Tidal Effects in Double White Dwarf Binaries
November 21, 2011
Abstract: Galactic double white dwarfs (DWD) in close orbits are the most numerous and guaranteed gravitational wave (GW) sources for the next generation of space-based interferometers, sensitive to low-frequency GWs (0.1 mHz – 1 Hz). Although the majority of them are expected to be circular, theoretical calculations predict a population of detectable eccentric DWDs. Here we investigate the potential for constraining the white dwarf (WD) properties through apsidal precession in these binaries. We analyze the tidal, rotational, and general relativistic contributions to this process using detailed WD models. We find that apsidal precession in eccentric DWDs can be detectable for certain frequency regimes, independently of the age of the WD components. More importantly we find that GW data analysis would suffer from extreme bias in the determination of the source masses if the different contributions to apsidal precession are not properly taken into account. We are also able to identify a statistically unique relation among the WD properties that will allow to use apsidal precession as an additional mass measurement tool. An even ÛÏcleanerÛ source of apsidal precession are eccentric binaries hosting a neutron star and a WD, which are observed in the field typically as radio pulsars orbiting WDs. In these sources, apsidal precession carries the unique signature of the WD, and could potentially provide a testbed for validating the reliability of our WD models. Beyond apsidal precession, where the effect of tides is non dissipative, we investigate the impact of tidal dissipation on the orbital evolution of DWDs through dynamic, nonadiabatic tides. To date, the GW forms from these sources are based on the underlying assumption that the orbital evolution of DWD is driven by GW emission alone calculated for point masses. Theoretical calculations of tidally-driven orbital evolution timescales confirm the validity of this approximation in the quasi-static tide limit (a low-frequency tidal forcing regime), where tidal energy is dissipated mainly through convective damping operating near the WDs surface. Consequently, as GW emission causes the WDs to spiral in, quasi-static tides will not be able to catch up with GW driven orbital evolution and synchronize the WDs rotation with the orbital motion. Such binaries will naturally enter a high-frequency tidal forcing regime, where the quasi-static tide approximation breaks down. In this new limit, energy dissipation through dynamic tides, and resonances between the WDs eigenfrequencies and orbital frequency must be taken into account. For this purpose we are developing a code based on the Riccati algorithm, proven to be successful in the study of WD oscillations.