in Compact Object Binaries

However, it is found that with companion masses of ∼10–105 M, there exist regions in the parameter space where the white dwarf can explode before tidal disruption. During inspiration, the system will go through a series of resonances between harmonics of the orbital frequency and the white dwarf normal mode eigenfrequencies.

WDCO Binary Formation Mechanisms

Primordial Binary Evolution
Tidal Capture
Three-Body Processes
Gravitational Bremsstrahlung

This assumption, which is necessary to ensure the stability of the newly formed bound system, reveals several problems with the tidal trapping mechanism. The tidal trapping mechanism meets this requirement by relying on the internal structure of one or both bodies to provide the additional degrees of freedom.

Intermediate Mass Black Holes

These sources, which are on average ~390 pc from the optical center of the host galaxy, have inferred isotropic X-ray luminosities in the range ~1037-40 erg s-1, which, if the sources are truly isotropic, makes them very bright. to be X-ray binaries with stellar-mass black holes. But despite the fact that the interpretation of ULX observations is a subject of considerable debate, it seems fair to claim that the existence of IMBHs seems much more plausible now than in the past.

WDCO Binary Populations

It should be noted that all of the above estimates should be treated with caution, as calculations of births and merger rates for binaries of compact objects tend to be exercises in the statistics of small numbers. The disadvantage is that due to the neglected nonlinear terms, the orthogonality of the normal modes is often violated at large amplitudes.

Non-Variational Formulations

The stars were assumed not to rotate, and orbital perturbations due to tidal excitation were ignored. Ivanov & Papaloizou (2004) considered the tidal interaction of massive planets in highly eccentric orbits in the context of the evolution of the planet's orbital parameters.

Variational Formulations

The problem is formulated in terms of action angle variables of the uncoupled mode orbit system. As a compromise, we develop the formalism in terms of complex variables throughout most of the chapter.

The Lagrangian

Overview of Variational Fluid Mechanics

There is great freedom in the choice of labeling coordinates, but it is practical to choose them so that they are related to the density of the liquid at. We can also include mass conservation (4.2) as a constraint in the Eulerian form (4.5) of the Lagrangian.

Homentropic Potential Flow

From our construction method it follows that all homentropic potential flows can be derived from (4.14) with φ as the velocity potential. Therefore, we can use (4.14) for problems involving non-rotating, homentropic stars, such as cold white dwarfs.

Equations of Motion and Normal Modes

Equations of Motion

Together with the equation of state for the fluid, (4.19)–. To determine the equations for the perturbations, we consider the variations of L with respect to φ1, ρ1 and Ψ1. The equation for the orbit, including the back-feedback of the perturbations, is obtained by taking into account the variation of L with respect to R:. where we used the fact that Z. this is equivalent to choosing the origin of the coordinatevex as the center of mass of the fluid).

Normal Modes

Note that with the gradient (4.23) it is easy to see that the normal modes ˆξn`m satisfy. It can be shown that the operator D is Hermitian with respect to mass (Chandrasekhar, 1964; Cox, 1980).

Displacement Formulation

It is convenient to rewrite the overlap integral fj by performing the integration by parts and using (4.24):. In terms of coordinates in the plane of the orbit, this can be written as

Conservation Laws

The energy and angular momentum associated with the normal modes also take relatively simple forms in terms of the time-dependent displacements xj. By performing an integration by parts, we get we find that the angular momentum associated with the mode is right. 4.61).

Summary

Simple Harmonic Oscillator
White Dwarf Oscillations
Gravitational Radiation
Equations of Motion

Note that it must be true that the oscillator energy cannot be negative. In other words, the driver can be considered harmonic with a well-defined frequency over several periods of the oscillator.

Table 5.1: Homogeneous, cold white dwarf models with µ e = 2, and properties of their quadrupolar f -modes.

Physical Considerations

The Tidal Limit
Importance of the ` = m = 2 f -Mode
Mode Damping
Time-Scales

The nonlinear evolution of large amplitude modes on a white dwarf is the subject of Part III. Finally, the third time scale of interest is the cooling time of the white dwarf. estimate for this is given by.

Resonant Energy Transfer

We see that the energy transfer decreases monotonically (with respect to the binding energy of the star) with mass. Since Ξjk(e)∝ e2(k−m), to the lowest order of eccentricity, energy transfer is usually a very sensitive function of eccentricity.

Discussion

Regime of Validity

This scenario suggests the possibility that feedback can force the resonant energy transfer to always be positive. In summary, feedback can be important in determining the magnitude and direction of resonant energy transfer.

Figure 5.2: The energy in the ` = m = 2 f -mode on a 0.6 M white dwarf is shown for a passage through the k = 15 resonance with different values of the parameter χ jk obtained by varying the eccentricity, and with q = 10, 000

Long-Term Evolution

We note that, in the irreversible reaction approximation, the energy transfer at a resonance can be both negative and positive, depending on the relative phase of the mode and the driver, and the initial amplitude. It was speculated that the inclusion of orbital perturbations from excited tides (back-reaction) could lead to qualitatively different results, even in the regime where back-reaction is not expected to significantly affect the magnitude of the energy transfer.

Figure 5.3: The regions in eccentricity-harmonic space where back reaction is and is not important (labelled as ‘BR’ and ‘No BR’, respectively) are delineated according to the χ jk criterion for a ` = m = 2 f -mode of a 0.6 M white dwarf, and various mass

The Hamiltonian Formalism

Two Elementary Systems

It remains to quantify the change in the action variable upon crossing the separatrix, and to determine the relationship between the action variable and the mode energy. Since the action variable can be shown to correspond asymptotically to the mode energy (within a scale factor), it follows that the energy transfer is always positive.

Resonant Tidal Excitation

Hamiltonian in terms of new variables is. where the uncoupled part is given by H0 =− q3. and the connecting part is given by H1 =X. Conservation of energy corresponds to the fact that the Hamiltonian itself is also an integral of motion.

Specialization to a Single Mode

Since G and L are integrals of the motion, the orbital degrees of freedom are completely decoupled from the modes, and the system is effectively reduced to two degrees of freedom for the motion of the mode variables. With this perspective, G and L are parameters of the two degrees of freedom system described by (6.31).

The Dynamics

Fixed Points
The Invariant Sub-Manifold
Approximate Trajectories
Action-Angle Variables
Gravitational Radiation

After distributing the coefficient of Φ2 (which corresponds to a choice of units), the Hamiltonian becomes. When Φ 1, the Φ2 term in the Hamiltonian can be neglected and the trajectory of the system obeys.

Figure 6.1 shows a sample phase portrait for the Hamiltonian (6.42) as a function of the canonical coordinates:

Resonant Energy Transfer

Resonances as Separatrix Crossings
Change in Adiabatic Invariant at a Separatrix Crossing
Energy Transfer at a Tidal Resonance
Orbital Evolution

Thus, if the system crosses the dividing line from region A to region C, the start and end values of the action variable are linked. More generally, to find δs we need to know when the energy of the system is equal.

Figure 6.3: A phase space trajectory showing a passage through the k = 15 resonance for the ` = m = 2 f-mode of a 0.6 M white dwarf in a system with q = 1000, and an initial eccentricity of 0.4

Discussion

Regime of Validity

In a non-rotating star the resonances of the quasi-static modes overlap with those of the higher m modes (and thus our results are not useful), but in a rotating star the resonances will be separated. A key assumption underlying our energy transfer calculation is that the adiabatic approximation is valid away from the separatrix.

Long-Term Evolution

To determine the extent and probability of such a scenario, it is necessary to understand the mode generation process in detail. Additionally, since we are restricting ourselves to adiabatic flow, it is convenient to use the entropy rather than the energy as a thermodynamic variable.

