Recent three-dimensional studies of the common-envelope phase of binary evolution have provided important insights into its theoretical description. The role of non-axisymmetric effects associated with gravitational torques is essential for understanding all aspects of the evolution. For successful ejection of the common envelope and survival of the remnant compact binary it is required that the orbital period of the progenitor system is long, so that one of the components of the system is in the red giant or red supergiant stage of evolution. Not only must there be sufficient energy released from the orbit to unbind the common envelope, but it is also necessary that a sufficiently steep density gradient exist above the evolved core of the giant. If these conditions are satisfied, the time scale for orbital decay in the region above the core exceeds the time scale for mass loss from the common envelope and merger is avoided. The implications of these results for the formation of cataclysmic variables (CVs), low-mass X-ray binaries (LMXBs), and the descendants of high-mass X-ray binaries (HMXBs) are discussed.

In addition to their possible relevance to gamma-ray bursts, coalescing binary neutron stars have long been recognized as important sources of gravitational radiation that should become detectable with the new generation of laser interferometers such as LIGO. Hydrodynamics plays an essential role near the end of the coalescence when the two stars finally merge into a single object. The shape of the corresponding burst of gravitational waves provides a direct probe into the interior structure of a neutron star and the nuclear equation of state. The interpretation of the gravitational waveform data will require detailed theoretical models of the complicated three-dimensional hydrodynamic processes involved. Here we review the results of our recent work on this problem, using both approximate quasi-analytic methods and large-scale numerical hydrodynamics calculations on supercomputers. We also discuss briefly the coalescence of white-dwarf binaries, which are also associated with a variety of interesting astrophysical phenomena.

Thorne-Żytkow objects (TŻOs) are red supergiants with neutron cores. The energy source in TŻOs with low-mass envelopes (≲8 M⊙) is accretion onto the neutron core, while for TŻOs with massive envelopes (≲14 M⊙) it is nuclear burning via the exotic rp process. TŻOs are expected to form as a result of unstable mass transfer in high-mass X-ray binaries, the direct collision of a neutron star with a massive companion after a supernova or the collision of a neutron star with a low-mass star in a globular cluster. We estimate a birth rate of massive TŻOs in the Galaxy of ∼2 10−4 yr−1. Thus, for a characteristic TŻO lifetime of 105–106 yr there should be 20–200 TŻOs in the Galaxy at present. These can be distinguished from ordinary red supergiants because of anomalously high surface abundances of lithium and rp-process elements, produced in the TŻO interior. The TŻO phase ends when either the star has exhausted its rp-process seed elements or the envelope mass decreases below a critical mass (∼14 M⊙). Then nuclear burning becomes inefficient and a neutrino runaway ensues, leading to the dynamical accretion of matter near the core onto the neutron star and its spin up to spin frequencies of up to ∼100 Hz. The fate of the massive envelope is not entirely clear. If a significant fraction can be accreted onto the core, the formation of a black hole becomes likely. Part of the envelope may collapse into a massive disk which may ultimately become gravitationally unstable and lead to the formation of planets or even low-mass stars. We discuss the various possible outcomes and suggest a possible link between massive TŻOs and soft X-ray transients.

I review models that try to explain dwarf nova outbursts and soft X-ray transients. The disc instability model for dwarf novae is still in a preliminary state of development: its predictions depend very strongly on the unknown viscosity mechanism. It is also doubtful that a pure disc instability phenomenon will be able to describe all types of dwarf nova outbursts, in particular superoutbursts. The disc instability model for SXTs suffers from the same difficulties but in addition its predictions are contradicted by observations of transient sources in quiescence. The mass transfer model, incorporating illumination of the secondary, cannot describe correctly the time scales of SXT events for main-sequence secondaries with masses less than 1 M⊙. The existence of at least three systems with Porb < 10 hr seems to rule it out as an explanation of the SXT phenomenon.

In this symposium, I have been given the task of summarizing our current understanding of the evolutionary history of spin periods of the neutron stars that we now see as binary and millisecond pulsars, i.e., recycled pulsars. We believe that a newborn, fast-spinning neutron star (with a rather high magnetic field ∼1011–1013 G) in a binary system first operates as a spin-powered pulsar, subsequently as an accretion-powered pulsar when accretion begins after the pulsar has been spun down adequately, and finally as a spin-powered pulsar for the second time after having been recycled to become a very fast-rotating neutron star (with a rather low magnetic field ∼108–1011 G) (see Ghosh 1994a, b, hereafter G94a, b).

A semi-analytic model for the reaction of a low-mass star to anisotropic irradiation of low incident flux is presented. By applying this model to the donor star of cataclysmic binaries (CBs) it is shown that CBs are likely to be unstable against irradiation-driven runaway mass transfer. The implications of this instability for the long-term evolution of CBs are examined. The possibility is discussed that because of this instability CBs evolve through a limit cycle in which phases of high and low mass transfer rate alternate on a time scale short compared to the evolutionary time scale.

Possible models for the matter source inside the eclipsing binary system PSR 1718–19, and for the evolution of this system are reviewed, including Zwitter's (1993) stripped main-sequence (MS) turnoff star model. Both the accretion induced collapse (AIC) scenario with a young neutron star, and the capture scenario with an old neutron star are discussed. Although Burderi & King (1994) claim that the size of the Roche lobe (∼0.5 R⊙) unambiguously rules out the AIC formation scenario, we show that in our evolutionary picture an AIC scenario will be possible.

A review is given concerning the current state of the theory of evolution of magnetic compact stars. The intrinsic evolution of the magnetized compact star is shown, both theoretically and numerically, to be the decisive factor in explaining observable properties, and in predicting yet unknown properties of high-energy radiation sources in our and other galaxies. The main results are given of recent evolutionary scenario simulations (Scenario Machine) by the Monte-Carlo method.

The presence of accreting black holes (BH) among the X-ray binaries has been recognized for many years. Traditionally, Cyg X-1 and the handful of other candidates have been thought of as cousins of the HMXB neutron star systems. Recent studies of the soft X-ray transients such as A 0620-00 have, however, shown that the dynamical evidence makes these low-mass systems very strong black-hole candidates. Further, analysis of the eventual end-states of various high-mass X-ray binaries suggest that some could end as observable BH-pulsar binaries, although the first such system is yet to be discovered.

The present understanding of type Ib/c supernovae and their connection to interacting binaries is reviewed. The problems of the classification and the lack of well-observed events exclude direct inference of progenitor characteristics. The absence of hydrogen lines in the observed spectrum, nevertheless, requires restricted evolutionary schemes to produce suitable progenitor stars for core collapse explosions with no hydrogen envelope. New relative statistics among the supernova types are presented which indicate that SN Ib/c are on average brighter than SN II, and with the dense sampling of supernova searches in nearby galaxies, a small intrinsic incidence of SN Ib/c is determined. The small rates might be in conflict with the observed ratio of massive stars in binaries in the Galaxy.

Supernovae are classified as type I and type II and further subdivided into Ia, Ib, Ic, II-P, II-L, and IIb. The origin of this observational diversity has not been well understood. The recent nearby supernovae SN 1993J and SN 1994I have provided particularly useful material to clarify the supernova — progenitor connection. For a progenitor of type IIb supernova 1993J, we propose that merging of two stars in a close binary is responsible for the formation of a thin H-rich envelope. As a progenitor of type Ic supernova 1994I, we propose a bare C+O star that has lost both its H and He envelope after a common-envelope phase. By generalizing these scenarios, we show that common-envelope evolution in massive close binary stars leads to various degrees of stripping off of the envelope of a massive star. This naturally leads to an explanation of the origin of type II-L, IIn, IIb, Ib, and Ic in a unified manner. The binary hypothesis to explain the diversity of supernovae can be substantiated with new information on SN IIb 1993J and SN Ic 1994I. Model light curves are compared with observations. Since extensive mass loss is essential for the binary scenario, circumstellar interactions are examined for comparison with X-ray observations.

Up to about 1985, supernovae (SNe) generally were placed into the two Minkowski classes, type I and type II, defined by the absence or presence, respectively, of hydrogen in their optical spectra. Around that time it was acknowledged that several type I SNe were systematically peculiar, both spectroscopically and photometrically (Elias et al. 1985; Wheeler & Levreault 1985; Uomoto & Kirshner 1985; Branch 1986; Filippenko 1986), by missing the characteristic Si II spectral feature near 6150 å, having distinct infrared light curves, being optically redder and subluminous, and showing radio emission (Sramek et al. 1984). These SNe were designated as type Ib (Elias et al. 1985; Branch 1986) to distinguish them from the classical type Ia. Harkness et al. (1987) identified He I lines in spectra of the SN Ib 1984L, but some subsequent examples showed no He in their spectra and were further subclassified as Type Ic (Wheeler & Harkness 1990). The two subtypes, however, are nearly indistinguishable at late times. In this Symposium the entire class has been referred to as type Ib/c SNe. A recent bright example is SN 1994I in M51 (Filippenko et al. 1994).

