Astron. Astrophys. 358, 417-424 (2000)

1. Introduction

Detection of gravitational radiation from astrophysical sources will mark a breakthrough in the history of astronomy (see, e.g., Thorne 1987 and Schutz 1996). Experimental efforts to search for these space-time wrinkles have been under development for the past twenty years (Thorne 1995 , 1996). With the advent of technological improvements in several crucial aspects of the detection process we will soon be ready to turn them a physical reality (Schutz 1996, Thorne 1995, Finn & Chernoff 1993).

In particular, the Laser Interferometric Space Antenna (LISA) is designed to detect low frequency gravitational waves in the frequency range Hz, which are not possible to detect on the Earth because of seismic noise. There is a lot of very interesting astrophysical phenomena which are believed to generate GWs in the frequency band detectable by LISA, namely: formation of supermassive black holes (SMBHs), SMBH-SMBH binary coalescence, compact stars orbiting around SMBHs (in, e.g., galactic nuclei), a wide variety of binaries, such as pairs of close white dwarfs (WDs), pairs of neutron stars, neutron star and black hole binaries, pairs of contacting normal stars, normal stars and white dwarfs (cataclysmic) binaries, and pairs of stellar black holes.

Due to the fact the GWs are produced by a large variety of astrophysical sources and cosmological phenomena it is quite probable that the Universe is pervaded by a background of such waves. Binary stars of a variety of stars (ordinary, compact or combinations of them), Population III stars, phase transitions in the early Universe, cosmic strings are examples of sources able to produce a background of GWs.

As the GWs possess a very weak interaction with matter passing through it unharmed, relic radiation (spectral properties for example) once detected can provide information on the physical conditions from the era in which they were produced. In principle it will be possible, for example, to get information from the epoch when the galaxies and stars started to form and evolve.

Concerning our galaxy, it presents a large number of binary systems, which produce a GW background named binary confusion noise (see Hils et al. 1990, Bender & Hils 1997). Some of the galactic binary sources are: close white dwarfs binaries (CWDBs), neutron star binaries (NSBs), unevolved binaries, WUMs binaries and cataclysmic binaries.

The binary systems are the most understood of all sources of GWs (see, e.g., Thorne 1987). Knowing the masses of the stars, the orbital parameters and their estimated distances, one can calculate the details of the GW produced.

The LISA's sensitivity as well as the binary confusion noise will determine in the end if one is able to discriminate the signal of a particular astrophysical source.

The first papers concerning the gravitational radiation from binaries systems were written by Mironowskii (1966), who studied in particular the W UMa stars, and by Forward & Berman (1967), approximately 30 years ago. After that many other studies concerning the evaluation of GWs background produced by various types of binary stars in the Galaxy followed (see, e.g., Douglass & Braginsky 1979; Lipunov & Postnov 1987; Lipunov et al. 1987; Evans et al. 1987; Hils et al. 1990; Bender & Hils 1997; Webbink & Han 1998; Hils 1998)

Here we are particularly interested in the cataclysmic variable binaries as sources of GWs, such a system is formed by a white dwarf and a low mass secondary star. The total number of such a kind of binary is estimated to amount in the Galaxy (see, e.g. Hils et al. 1990). These systems produce low frequency GWs, namely, , which could be detected by LISA.

We are not concerned here with the calculation of a confusion noise produced by such binaries, our aim is similar to the study by Douglass & Braginsky (1979) who evaluate the dimensionless amplitude h for a series of specific low frequency GW binaries. Based mainly on the 6th edition of the catalogue of cataclysmic binaries, low mass X-ray binaries and related objects (Ritter & Kolb 1998) we have catalogued almost 160 CV systems for which it is possible to evaluate the GW amplitude. We have catalogued firstly those CVs with known distances, orbital period and masses, quantities necessary to evaluate the GW amplitude produced by such objects; secondly we have catalogued those systems for which the distances and the orbital periods are known, the masses being obtained from a mass-period relationships.

The remainder of the paper is as follows: Sect. 2 deals with the cataclysmic variables. Sect. 3 addresses the gravitational waves from cataclysmic variables. The discussion and conclusions are summarized in Sect. 4.

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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