J. Rahe of Bamberg reported on IUE observations of comets Sargent (1978) and Bradfield (1980).

Observations of the Pluto-Charon system using the Hubble Space Telescope are soon to provide another test of the modern Laplacian theory of solar system origin. According to this theory, Pluto, Charon and Neptune’s moon Triton are condensed remnants of the gas ring which was shed by the proto-solar cloud at Neptune’s orbit, and from which Neptune formed. Each body should have the same bulk chemical composition provided that the influence of secondary events such as physical collision can be neglected. Here we report a model for the time-dependent, surface-catalysed production of CH4, solid carbon C(s) and CO2 within the outer layers of the proto-solar cloud. This model is used to determine the condensate mix which best fits the Voyager 2 spacecraft measurements of Triton’s mean density. We suggest that Triton is a chemically homogeneous structure consisting, by mass, of 44.2% anhydrous rock, 2.1% graphite, 33.0% water ice and 20.7% dry ice (i.e., solid CO2 ice, of which a fraction nearly 2/3 is present as the clathrate hydrate CO2·5.75 H2O). On the basis of this composition, the individual mean densities of Pluto and Charon are predicted to be 2.02 ± 0.02 g/cm3 and 1.94 ± 0.02 g/cm3, respectively, assuming a mean surface temperature of 35 K. The mean density of the combined system is predicted to be 2.01 ± 0.02 g/cm3.

If the relative velocity between a plasma and a molecular cloud impinging on it exceeds the Alfvén critical ionization velocity, then the surface of the molecular cloud can be ionized. This phenomenon has been used to model some of the ionized regions at the Galactic Centre. A review of the mechanism behind Alfven’s critical ionization phenomenon is presented here, and its use at the Galactic Centre is discussed.

In the first part the UV and optical observations of the circumstellar gas of SN 1987A are reviewed, and the physical properties of the gas discussed. In the second part models of the emission are treated, as well as their use as a diagnostic of the radiation from the supernova during the first, unobserved hours after the explosion.

Zeeman Doppler Imaging (ZDI) is a recent technique for measuring magnetic fields on rapidly rotating, active stars. ZDI employs spectropolarimetry taken at different rotational phases to derive information on the magnetic field distribution over the stellar surface. The Zeeman effect is used to identify the presence of a magnetic field, and variations in Doppler wavelength shifts across the rapidly rotating star allow fields to be resolved on different parts of the visible disk. Analysis of the spectra can be used to produce both thermal and surface magnetic images. ZDI requires very high S/N spectra to be acquired within a time interval short compared to the stellar rotation period. As a result, a large-aperture telescope is needed. Since an initial successful test in 1989, the 3·9 m Anglo-Australian Telescope has been used to obtain ZDI spectra of active stars of different evolutionary stages. The observations have concentrated on the K subgiant in the RSCVn system HR 1099 to monitor changes on this bright and active star. With the advent in 1991 of ZDI spectropolarimetry with the AAT échelle spectrograph, it has become possible to co-add the polarisation signature from the many magnetically sensitive lines recorded simultaneously. As a result, stellar magnetic field detections of unprecedented quality have been obtained. The aims of this paper are to briefly outline the principles of ZDI, describe the instrumental setup at the AAT and present some preliminary results from recent observations.

A simple circuit has been designed for reading out at TV rate the two-dimensional CCD on the Automated Patrol Telescope. Because it is controlled by a programmable Cathode Ray Tube Controller integrated circuit it is very flexible and applicable to frame transfer CCDs having a wide range of formats.

The effects of the specific geometry of the magnetic field (such as field lines with torsion) on curvature emission and absorption in pulsar magnetospheres are discussed. Curvature maser emission can arise from two effects: the curvature drift, as has already been discussed in the literature, and field line torsion as discussed here in detail for the first time. Maser emission due to field line torsion can operate only when the Lorentz factor is larger than a certain value. However, when the Lorentz factor of electrons or positrons is sufficiently high, curvature masering is due to both curvature drift and magnetic field line torsion. The optical depth in the case of field line torsion is estimated. It is shown that if torsion is due to rotation, the resultant luminosity should be dependent on the rotation period in such a way that shorter periods correspond to larger luminosities.

A survey of clusters of galaxies from Abell’s catalogue has been carried out using the Mk II 11 cm receiver recently installed at Parkes. For the present preliminary report only sources with flux densities down to about 0.3 f.u. will be considered, although the sensitivity limit of the receiver is about one-sixth of that value.

