The Kukarkin-Parenago relation for dwarf novae and novae

The period-amplitude relation and the period-energy relation for dwarf novae and novae are discussed. The total outbursts energy is shown to be a more suitable characteristic of the outburst than its amplitude.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Recurrent novae

A brief review of recurrent novae studies which have been undertaken in South Africa during the last ten years is given. These works have substantially altered our view of these objects as distinct sub-classes of cataclysmic variable stars. Observational results which led to new developments in our understanding of the recurrent novae are reviewed in some detail. Observations of V1017 Sgr, which had been suggested as a recurrent nova, are also mentioned.     

A brief overview of dwarf novae and X-ray novae

Dwarf novae and X-ray novae have proven to be important in advancing our knowledge of accretion disk physics, or at least of posing challenging questions. The myriad behavior of the various classes and subclasses, including aspects associated both with outburst and quiescence, are constantly providing stimulus to our efforts at modeling accretion disks. I will briefly review some of the main properties of these two classes of objects – including discussions of their outburst characteristics and physical differences. I will also try to point out areas in which we do not have a detailed understanding of the basic model within which to interpret the observations.     

Extragalactic Cepheid database

We present in this paper an exhaustive compilation of all published data of extragalactic Cepheids. We have checked every light curve in order to characterize the different types of Cepheid and detect potential overtone pulsators, or to estimate the quality of the data. This compilation of about 3000 photometric measurements will constitute a very useful tool for astronomers involved for instance in the extragalactic distance scale.     

Nucleation in novae

We consider the possibility that dust grains in novae could nucleate on ions. We examine how different ejected masses and different elemental abundances effect the availability of nucleation centres, and we show how this model could explain why only novae of intermediate speed classes form dust.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Understanding Extragalactic Hydroxyl

Hydroxyl (OH) is one of the few molecules in space showing both maser emission and absorption. In the Milky Way, hydroxyl has been found in various environments such as stellar envelopes, star-forming regions, and HII regions, tracing the physical states of the interstellar medium. Extragalactic OH has been found to provide essential information about the dusty and obscuring material within the inner kiloparsec region of active galaxies. At angular resolution of a few tens of parsecs, OH shows a rather complicated spatial distribution and the interpretation of the circumnuclear environment is rather difficult. Based on results obtained from low- and high-resolution observations, the diagnostic tools of OH will be reviewed and the strategies to further investigate extragalactic OH will be discussed.     

Extragalactic HI surveys

We review the results of HI line surveys of extragalactic sources in the local Universe. In the last two decades major efforts have been made in establishing on firm statistical grounds the properties of the HI source population, the two most prominent being the HI Parkes All Sky Survey and the Arecibo Legacy Fast ALFA survey. We review the choices of technical parameters in the design and optimization of spectro-photometric “blind” HI surveys, which for the first time produced extensive HI-selected data sets. Particular attention is given to the relationship between optical and HI populations, the differences in their clustering properties and the importance of HI-selected samples in contributing to the understanding of apparent conflicts between observation and theory on the abundance of low mass halos. The last section of this paper provides an overview of currently ongoing and planned surveys which will explore the cosmic evolution of properties of the HI population.     

Observations of eclipsing dwarf novae

High Speed Photometry of four eclipsing dwarf novae, z Cha, OY Car, HT Cas and IP Peg, is used to determine the orbital parameters and the masses of the two stars in each system. The mass ratio, q, and inclination, i, are found by constraining theoretical stream trajectories (Lubow and Shu 1975) to pass through the observed light centre of the bright spot eclipse. The light centre is defined as the point corresponding to the half flux points in the ingress and egress of the eclipse of the compact part of the bright spot. The theoretical stream width indicates that the centre of the stream must intersect the bright spot close to this point.The length of ingress and egress in the primary eclipse, constrains the radius of the central object relative to the orbital separation. The shape of this eclipse and the temperature of the central objects in z Cha, OY Car and HT Cas imply that the measuredPaper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Balmer decrements in dwarf novae

We investigate the question of the possible variable conditions in dwarf novae. The Balmer decrements are used as variability indicators due to their form being strongly dependent on the conditions in the radiating medium. It is shown that the decrements are usually anomalous and variable in time at minimum light, which indicates a variability of the conditions in the radiating medium. The spectrum changes significantly during the outburst state, the decrement becomes steep and has no anomalies. It appears that the outburst makes the emitting region more homogeneous and sparse. Another possible explanation is that the radiation is emitted by some other region, unseen at minimum light.     

The nature of recurrent novae

The observational properties and outburst histories of each member of the small class of recurrent novae are reviewed, with the objective of identifying the outburst mechanisms for these systems. Examples are found of both accretion-powered outbursts (T CrB and RS Oph) and thermonuclear-powered outbursts (T Pyx and U Sco). Outburst properties which might be used in future to distinguish between these mechanisms are identified. Many erstwhile recurrent novae (VY Aqr, RZ Leo, V616 Mon, V1195 Oph, V529 Ori, WZ Sge, and V1017 Sgr) appear to be misclassified examples of other types of cataclysmic variables.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G. 16–19 June, 1986.     

