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Title: ULTRACAM observations of interacting binaries
Author: Cabral de Barros , Susana Cristina
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2008
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Abstract:
This thesis focuses upon two different aspects of interacting binary stars, the study of the shortest period binary stars known V407 Vul and HM Cnc and the study of stochastic variability in cataclysmic variable stars. V407 Vul and HM Cnc are X-ray emitting stars with X-ray and optical light curves that are modulated on periods of 569 and 321 s, respectively. In chapter 4 we consider geometrical constraints upon the unipolar inductor model for these stars, in particular what parameter values (component masses, orbital inclination and magnetic colatitude) can describe the X-ray and optical light curves. We find that for a dipole field on the primary star, the unipolar inductor model fails to match the data on V407 Vul for any combin?tion of parameters, and can' only match HM Cnc if the sparser set of observations of this star have been unluckily timed. In chapter 5 we present optical light curves of V407 Vul and HM Cnc. The optical and X-ray light curves of HM Cnc have been reported as being in antiphase, but we find that in fact the X-rays peak around 0.2 cycles after the maximum of the optical light, as seen also in V407 Vul. The X-rayjoptical phase shifts are well explained under the accreting models of the systems if most of the optical modulation comes from the heated faces of the mass donors and if the X-ray emitting spots are positioned in advance of the mass donors, as is expected given the angular momentum of the accreting XIV l II , Supplied by The British Library - 'The world's knowledge' 1 material. Some optical emission may also come from the vicinity of the X-ray spot. and we further show that this can explain the non-sinusoidal light curves of HM Cnc. The only significant difference between the two stars is that V407 Vul is observed to have the spectrum of a G star. The variation in position on the sky of a blend of a variable and a constant star can be used as a measure of their separation. and is sensitive to values well below the limit set by seeing. We apply this 'pulsation astrometry' to deduce that .the G star is separated from the variable by about 0.027 arcsec and hence plays no role in the vadability of V407 Vul. We show that light travel time variations could influence the period change in V407 Vul if it forms a triple system with the G star. In chapter 6 we present the study of flickering. Flickering is a characteristic of accreting systems. It is thought that the maximum frequency present in an accretion disc is the dynamic frequency at the inner accretion disc radius. In cataclysmic variable stars this would appear as a break in the power spectrum on frequencies 0.01 - 3 Hz. We use the high speed CCD Camera ULTRACAM to obtain high time resolution data on 14 cataclysmic variables in the hope of seeing the expected break in their power spectrum. We did not find such a break because the power spectra of the cataclysmic variable stars observed was steeper than was expected. We measure a power spectrum proportional to j-2.5 while previous studies reported it to be proportional to j-2. We compared flickering in cataclysmic variable stars with stellar flares and concluded that they have the same colour behaviour so they have could the same origin Le. magnetic reconnection. We also compare the power spectra of cataclysmic variable stars and of X-ray binaries in the optical and concluded that the latter are much shallower than cataclysmic variable stars. We argue that this implies that flickering in X-ray transients comes from the inner accretion disc in both systems since their outer discs are thought to be similar. Supplied by The British Library - 'The world's knowledge'
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Warwick, 2008 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.491889  DOI: Not available
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