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Title: Estimating the space density of ultracompact binary stars
Author: Carter, Philip J.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2014
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The AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultrashort period, mass-transferring white dwarf binaries, some of which may be Type Ia supernova progenitors. These systems represent the end product of several finely-tuned evolutionary pathways, and as such are of great interest for binary stellar evolution theory. They are also some of the strongest known sources of low-frequency gravitational waves. Establishing their space density is important for constraining evolutionary models, and the signals we expect to detect from the Galactic population. This has been difficult as much of the known population has been discovered in a heterogeneous manner. In this thesis I present the latest results from a spectroscopic survey designed to uncover the hidden population of AM CVn binaries in the photometric database of the Sloan Digital Sky Survey (SDSS). This small, colour-selected sample of ∼2000 candidates, is expected to contain the majority of all AM CVn binaries in the SDSS. The survey is now approximately 70 per cent complete, and the discovery of only seven new AM CVn binaries indicates a lower space density than previously predicted. Characterisation of the sample requires detailed follow-up observations after the initial discovery, in order to determine the orbital period. I present time-resolved spectroscopy of the recently-discovered AM CVn binary SDSS J173047.59+554518.5, and the dwarf nova SBSS 1108+574. The orbital period I measure for SDSS J1730 confirms its ultracompact binary nature. The quiescent spectrum of SBSS 1108+574 is unusually rich in helium compared to typical cataclysmic variables (CVs). I find that its orbital period is significantly below the normal CV period minimum. This indicates that the donor in SBSS 1108+574 is highly evolved, making it a candidate for the often-dismissed ‘evolved CV’ channel for AM CVn binary formation. I discuss a wider search for new AM CVns in the SDSS spectroscopic database, and present two new systems discovered in SDSS-III spectroscopy. Since this search has revealed only these two new systems, it is unlikely that we have missed a large population of AM CVn binaries, and their discovery should have little effect on our previous calculations of the AM CVn space density.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy