White dwarfs, black holes and neutron stars in close binaries

Gijs Nelemans

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PhD thesis, University of Amsterdam. March 2001
Supervisors: Ed van den Heuvel (University of Amsterdam), Frank Verbunt (Utrecht University)


In this thesis some aspects of the formation and evolution of binaries containing white dwarfs, black holes and neutron stars are investigated.

In the first part the formation of observed single undermassive white dwarfs and double helium white dwarfs is studied. I conclude that the formation of single undermassive white dwarfs can be explained by the evolution of `binaries' consisting of a solar-like star and a massive planet or brown dwarf. A tidal instability causes the low-mass companion to be drawn into the solar-like star when it evolves up the giant branch, expelling the giants envelope, leaving a single undermassive white dwarf. I reconstructed the evolution of three observed double helium white dwarfs, using the unique core mass -- radius relation for giants with degenerate cores, to find the pre-mass-transfer orbital separations and came to the following conclusions: (i) The last mass-transfer phase can be described with the spiral-in formalism with high common-envelope efficiency. (ii) The first mass-transfer phase cannot be described by a spiral-in nor by stable mass transfer but can be described very well with a formalism based on the angular momentum balance, with one free parameter which for the three observed systems has a very similar value.

Further, I calculated the current population of close double white dwarfs and interacting white dwarfs (AM CVn stars) and concluded: (i) The recently proposed cooling curves for helium white dwarfs overestimate the luminosity for the lowest mass helium white dwarfs (ii) The fraction of double white dwarfs among all white dwarfs can only be brought into agreement with observations if the initial binary fraction is not above 50 % (iii) The model with an exponentially decaying star formation rate gives a slightly better fit to the observed period distribution for double white dwarfs than a constant star formation rate. For the AM CVn stars I conclude that in order to distinguish between different models and formation channels both the theory of helium accretion disks and the homogeneity and completeness of the observations (particularly regarding the distances to the AM CVn stars) need to be improved. I already started this by reducing and analyzing high-speed spectroscopic data of AM CVn itself and found, for the first time, a clear direct signature of the binary nature of AM CVn in its spectrum.

The study of black hole binaries led to two conclusions. The first is that the observed space velocities of black hole binaries imply that in the supernova in which the black hole was produced, some 30 -- 50\% of the mass of the exploding helium star was ejected from the system if the explosion was symmetric. The second is that the mass-loss rates for for Wolf-Rayet stars currently used in stellar evolution calculations still overestimate the mass loss, yielding very low masses for massive stars when they explode. A mass-loss law more in agreement with the observed values significantly increases these final masses, improving the possibility for the formation of black holes in binaries.

Finally I describe the population of binaries consisting of two compact objects, either white dwarfs, neutron stars or black holes and use these to calculate the unresolved noise background produced by double white dwarfs and calculate the population of resolved binaries and binaries with signals sufficiently strong that they may be detected above the noise level for the low-frequency gravitational wave detector in space (LISA).

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