White dwarfs: perfect laboratories for understanding non-radial pulsations and revealing secrets on the single degenerate pathway towards Supernova type Ia

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Abstract

White dwarfs are elderly stars that represent the endpoint of stars with masses lower than ≈9MO, 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. G29-38 is a non-radial pulsating white dwarf that shows infrared excess due to a dusty debris disc that formed from the tidal disruption of a planetesimal. The ongoing accretion from this debris disc pollutes the atmosphere of G29-38. The analysis of the photospheric contamination provides a direct measurement of the bulk composition of the disrupted planetesimal. However, the geometry and the process of the accretion from the debris disc onto the white dwarf are not yet well understood. In Chapter 4, I make use of the pulsations as a spotlight to investigate the metal distribution across the white dwarf surface which provides constraints on the geometry of the accretion process. In Chapter 5 I present my work on the dwarf nova GWLibrae in which the white dwarf drives nonradial pulsations. GWLibrae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 outburst. Therefore, GWLibrae offers the opportunity to investigate the response of these modes to changes in the white dwarf temperature. Here I report the presence of a high-amplitude variability on a ' 4:4 h time-scale, which I demonstrate to be the result of an increase of the temperature of a relatively small region on the white dwarf surface. Cataclysmic variables undergoing thermal time scale mass transfer are known as super-soft X-rays sources which provide a pathway towards Supernova type Ia through the single degenerate channel. The study of these systems is very difficult due to the small number of systems known, moreover the extremely hot white dwarfs outshine their companions. In Chapter 6 I present the analysis of HS0218+3229 and QZSer which present low abundance ratios of carbon to nitrogen which is the signature seen in descendants of super-soft X-rays sources. I also present MESA simulations that constrain the parameter space for the formation of these failed supernova type Ia. Finally, I present the concluding remarks of this thesis in Chapter 7.

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