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Abstract:

Multiple systems play an important role in the evolution of star clusters. First we discuss several formation mechanisms which depend on the presence of binaries, either primordial or of dynamical origin. Hierarchical configurations are often stable over long times and yet may experience evolution of the internal orbital parameters. We describe an attempt to model the eccentricity change induced by the outer component using an averaging method, together with the effects due to tidal dissipation and apsidal motion acting on the inner binary. This treatment is adopted for systems with high induced eccentricity which gives rise to some interesting outcomes of significant period shrinkage.

Abstract:

We have systematically analyzed the evolution of a wide variety of binary systems that ultimately lead to the formation of detached or semi-detached helium degenerate dwarfs (HeDD's). Specifically, we have explored some of the more important dimensions of parameter space including various: (i) metallicities; (ii) mass-transfer rates; and, (iii) states of evolution of the donor at the onset of mass transfer (equivalent to choosing different initial orbital separations). As has been shown in other papers, there is a sharp bifurcation in the evolutionary phenomena that lead to the formation of HeDD's. If the donor star is sufficiently evolved at the onset of mass transfer, then the binary will ultimately evolve to long orbital periods and the HeDD will detach from its Roche Lobe and cool indefinitely. In the other case, the donor continuously loses mass as the binary evolves to very short orbital periods. By examining the evolutionary results for a comprehensive range of starting conditions, the initial properties of HeDD's that undergo cooling can be determined. This is especially important since HeDD's initially cool very quickly and are most likely to be detected when they are relatively luminous. Moreover, many HeDD's are observed as companions in binary millisecond pulsar (BMSP) systems, and thus an accurate evaluation of their initial properties (e.g., surface temperature and luminosity) is central to the estimation of the ages of these systems.

Abstract:

Of the several processes that have been suggested for the formation of binary systems - fragmentation, fission, capture, and collision-induced processes - fragmentation during the collapse of a rotating proto-star appears to be a key ingredient. Numerical simulations of this process do indicate that under certain conditions fragmentation does occur; however the simulations so far have not reached the point where they can explain the statistical properties of observed binary and multiple star systems.

Abstract:

Studies of the observed characteristics of black-hole (BH) X-ray binaries can provide us with valuable information about the process of BH formation. In this paper I address some of the aspects of our current understanding of BH formation in binaries and point out some of the existing problems of current theoretical models. In particular, the measured orbital periods and donor-star properties indicate that a common-envelope phase appears to be a necessary ingredient of the evolutionary history of observed BH X-ray transients, and that it must be associated only with a modest orbital contraction. The timing of this common-envelope phase is crucial in determining the final BH masses and current evolutionary models of mass-losing massive stars place strong constraints on the possible masses for immediate BH progenitors and wind mass loss from helium stars. Last, it is interesting that, even in the absence of any source of mass loss, the highest helium-star masses predicted by current evolutionary models are still not high enough to account for the measured BH mass in V404 Cyg (> 10 Mcircle dot). An alternative for the formation of relatively massive BH may be provided by the evolutionary sequence proposed by Eggleton & Verbunt (1986), which invokes hierarchical triples as progenitors of BH X-ray binaries with low-mass companions.

Abstract:

Using Eggleton's stellar evolution code, we carry out 150 runs of Pop 1 binary evolution calculations with initial primary mass between 1 and 8M(circle dot), the initial mass ratio q = M-1/M-2 between 1.1 and 4, and the onset of Roche lobe overflow (RLOF) at an early, middle, or late phase in the Hertzsprung gap. We assume that the RLOF is conservative in the calculations and find that the remnant mass of the primary may change by more than 40 per cent over the range of initial mass ratio or orbital period for a given primary mass. This is contrary to the often held belief that the remnant mass depends only on the progenitor mass if mass transfer begins in the Hertzsprung gap. We fit a formula, with an error less than 3.6 per cent, for remnant (i.e., white dwarf) mass as a function of the initial mass M-li of the primary, the initial mass ratio q(i), and the radius of the primary at the onset of RLOF. We also find that a carbon-oxygen white dwarf with mass as low as 0.33M(circle dot) may be formed if the primary's initial mass is around 2.5M(circle dot).