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Session 17 - CMB Radiation, Cosmology and Gravity.
Oral session, Monday, June 08
The sensitivity of the Laser Interferometer Space Antenna (LISA) for studying gravitational waves is expected to be limited over the frequency range from roughly 0.1 to 3 millihertz by confusion noise due to unresolvable signals from very many close binaries throughout the galaxy. The most important contributors to the confusion noise are expected to be of two types. One is close white dwarf binaries (CWDBs). The second is AM CVn binaries and their progenitors, which we will refer to as helium cataclysmics (HeCVs). We earlier estimated the confusion noise level due to the CWDBs within wide uncertainties due to the lack of definitive information on their space density. However, the contribution of the HeCVs was not considered because of major uncertainties about their evolutionary history.
Despite the uncertainties, it seems desirable to adopt some plausible model and include the HeCVs. We have used a model where the progenitors are assumed to consist of CO white dwarf primaries and low mass He star secondaries. This model coincides roughly with the types of models considered by Savonije et al (1986), Tutukov and Fedorova (1989) and Iben and Tutukov (1991). After the He star fills its Roche lobe and mass transfer begins, the period decreases to about 10 minutes, and then increases again. Evolution slows down as the now semi-degenerate He star reaches the mass range characteristic of AM CVn binaries, a few hundredths of a solar mass. The contributions to the confusion noise level are limited to frequencies below about 3 millihertz because of the period minimum. With the space density normalized to the rough AM CVn binary space density estimated by Warner (1995), the effect on the confusion noise level estimate at lower frequencies is rather small. However, the possibility of a higher space density should be kept in mind.
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