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AM CVn stars

AM CVn stars are hydrogen-poor cataclysmic variables: binaries in which hydrogen-poor material is transferred to a white dwarf accretor. This means that the donor star is also a compact object: a white dwarf or (low-mass) helium star. The binary itself is also very compact; orbital periods are usually an hour or less, and may be as short as a few minutes.

We are finalising our population-synthesis code which can evolve a binary through two subsequent common-envelope phases. If the binary survives such evolution, it will have a short orbital period and consist of either white dwarfs or low-mass helium stars. These binaries are thought to evolve into AM CVn systems under the influence of angular-momentum loss due to the emission of gravitational radiation.

We have detailed plans to use the population of detached compact binaries that comes out of our double-common-envelope population-synthesis code as an input for the evolution code which is being developed by Dr. Deloye at Northwestern University. This code is supposed to follow in detail the evolution of these binaries through the AM CVn phase.

In this study, we hope to gain more insight in the questions concerning these binaries. For example, it seems that the number of observed AM CVn stars is about ten times smaller than predicted by theoretical models. In addition, these models predict that some of the donors in AM CVn stars should be 'hot', whereas others should be 'cold'. However, only hot donors are observed. Thus the question arises whether all observed donors are helium stars and hence there are no white-dwarf donors, or whether other explanations must be found. If the population of these binaries and perhaps their progenitors, such as the double white dwarfs is indeed different than previously thought, this could have strong consequences for e.g. the number of systems that can be observed by the LISA mission.