Theory predicts that a large fraction of CVs should have passed through the minimum period. The Sloan Digital Sky Survey (sdss) sample is finally unearthing these systems in large numbers. But due to their faint donor stars, the orbital period is often the only measurable system parameter for most CVs. The indirect measurable of the superhump period, and hence superhump excess, could potentially provide an indication of the mass ratio of the systems via the empirical relation between the two observables. While this relation is potentially very useful for the determination of mass ratios, the large scatter in the calibrators, especially at the low mass ratio end, prohibits a direct conversion between easy to measure light curve variability and the much sought after mass ratio. To place a short period CV firmly on the evolutionary track (e.g pre- or post bounce systems), more direct methods to determine the mass ratio are required, as well as a better calibration and validation of the relation between the superhump excess and mass ratio. We can achieve this, by constraining the mass ratios of short period CVs using dynamical constraints on the radial velocities of the binary components. The radial velocity of the WD (K1) is only occasionally directly measurable as the WD features are typically swamped by the strong disc features. As the disc is centred on the WD, measuring the disc radial velocity can give an indication of the WD radial velocity, but these measures tend to be biased by hotspots and other asymmetries in the disc. Measuring the radial velocity of the donor star (K2) is less straightforward and normally performed by either measuring the radial velocity of the donor absorption lines for earlier type donor stars, or via emission lines associated with the donor star, if irradiated by the disc and WD. The first method fails in short period CVs as the faint features from the late type donors in these systems are concealed in the accretion and WD dominated optical spectrum, even at very low mass loss rates. The second method comes with tight timing constraints as the irradiated donor is generally only visible on top of the double peaked disc emission shortly after outburst and data needs to be obtained via target of opportunity programs. In this thesis, we present a spectroscopic survey of short periods CVs and explore new techniques in addition to the traditional methods for the determination of the radial velocity components. We combine these new methods with the exploitation of the more `exotic' Ca ii triplet lines in the I-band in addition to the commonly used Balmer lines. We will show that, while it suffers from some of the same systematics as the Balmer lines, we can measure K1 better in Ca ii than in Balmer, especially when exploiting Doppler maps for these measures. More importantly for many systems, donor emission is visible in the Ca ii lines, which provides us with measures for the radial velocity amplitude of this feature (Kem). These, combined with K-correction models, yield a good measure of K2. We find that the determination of Kem is easy in Doppler maps, and that the K-correction, via irradiation methods, is fairly solid. We use these values to dynamically constrain the mass ratios for 13 CVs, including several eclipsing systems to test the validity of our method. The survey includes well known systems such as GW Lib, WZ Sge, OY Car and IP Peg.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:582192 |
| Date | January 2011 |
| Creators | van Spaandonk, Lieke |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/55659/ |
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