Pre-Phase A Report - Lisa - Nasa

1.2 Low-frequency sources of gravitational radiation 27
are best understood (AM CVn, V803 Cen, CP Eri, and GP Com) are shown in Figure 1.3
as the unlabelled squares at and above 1 mHz. The frequencies are known for AM CVn
and GP Com, and are probable for the other two.
The initial conditions and evolutionary paths that produce HeCVs are not well known.
One plausible assumption [31] is that they evolved through a stage where a CO white dwarf
had formed and the secondary was burning He when it made Roche lobe contact. The
orbital period would have shortened rapidly due to gravitational radiation until it reached
a minimum of about 600 s when the secondary mass was reduced to roughly 0.2 M⊙. Later
evolution due mainly to mass transfer would be to longer periods, and the rate of period
change would become slower as the secondary mass decreased.
Hils and Bender [32] recently have estimated the HeCV signal strength under the above scenario,
with the space density of the sources normalized to an estimate given by Warner [30].
Because of the rapid evolution of these binaries before they reach the AM CVn stage with
low secondary masses, and the weakness of the signals from then on, the resulting contributions
to the rms binary signal strength as a function of frequency are fairly small.
However, the estimated number of such sources in the frequency range of interest is large,
so they fill many of the frequency bins that otherwise would be open between roughly 1
and 3 mHz. Thus a curve for the confusion noise including the HeCV contribution as well
as those from CWDBs and other binaries is given in Figure 1.3 and in later figures.
Another estimate for the HeCV space density based on a different assumption about the
nature of their progenitors has been given by Tutukov and Yungelson [33]. It considers
the helium star secondary to already be degenerate or semi-degenerate at the time of
Roche lobe contact. The resulting estimated space density of AM CVn binaries is much
higher than the estimate of Warner [30]. Until the likelyhood of the different assumptions
is better understood, the uncertainty in the contribution of HeCVs to the confusion noise
should be remembered. However, they will not contribute much at frequencies above
about 3 mHz in any of the assumed scenarios.
Normal detached binaries, contact binaries, and cataclysmic variable binaries.
These three types of binaries have been discussed in some detail [13]. By normal detached
binaries we mean binaries consisting of normal, “unevolved” stars whose Roche lobes are
not in contact. “Unevolved”, as used here, means that the stars have not yet reached a
giant phase or started helium burning. Contact binaries are the W UMa binaries studied
first by Mironowskii [10], which are two unevolved stars with their Roche lobes in contact.
A cataclysmic variable binary consist of a white dwarf which accretes mass spilling over
from a low mass hydrogen-burning secondary.
Some individual binaries of each of these types will be close enough and at high enough
frequency, so their signals will be resolvable. This includes the normal detached binary
ι Boo and the cataclysmic binary WZ Sge, which are the two lowest-frequency circles
shown in Figure 1.3 . The expected confusion limits for the W UMa and cataclysmic
binaries are comparable with the LISA noise budget level over the frequency range from
0.1 to 0.4 mHz. Thus, if the spectral amplitudes for the CWDBs andIWDBs should turn
out to be low enough, the abundance of these other types of binaries could be determined.
Corrected version 2.08 3-3-1999 9:33