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A significant number of X-ray binaries are now known to exhibit long-term periodicities on timescales of ~10 - 100 days. Several physical mechanisms have been proposed that give rise to such periodicities, one of which is radiation-driven warping and precession of the accretion disk. Recent theoretical work predicts the stability to disk warping as a, function of the mass ratio, binary radius, viscosity and accretion efficiency. We investigate the stability of the superorbital periodicities in the neutron star X-ray binaries Cyg X-2, LMC X-4, SMC X-l and Her X-l, and thereby confront stability predictions with observation. We find that the period and nature of the superorbital variations in these sources is consistent with the predictions of warping theory.
Making use of a new N-body model to describe the evolution of a moderate-size globular cluster, we investigate the characteristics of the population of black holes within such a cluster. This model reaches core-collapse and achieves a peak central density typical of the dense globular clusters of the Milky Way. Within this high-density environment, we see direct confirmation of the merging of two stellar remnant black holes in a dynamically formed binary, a gravitational wave source. We describe how the formation, evolution, and ultimate ejection/destruction of binary systems containing black holes impacts the evolution of the cluster core. Also, through comparison with previous models of lower density, we show that the period distribution of black hole binaries formed through dynamical interactions in this high-density model favours the production of gravitational wave sources. We confirm that the number of black holes remaining in a star cluster at late times and the characteristics of the binary black hole population depend on the nature of the star cluster, critically on the number density of stars and by extension the relaxation timescale.
At least 60% of stars appear to be binary and about half of these are close enough to interact. Because of the enormous expansion on the AGB, many of these interactions will involve an AGB star and a relatively compact companion, anything from a low-mass main-sequence star to a degenerate remnant. Mass loss plays the dominant role in determining the lifetime and the extent of nuclear processing of the AGB phase. Binary interaction will increase the mass loss from the AGB star and curtail its evolution, either through Roche-lobe overflow, common-envelope evolution or the driving of an enhanced stellar wind. These processes will tend to reduce the metals, particularly carbon, returned to the inter-stellar medium. On the other hand merged systems or companions that accrete a substantial amount of mass themselves evolve into AGB stars that can synthesize and return more carbon than the two individuals would have alone. By synthesizing large populations of stars, with nucleosynthesis and binary interaction, we estimate a reduction in carbon yield owing to binary star evolution of as much as 15%.
The progenitors of cataclysmic variables (CVs) undergo a common envelope phase, during which their initial orbital period is reduced to a few hours. After this phase the radiative core of the secondary might rotate at a lower rate than the tidally coupled envelope. A shear is then generated at the core-envelope interface which can give rise to a boundary layer dynamo and, consequently, to the magnetic braking of the binary system. The temporal variation of the shear energy content can be obtained by comparing the rate of magnetic energy production with the input rate of orbital energy in the convective envelope. We assume for the flux tubes a filling factor, fV, owing to the expected size of the tubes and the fact that the dynamo action may be concentrated inside an equatorial belt of the boundary layer shell. We also assume that the production of toroidal field is efficient only inside the boundary layer, while the poloidal component of the field can be enhanced also across the outer convective envelope (see also Zangrilli & Bianchini 1995). We find that the decrease in the shear energy, caused by the continuous production of magnetic field and by the progressive reduction of the core moment of inertia, leads the core coupling with the envelope, thus switching off the dynamo at the core-envelope boundary. Further details of this core-envelope decoupling dynamo model will be given in a forthcoming paper (Zangrilli, Tout & Bianchini 1996).
