It has been proposed that the Type I supernova (SNI) explosion starts in a white dwarf (WD) star that is accreting mass from a companion star in a close binary orbit. Others’ computations have described this explosion, but how an accreting WD can ever become an SNI if it first ejects the accreted envelope in one or more nova outbursts is still unresolved. My calculations of WD evolution include the effects of mass accretion, convective mixing, nuclear burning, and gravitational settling. These calculations test the Starrfield, Truran, and Sparks (1981) proposal that at low accretion rates, settling of the CNO nuclei will lead to steady-state hydrogen burning, not nova outburst. The preliminary results of this study, consisting of two evolutionary sequences of an initially cool pure 12C 1.0 Mʘ WD show that nearly half of the carbon near the base of the accreted layer, initially of solar composition, gravitationally diffuses out in 2 x 105 years.

After the first suggestion by Schatzman (1963) that accreting white dwarfs, close to the limiting mass, might be the progenitors of type I Supernovae, the problem was not studied until Schatzman (1974) described the competition between collapse dominated by beta decay, which would lead to neutron star formation and collapse dominated by nuclear reactions leading to stellar disruption.

Until 1976, cataclysmic variable star research proceeded with few requirements for the inclusion of magnetic fields in theoretical models. Although models for low-mass X-ray binaries stressed the importance of magnetic fields (Lamb et at. 1973) and there was an increasing number of known magnetic single white dwarfs (Angel 1977), and a magnetised white dwarf had been one of the models proposed to explain the rapid oscillations in DQ Her (Herbst et al. 1974, Katz 1975), there was no anticipation of the more general role that magnetic fields now seem destined to play. The two major reviews of the time (Robinson 1976, Warner 1976) scarcely considered the presence of magnetic fields.

We present the energy distribution of three hard X-ray sources H2252-035, 4U1849-31 and 2A0526-328 identified with intermediate polars. The ultraviolet and optical continuum can be well described by the standard disc model but this requires rather large discs and high accretion rates. Other possible contributions to the energy distribution are estimated. The predicted high accretion rates as well as the amplitude of the optical pulsations are in disagreement with the observed X-ray luminosity suggesting the existence of an unseen soft X-ray or extreme UV component.

The problem of column accretion on to white dwarfs in the AM Her and DQ Her systems is reviewed. Particular attention is paid to recent progress in explaining the large soft X-ray - EUV fluxes observed in these systems in terms of nonlocal electron energy transport into the white dwarf photosphere.

In this talk, we consider cataclysmic variable systems containing a white dwarf with a strong magnetic field. These include systems like AM Her (see, e.g., Chiappetti, Tanzi, and Treves 1981) in which the white dwarf rotates at the orbital period and systems such as AE Aquari in which the white dwarf rotates much faster than synchronously (see Patterson et al. 1980). The magnetic field in all of these systems is strong enough to disrupt the accretion disk at a point far above the surface of the white dwarf and may prevent the formation of a disk altogether. We will present theoretical models for the temperature, density, and velocity structure of the accretion flow in the region near the surface of the white dwarf where the kinetic energy of the flow is thermalized and radiated in the form of X-rays and ultraviolet radiation. This information is required to calculate accurate model spectra, and the results also have immediate consequences for the interpretation of observations.

Circular polarimetry in the red and simultaneous photometric observations in the UBVRI bands during the period June 1-3, 1981, are discussed. The peak value of negative circular polarization PV ~ -15 % is stronger than observed in 1976-79. Variations in the shape of the polarization and light curves occur from night to night. The positive crossover and reversal of the sign of the circular polarization are only marginal. A probable explanation of the short term variations seems to be the changing shape and position of the accretion columns with respect to the magnetic axis

Five symbiotic stars have been observed for linear polarization (UBVRI) in September 1981. Three systems, CH Cyg, CI Cyg and AG Peg show intrinsic polarization while in the case of Z And and AX Per the observed polarization seems to be mostly of interstellar origin. The position angle of polarization of CI Cyg and AG Peg rotates strongly vs. wavelength, as observed also for CH Cyg in 1977-80. The polarization of CH Cyg has decreased since May 1980, especially in the I, R and U bands, so that the maximum polarization is now in the blue (PB ~ 0.3 %). Probably we are monitoring the formation, growth and disappearance of dust particles in the atmosphere of this star. Two related systems, PU Vul (Nova Vul 1979) and R Aql (Mira) have polarization behaviour rather similar to that of symbiotic stars which suggests that the M type giant present in these systems is responsible for most of the intrinsic polarization.

Calculations of polarized cyclotron radiation from magnetized hot plasmas are modified to include the effects of collisions and photon scattering. These effects produce a general reduction in both the linear and circular polarization. The background unpolarized flux which had previously been invoked, in an ad hoc manner for the collisionless case, is no longer required to bring agreement with observations. The most striking effect is at small viewing angles with the magnetic field, where the fractional circular polarization is reduced from nearly 100% to close to 0%. The calculations also show that the shape of the linear polarization pulse is essentially unchanged.