The presence of a close binary companion can affect the evolution of a massive star through one or more episodes of mass transfer, or by merger in a common-envelope phase. Monte Carlo calculations indicate that ∼20–35% of all massive supernovae are affected by such processes, and that a substantial fraction of these events will be supernovae of type II. The properties of the progenitor star, the distribution of circumstellar material, the peak supernova luminosity, the shape of the supernova light curve, and other observable features of the supernova event can be affected by prior binary membership. Binary interactions may be the cause of much of the variability among type II supernova light curves. In particular, many of the peculiarities of SN 1987A and SN 1993J may well have resulted from the prior duplicity of the progenitors.

According to general relativity theory, compact concentrations of energy (e.g., neutron stars and black holes) should warp spacetime strongly, and whenever such an energy concentration changes shape, it should create a dynamically changing spacetime warpage that propagates out through the Universe at the speed of light. This propagating warpage is called gravitational radiation — a name that arises from general relativity's description of gravity as a consequence of spacetime warpage.

A discovery of three terrestrial-mass planets orbiting the millisecond pulsar PSR B1257+12 and a subsequent detection of the predicted effect of gravitational interaction between the two more massive planets confirms that the first extrasolar planetary system has been identified and that pulsars can be successfully used as probes of planetary dynamics. In the absence of detections of planet-sized objects around Sun-like stars, planets orbiting a precise pulsar clock represent a unique source of information concerning the origin and evolution of planetary systems.

Arrival times of some millisecond pulsars can be measured with sub-microsecond precision. This allows unprecedented measurements of the rotation of these stars, astrometric parameters including proper motion and parallax, motion in binary and complex dynamical systems, perturbations resulting from propagation through small scale turbulence in the interstellar plasma and other effects. In addition to the independent parameters which are required to model each pulsar, there is a set of global parameters that affect the array of pulsars in a correlated manner. These parameters involve the international atomic time scale, the ephemeris of the Earth's orbit and the gravitational wave background and have monopole, dipole and quadrupole and higher order angular signatures, respectively.

Lyne & Lorimer (1994) recently demonstrated that revisions to the pulsar distance scale, coupled with new interferometric measurements of pulsar proper motions and a better treatment of selection effects, indicate that typical pulsar velocities are of the order 450 km s−1. This is between a factor of 2–4 greater than most estimates made over the last decade. This paper looks at the implications of these higher velocities for the various theories about their origin. An extremely simple argument is used to place a fairly rigid upper limit for the rate at which neutron star pairs merge of 10−5 yr−1 in the Galaxy. It appears inevitable that an extremely large fraction of binaries containing neutron stars coalesce during the common-envelope stage of massive binary evolution.

Since the discovery of the first millisecond pulsar in 1982, the main discoveries of more of these exotic objects have come in two bursts. The first followed the realization late in the decade that they might be formed readily in globular clusters and resulted in the discovery of about two dozen objects in these clusters between 1987 and 1990. The second burst started shortly after this as improved computer technology permitted widespread searches of the Galaxy with high sensitivity and is continuing at the present time. This paper describes the main successful surveys for these galactic millisecond pulsars of which 29 are now known. 21 of these are in circular orbits with low-mass companion stars. The large number of objects now available allows a number of firm statistical trends to be seen which all point to an evolutionary phase of these systems involving the spin-up of old neutron stars during the accretion of matter from less evolved companions during their giant phase. Surveys of the present generation are still very insensitive to short period pulsars or highly accelerated pulsars in close or massive binary systems, limitations which should be removed in future years.

Using a fully self-consistent approach to account for known survey selection effects, we constrain the number and scale height of low-mass binary pulsars (LMBPs) in the local solar neighbourhood. Our results show that the local surface density of LMBPs with luminosities above 2.5 mJy kpc2 is ∼20 kpc−2. Assuming that these are long-lived (≲1010 yr) objects, their local birth rate is at least 2 10−9 kpc−2 yr−1. Whilst this is in excellent agreement with the birth rate of their proposed progenitors, the low-mass X-ray binaries, there are several uncertainties involved which could significantly increase our derived birth rate, perhaps by an order of magnitude. Models in which the scale height of LMBPs above the galactic plane exceeds 500 pc are found to be most consistent with the data. The mean space velocity at birth required to produce scale heights of this order in 1010 yr is found to be ≳ 80 km s−1.