Thomas Kupfer, Radboud University Nijmegen
The evolution of compact binaries – Binary white dwarf systems as strong gravitational wave sources

Abstract: Hot subdwarf B stars (sdBs) are hot core helium-burning stars with a large fraction found in short-period binaries with white dwarf (WD) companions. During their further evolution, these systems will become compact double white dwarf binaries and due to gravitational wave radiation the orbit will shrink further where they will go into contact at an orbital period of about 5 min. Having the correct mass ratio a merger of the system is prevented and an ultracompact AM CVn type binary is formed. AM CVn systems are a small group of mass transferring ultracompact binaries evolving towards longer periods with decreasing mass accretion rates. They are important as strong low-frequency Galactic gravitational wave sources and the most compact ones will act as verification sources for upcoming space based gravitational wave detectors like eLISA/NGO.

In this talk I will report on the results of a population study of compact sdB binaries and show that the average mass of WDs in those binaries is significantly lower than found for single WDs with some being good candidates to evolve into an AM CVn type binary. Furthermore, I will discuss the latest results on some particularly interesting AM CVn type binaries and show a possible way to detect the most compact systems with orbital periods well below 10 minutes which will belong to the strongest Galactic gravitational sources in the low-frequency regime.