It has been suggested that the recently discovered radio emission from AM Her arises as a result of gyrosynchrotron radiation from electrons at energies ≈400 keV in the magnetosphere of the white dwarf. However, no mechanism for producing such energetic electrons was discussed. In this paper, we argue that small departures from synchronous rotation can cause the companion star to act as a unipolar inductor. This leads to high voltages being produced across the companion star, which provides the necessary acceleration mechanism. This also implies that if the magnetic white dwarf was formed with a rapid rotation, synchronization would be achieved on a time scale ~104 yr.

We have carried out a detailed comparison of the spin-up behavior of the recently discovered faint long-period X-ray sources, using the accretion torque theory of Ghosh and Lamb. We confirm that these sources are degenerate dwarfs. We find that they have magnetic fields B ~ 1 x 106 G, an order of magnitude smaller than those of AM Her stars. On this basis, we predict that they will show significant polarization in the infrared.

Some cataclysmic binaries may be products of Case C evolution of low mass stars (orbital period ~ 1 yr; masses ~ 1 - 4 Mʘ), involving a common envelope phase. Other mechanisms, probably involving late Case B and even early Case B, but with significant loss of angular momentum, may be necessary to account for some evolved binaries such as AA Dor or V Sge. Further angular momentum loss, probably by magnetic braking coupled with tidal friction, causes secular evolution in cataclysmic binaries. It is suggested that tidal friction may account for the shortage of cataclysmics with periods ≲ 1.3 hr; but this cutoff, as well as the gap in the period distribution between 2 and 3 hrs, is hard to explain and imposes more severe constraints on possible theories than is commonly acknowledged.

It is shown that the secondary components of cataclysmic binaries with orbital periods of less than ~10 hours are indistinguishable from ordinary low-mass main-sequence stars and that, therefore, they are essentially unevolved. On the other hand, it is shown that, depending on the mass ratio of the progenitor system, the secondary of a cataclysmic binary could be significantly evolved. The fact that nevertheless most of the observed secondaries are essentially unevolved can be accounted for by assuming that the probability distribution for the initial mass ratio is not strongly peaked towards unity mass ratio.

Recent observations of four close binaries have established that there is a group of very-short orbital-period (VSOP) binaries whose orbital periods are less than 60 minutes. The VSOP binaries consist of both x-ray close binaries (4U1626-67; Middleditch et al. 1981 and 4U1916 -0.5; White and Swank 1982) and cataclysmic variables (AM CVn; Faulkner et al. 1972 and G61-29; Nather et al. 1981). Their orbital periods are too short to have a main-sequence companion. However, four binaries, none of them belongs to any globular cluster, are too abundant to be explained by capturing mechanism of a white dwarf. Therefore it seemed to be worth to present an evolutionary scenario from an original binary system which can be applied for all of VSOP binaries.

We investigate the possibility of forming a low mass cataclysmic binary from a star-planet system. Using certain simplifying assumptions it is shown that an analytic solution for the evolution of the separation can be obtained. The fate of the system is determined by the competition between mass loss and accretion drag. While it is found that some fine tuning is required for the formation of a close binary, such a binary can still be formed, for parameters that are not too exotic.

Cataclysmic binaries with a black-dwarf secondary can in principle be distinguished from those containing a low-mass main-sequence secondary because of the differences of the corresponding eclipse light curves. However, due to the very low intrinsic luminosity of the systems containing a black dwarf, the probability of detection turns out to be smaller than the corresponding probability for an ordinary cataclysmic binary by about 1.5 to 3 orders of magnitude. Therefore, despite the fact that the predicted space density of these objects is quite high, systems containing a black-dwarf secondary must remain essentially undetectable.

The properties of coherent oscillations with periods of tens of milliseconds that have been observed in seven variable X-ray sources and bursters are reviewed. Theoretical models for the generation of the oscillations are discussed. Difficulties with a magnetic rotator model are indicated. It is suggested that at least in several cases, g-mode oscillations of the neutron star provide a satisfactory explanation of the observations.

Basic features of observed Type I X-ray bursts are thought to be interpreted in terms of helium shell-flash near the surface of the accreted neutron stars (Lewin and Joss 1981). Numerical models of the helium shell-flash show that the luminosity grows very close to the Eddingotn luminosity just after the peak of energy generation.

We review X-ray observations of cataclysmic variables and theoretical work on X-ray emission from them, emphasizing recent developments.

Colloquium IAU 72 is the first IAU-sponsored activity in this country, and it therefore seemed appropriate to include a paper on Astronomy in Israel. With the highly singular history of this nation, the scene will move from Eretz-Israel to Babylon (present day Iraq) and to Europe before returning to this area.