thesis I performed the analysis of spectroscopic data taken with the Cosmic Origins Spectrograph and the Space Telescope Imaging Spectrograph, therefore I explain their performance and capabilities in Chapter 2. The analysis of the Hubble spectroscopy consists mainly in determining the white dwarf atmospheric parameters. The technique used in this thesis is fitting the data with synthetic white dwarf atmospheres using the Markov Chain Monte Carlo for Bayesian inference, which I explain in Chapter 3.