Pre-Phase A Report - Lisa - Nasa

26 Chapter 1 Scientific Objectives
pass through at least one and sometimes two stages of common envelope evolution before
CWDBs are formed. However, it is not known how to calculate the loss in angular
momentum during the common evolution stages, and thus the calculated space density
of CWDBs with periods in the range of interest for LISA is more like an upper estimate
than an expected value. These estimates suggest that LISA will suffer an embarrassment
of riches from this population, so many systems that they will not be resolvable by LISA
below a few millihertz frequency, and they will obscure other, rarer systems.
Until recently, searches for this type of binary with orbital periods less than a day have
been unsuccessful, but the total number of known non-interacting CWDBs with periods
longer than 1 day jumped not long ago from two to six, and two additional ones with
periods of 3.47 hr and about 4 hr have been found [28, 29]. These last two have lifetimes
before merger of roughly a quarter of the Hubble time, and so further analysis and observations
may lead soon to better understanding of the binary population in the LISA
range.
In view of the theoretical and observational uncertainties, the detailed estimates of Hils et
al. [13] of the gravitational wave background that could be expected from this population
used a space density for non-interacting CWDBs which was a factor 10 less than the evolutionary
rough limit. These binaries give the dominat contribution to the gravitationalwave
confusion noise level shown in Figure 1.3 . The part of the curve above about 3 mHz,
where the level has dropped off sharply, is due to the estimated integrated effect of CWDBs
in all other galaxies. It should be stressed that the actual confusion noise level might be
significantly higher or lower than the level shown. LISA will be able to distinguish the
galactic binary background from a cosmological background or instrumental noise because
there will be a large number of “outliers” — binaries at high enough frequencies to be
individually resolvable. By studying the resolvable systems, one should be able to predict
what the background level is, and infer from it what the space density of CWDBs isin
other parts of the Galaxy.
The strengths of the signals from the resolvable CWDBs at the galactic centre are shown
by the solid curve with that label in Figure 1.3 . The curve rises with respect to a constant
slope curve above 15 mHz as the lighter CWDBs consisting of two He white dwarfs coalesce,
and only the heavier ones consisting of two carbon-oxygen (CO) white dwarfs are left. The
dashed curve labeled 5 % gives the rms strength for CWDBs at a distance from the Earth
such that only 5 % of those in the galaxy are closer, and the 95 % curve is defined in
a similar way. Thus 90 % of the galactic CWDBs give signal strengths between the two
dashed curves. Based on the CWDB space density assumed in calculating the binary
confusion noise estimate, roughly 5000 CWDBs would be resolvable at frequencies above
about 3 mHz.
Helium cataclysmic variables. These are systems where a low-mass helium star fills
its Roche lobe and transfers mass onto a more massive white dwarf [30]. Such systems have
close orbits that place them in the LISA frequency band. Six HeCVs within about 100 pc
of the Earth are known, and all have likely gravitational-wave frequencies near 1 mHz.
The He star in these cases has been reduced by mass transfer to a few hundredths of
a solar mass, so that the strength of the signals is quite low. These sources frequently
are called AM CVn binaries, and also interacting white dwarf binaries, even though the
He star may be only semi-degenerate. The estimated signal strengths for the four which
3-3-1999 9:33 Corrected version 2.08