Green, Matthew J. (2019) The Evolution of AM CVn Binary Systems. PhD thesis, University of Warwick.

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Abstract

AMCVn-type binary systems are a class of ultracompact, accreting binary system. Each system consists of a white dwarf accreting helium-dominated material from a compact donor star, which may be degenerate or semi-degenerate. Due to the position of these binary systems within late binary evolution, they provide an opportunity to constrain poorly understood aspects of binary evolution such as common envelope evolution and the merger rate of double white dwarf binaries (the favoured progenitors of type Ia supernovae). Despite the growing number of known AMCVn systems, details of the prior evolution of these systems remain poorly constrained, with several competing models. The objective of this thesis is to use observations of AMCVn binaries to explore their prior evolution.

I present an in-depth study of Gaia14aae, the only known fully-eclipsing AMCVn system. High-speed photometry allows me to model the system and deduce its component stellar masses and radii. Phase-resolved spectroscopy provides an independent test of the photometric results, and allows me to probe the origin of the line emission seen in many AMCVn systems. The properties of Gaia14aae make it clear that the donor is not degenerate, and suggest an unusual evolutionary origin in which the system is descended from a short-period cataclysmic variable.

I present the discovery of SDSS J1351-0643, a 16 minute orbital period binary which was observed by the K2 project. Several periods with physical origins are identified, allowing me to estimate the mass ratio of the component stars. From the literature, I compile all known mass ratio measurements and compare these with evolutionary models. The results suggest that AMCVn donor stars across the population are more inflated than has been previously expected, and that few, if any, donor stars are fully degenerate. Acknowledging the need for a greater number of well-characterised systems in order to fully explore this suggestion, I present spectroscopy of five AMCVn binaries and short-period cataclysmic variables. Four of these systems show potential to be well characterised with further observations.

The results in this thesis, in line with other recent results, suggest that the importance of the helium star donor channel and the evolved CV channel of AMCVn formation have historically been underestimated. The importance of the white dwarf donor channel has either been overestimated, or mechanisms that heat and inflate the donor have been overlooked. Future studies to find and characterise new AMCVn binaries will help to further explore these questions.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QA Mathematics Q Science > QB Astronomy
Library of Congress Subject Headings (LCSH):	Binary system (Mathematics), Astronomy, Dwarf stars
Official Date:	August 2019
Dates:	Date Event August 2019 Published
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Sponsors:	Marsh, Tom; Steeghs, Danny
Format of File:	pdf
Extent:	x, 184 leaves: illustrations, charts, plates
Language:	eng

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