Differencing Scheme

Advection

Donor Cell Upwinding
van Leer Upwinding
PPA Upwinding

Artificial Viscosity
Momentum Source Terms
Courant-Friedrichs-Lewy Time Step
Boundary Conditions
Parallelization

In terms of the van Leer slopes, the inverse of the quantity at the cell boundary is given by Therefore, we take the "zero" density to be a small fraction (typically, 10-8) of the maximum initial density.

Figure 7.1: The geometry of a zone-centered, uniform Cartesian grid is shown. Here, λ can be any of the five evolved quantities (ρ, s, and J ) or the gravitational potential (Φ).

Solving the Poisson Equation

The DST is most efficiently parallelized in terms of a slab decomposition of the grid, as opposed to the ideal decomposition for the obtained advection step (which is cubic). As a result, a significant amount of interprocess communication is required to prepare for solving the Poisson equation at each source substep.

Figure 7.2: A square pulse that has been advected five times its initial width (50 cells) using the donor cell (open circles), van Leer (filled triangles), and PPA (open squares) upwinding schemes

Test Problems

Advection

To quantify these errors for diffusion due to the winding scheme, a sine wave was advected with periodic boundary conditions for 100 times its wavelength. In both the square pulse and the sine wave, a noticeable asymmetry (which is determined by the direction of propagation) develops as a result of higher order effects in the winding schemes.

Sod Shock Tube

By this time, the winding scheme of the donor cell has completely diffused the sine wave, so only Van Leer's and PPA's methods are shown in Figure 7-3. Norman (1992) this is a real result, arising from the numerical viscosity inherent in any finite difference code.

Pressure-Free Collapse

Application to a Pulsating White Dwarf

Hydrostatic Equilibrium

Note that the inclusion of the isothermal mantle does not change the mass appreciably, while the radius increases significantly. The new gravitational potential and the initial density guess are then used to calculate the Bernoulli constant at the center of the star.

Figure 7.5: The numerical (open circles) and analytical (solid line) solutions for the density as a function of distance along a radial section for the pressure-free collapse of a uniform density sphere are shown

Oscillation Modes

In the power spectrum of the even quadrupole moment m= 2, there is a peak which extends five orders of magnitude above the rest of the spectrum. Therefore, we face the problem of distinguishing the physical effects from the by-products of the numerical scheme.

Figure 7.8: Same as Figure 7.7 for the case when a quadrupolar perturbation is present (note the difference in scales in comparison to that figure)

Resonant Excitation of Modes

Progenitors

Here, the initial orbit of the system will be highly eccentric, and its evolution is somewhat uncertain (cf. the discussion in Section 1.1.2). In principle, the formalism of Press and Teukolski (1977) can be used to calculate the initial evolution of the orbit assuming that the oscillations are damped on the orbital time scale.

Tidal Heating: Bombs vs. Duds

In this way, we can identify trajectories that are potentially suitable for the detonation of a white dwarf. It is immediately apparent from Figure 9.2 that regardless of the mass of the white dwarf, tidal detonation with resonant excitation of f-modes is not possible with an accompanying mass of 1.4 M or less, as the required temperature rise cannot be achieved before tidal disruption.

Table 9.1: The binding energies (B.E.) and heat capacities (C V ) for several Chan- Chan-drasekhar white dwarfs, assumed to be equal carbon-oxygen mixtures

Detonation and Aftermath

Using this criterion, the conditions for the exhaust to be trapped are:. For the limiting case, β = 1, the resulting conditions for the orbital period at the time of detonation are for the exhaust to remain bound.

Figure 9.3: The orbital period as a function of eccentricity for gravitational inspiral.

Comments and Caveats

The formalism developed in Chapter 6 assumes that the mode amplitude is in the linear regime. Of the companion masses we have considered, 103 and 105 M are purely speculative (except perhaps for some galaxies that have black holes at their centers with masses several times greater than 105 M).

Gravitational Wave Sources

Resonant excitation of f-modes on the white dwarf turned out to be unimportant in this context due to the large orbital periods of the last stable orbits. The exact size of the errors depends on the white dwarf model and orbital parameters.