The outbursts of dwarf novae

We present a numerical scheme for the evolution of an accretion disc through a dwarf nova outburst. We introduce a time-varying artificial viscosity into an existing smoothed particle hydrodynamics code optimized for two- and three-dimensional simulations of accretion discs. The technique gives rise to coherent outbursts and can easily be adapted to include a complete treatment of thermodynamics. We apply a two-dimensional isothermal scheme to the system SS Cygni, and present a wide range of observationally testable results.     

Spectroscopic studies of three novae

We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars.     

Alternating cycle durations in dwarf novae

The cycle durationC in some dwarf novae underwent abrupt significant changes, as was found by using our own photometric data and the published moments of maxima. For the analysis, the DMRT method (Dumontet al., 1978) as well as least-squares (LS) routine were used. The DMRT method is more appropriate for a search for mean period, if the cycle numeration is unknown. Such changes may be possibly explained by a solar-type activity of convective secondary stars in cataclysmic binaries. However, the abrupt character of the changes ofC (i.e., the time of switchings from one cycle to another is sufficiently small as compared with the lifetime of a given cycle) is an observational puzzle which is to be interpreted theoretically.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Extragalactic eclipsing binaries: astrophysical laboratories

Extragalactic eclipsing binaries open a new perspective on the study of stellar structure and evolution. Stars in different galaxies have formed and evolved in environments with chemical histories that may differ from those of the solar neighborhood. For example, the LMC and SMC contain low-metallicity, young massive stars that are no longer found in our Galaxy. Eclipsing binaries, yielding accurate determinations of masses, radii and temperatures, allow for critical tests of, e.g., convective overshooting, mass loss, and internal structure. In addition, they provide an empirical calibration of the mass-luminosity relationship. In this paper, I present some results from ongoing programs on the determination of physical properties and distances to eclipsing binaries in the LMC, the SMC, and M31. In particular, I focus on aspects relevant to stellar astrophysics, and on the contributions of these eclipsing binaries to our understanding of the structure and chemical evolution of the host galaxies.     

Extragalactic binaries as core-collapse supernova progenitors

Binary star systems are likely the progenitors of many core-collapse (Type II, Type Ib/c) supernovae (SNe). We present observational investigations using ground-based and Hubble Space Telescope optical imaging and radio monitoring of SNe and their environments, which either indicate or attempt to constrain the possible binary nature of the SN progenitors. For example, from radio observations with the Very Large Array of the Type II-linear SN 1979C in M100 we conclude that the progenitor was possibly in a massive, highly eccentric binary, similar to the VV Cephei systems. The Type IIb SN 1993J in M81 is presumed to have a massive progenitor in an interacting binary system, and from Hubble imaging we cannot yet constrain the nature of the presumed massive, blue companion. We will present additional results for other Type Ib/c and II SNe.     

Extragalactic sources with one-sided radio structure

A list has been compiled of 49 extragalactic sources, most of them identified with quasars, that appear to have a one-sided (D₂ type) radio structure characterized by a single outer component displaced from a compact central (nuclear) component coincident with the optical object. The observed properties of a subsample of 28 D₂ quasars that have an overall angular size larger than 5 arcsec are briefly discussed and compared with those of normal (D1 type) double quasars. It is found that the central components in most D₂ sources account for more than half the total flux density at high frequencies in contrast to the D₁ quasars which generally have less than 20 per cent of their total flux density in a central component. This makes it very unlikely that D₂ sources are just those D1s in which there is a large intrinsic difference in the flux densities or separations of the two outer components. The observed properties of D₂ sources are easier to understand in the relativistic beaming interpretation in which their axes are inclined at smaller angles with the line of sight compared to D₁ sources.     

New Aspects of Extragalactic Globular Clusters

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Extragalactic science with the FRESNEL imager

The space Fresnel Interferometric Imager, originally proposed to the ESA Cosmic Vision plan, is an innovative concept providing enhanced capabilities in terms of spatial resolution and dynamical range. In this paper we describe some of the most promising applications of the Fresnel Interferometric Imager concept in extragalactic studies. There are two different topics where the FII could make major progress. The first one is the mapping of star-formation in galaxies, from the local universe to ??75?C85% look back time, based on the first version of FII which is optimized for the UV and optical domains. The second topic is a test case for a subsequent improved version of FII, with a larger collecting area and optimized for the near-IR, focusing on the physical properties of the first galaxies.     

依巴谷与河外参考架连接方法和进展

给出依巴谷星表与河外参考架连接的数学原理及实现这种连接的各种方法，介绍在该领域的最新进展和初步结果．     

Photometry of recent novae and supernovae

Light curves of recent novae and the supernova 1991T are presented primarily to demonstrate the paucity of photoelectric or calibrated CCD measurements, particularly in the critical maximum phase. There is a an opportunity for a very useful contribution to the study of these objects by photometrists with modest telescopes, particularly if they can respond quickly.