This work is concerned with binary systems that we call ‘moderately close’. These are systems in which the primary (by which we mean the initially more massive star) fills its Roche lobe when it is on the giant branch with a deep convective envelope but before helium ignition (late case B). We find that if the mass ratio q(= M1/M2) < qCrit = 0.7 when the primary fills its Roche lobe positive feedback will lead to a rapid hydrodynamic phase of mass transfer which will probably lead to common envelope evolution and thence to either coalescence or possibly to a close binary in a planetary nebula. Although most Algols have probably filled their Roche lobes before evolving off the main-sequence we find that some could not have and are therefore ‘moderately close’. Since rapid overflow is unlikely to lead to an Algol-like system there must be some way of avoiding it. The most likely possibility is that the primary can lose sufficient mass to reduce q below qcrit before overflow begins. Ordinary mass loss rates are insufficient but evidence that enhanced mass loss does take place is provided by RS CVn systems that have inverted mass ratios but have not yet begun mass transfer. We postulate that the cause of enhanced mass loss lies in the heating of the corona by by magnetic fields maintained by an α-ω dynamo which is enhanced by tidal effects associated with corotation. In order to model the the effects of enhanced mass loss we ignore the details and adopt an empirical approach calibrating a simple formula with the RS CVn system Z Her. Using further empirical relations (deduced from detailed stellar models) that describe the evolution of red giants we have investigated the effect on a large number of systems of various initial mass ratios and periods. These are notable in that some systems can now enter a much gentler Algol-like overflow phase and others are prevented from transferring mass altogether. We have also investigated the effects of enhanced angular momentum loss induced by corotation of the wind in the strong magnetic fields and consider this in relation to observed period changes. We find that a typical ‘moderately close’ Algol-like system evolves through an RS CVn like system and then possibly a symbiotic state before becoming an Algol and then goes on through a red giant-white dwarf state which may become symbiotic before ending up as a double white dwarf system in either a close or wide orbit depending on how much mass is lost before the secondary fills its Roche lobe.
The WGARG was created in 2001 to oversee the rapid growth of the quantitative determination and understanding of the abundance patterns seen in red-giant stars. As the field progresses we are regularly reminded of how broad and multi-disciplinary is this area of research.
The main activity of the WG on Abundances in Red Giants has been to propose a JD for the IAU GA in 2009. The increasing evidence for distinct populations within globular clusters is leading to the view that there is a continuum between globular clusters and the smallest of the galaxies. Our JD was designed to investigate this link. However, our JD was incorporated into IAU Symposium No. 266 Star Clusters: Basic Building Blocks throughout Time and Space for the IAU XXVII in Rio de Janeiro, 2009. We will be responsible for organising one session in the Symposium to cover the agenda put forward in our JD proposal.
The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.
Unfortunately the Business Meeting clashed with interesting sessions on stellar convection theory that were very relevant to most members of this Working Group. Hence the attendance was very small, and some preliminary discussions were later followed up by email among the Organising Committee members.
We present a summary of the extensive problems posed by the
curious abundance correlations seen in stars in Galactic globular clusters.
We discuss three scenarios, and conclude that only one seems to be
consistent with the observations. Even this is still rather unsatisfying
from many viewpoints. We determine the kind of nucleosynthesis required
and discuss AGB stars as the possible source. Some calculations seem to
rule out AGB stars and others seem to favour them. The reasons for
the differences are discussed. No satisfying conclusion is reached!
Binary stars are abundant. We describe their orbits and the effects of
tides and introduce the concept of the Roche lobe and mass transfer.
We illustrate binary star evolution with Algols, cataclysmic variables
and type Ia supernovae as examples.
Determining and understanding the abundances seen in red-giant stars has taken a central role in our understanding of many branches of modern astrophysics. Activity in the area continues apace, both in terms of the fundamental physics of the stellar nucleosynthesis as well as its implications for wider fields. A major role of the Working Group has been to facilitate meetings where the fundamental role of these stars can be further understood and exploited by other researchers.
We examine the effect of binary companions and magnetic fields on
stellar evolution. Today our understanding of the former is good
though far from complete. We single out common envelope evolution as
the least satisfactory and illustrate the effects of our lack of
knowledge on the population synthesis of type Ia supernovae. We
currently fare much worse with magnetic fields and so concentrate on
what we do know and where this breaks down. Combining both aspects we
examine what role magnetic fields might play in common envelope
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