We describe bright microwave events that were first detected with the Parkes 64-m telescope at 8.4 or 22 GHz from six active-chromosphere stars. In some flares spectral data were obtained over a large frequency range from simultaneous measurements with the Parkes reflector (8.4 or 22 GHz), the Tidbinbilla interferometer (8.4 and 2.29 GHz), the Fleurs synthesis telescope (1.42 GHz) and the Molonglo Observatory synthesis telescope (0.843 GHz). Data on circular polarization were obtained from the Parkes observations at 8.4 GHz.

The stars were in a wide variety of evolutionary states, ranging from a single pre-main-sequence star (HD 36705), two RS CVn binaries (HD 127535, HD 128171), an Algol (HD 132742) and two apparently single K giants (HD 32918 and HD 196818). Their high brightness temperatures, positive spectral indices and low polarization are consistent with optically thick gyrosynchrotron emission from mildly relativistic electrons with average energies 0.5 to 3 MeV gyrating in inhomogeneous magnetic fields of 5 to 100 G.

Initially some simple analytical properties based on the annual Zürich relative sunspot number are established for the 22-year Hale solar magnetic cycle. Since about AD1850, successive maximum sunspot numbers in a Hale cycle are highly correlated. Also, a regression model for the reconstruction of the 22-year Hale cycle has been formulated from proxy tree-ring data, obtained from spruce trees growing at a high altitude site in White River National Forest in Colorado. Over a considerable fraction of the past 300 years to AD1986, the ring-index time series power spectrum exhibits a strong 22-year periodicity, and more recently a significant spectral peak (at the 95% confidence level) at approximately 11 years. The model shows that the greatest variation in ‘amplitude’ in the magnetic cycle occurs over the early decades of the eighteenth century, when the sample size is small. Thereafter, a nearly constant amplitude is maintained until about AD1880 when a break occurs in both phase correspondence and amplitude, extending over the next three cycles. From AD1950 the signal recovers phase with the solar cycle, with reduced but increasing amplitude.

The low-mass X-ray binary source Cyg X–3 has been extensively observed from radio to ultra-high-energy (UHE) gamma-ray energies (i.e., energies >1015eV). In the radio, Cyg X–3 exhibits intense non-thermal outbursts (flares) and a double-sided relativistic jet morphology. Interestingly, at energies above 1 TeV (1012eV), the gamma-ray emission is highly variable and possibly correlated with the radio outbursts. This emission results primarily from the radiative decay of π°-mesons generated in inelastic collisions between relativistic nuclei (predominantly protons) and the surrounding matter. The observed flux of UHE gamma-rays from Cyg X–3 implies that Cyg X–3 is a localised accelerator of such particles.

We propose a model of Cyg X–3 wherein particles expelled by the source are accelerated by Shockwaves in the relativistically expanding jets. Intense flaring episodes then lead to time variations in the ambient particle flux which account for the observed features at UHE energies.

High-resolution surveys of the galactic radio emission show detailed structure both near the galactic plane and at high galactic latitudes. Some of this structure is describable as ‘spurs’ while the rest comprises isolated features. We describe here measurements of the spectral index of the background radiation with a resolution of 3°.5. These measurements are aimed at a detailed understanding of the high-latitude synchrotron emission.

It has generally been accepted that moving type IV bursts are generated as synchrotron radiation from energetic electrons high in the solar corona (Boischot and Denisse 1957). At 80 MHz the peak brightness temperature is usually ~ 108 K and the radiation becomes highly circularly polarized as the burst decays. This has led several authors (Kai 1969; Dulk 1970, 1973; Schmahl 1972; Robinson 1974, 1977; Nelson 1977) to the conclusion that the radiation comes from mildly relativistic (~ 100 keV) electrons and occurs at low harmonics of the gyro-frequency (gyro-synchrotron radiation). We present evidence of moving type IV bursts at 43, 80 and 160 MHz with brightness temperatures of ~ 109 K, and one at 43 MHz as high as 1010 K. The number (~ 1033) of energetic (≥ 1 MeV) electrons which is required in order to explain such high brightness temperatures by incoherent gyro-synchrotron emission is very large and near the upper limit for the number of fast electrons accelerated in the second phase of a solar flare. If amplification takes place a smaller number of electrons with energies ~ 100 keV would be required.

Following a successful program to investigate the physics of ultra-high-pressure proportional counters, a counter array has been developed for hard X-ray astronomy. A parallel investigation has evaluated the performance of a large-area phoswich scintillator detector for the same purpose. The two detectors have been integrated in a balloon-borne payload, the Astrophysical X-ray Experimental Laboratory (AXEL). This paper describes the instrumentation aboard the payload.

Non-thermal radio emission has been detected from dMe stars, RS CVn binaries and W T Tauri stars. Polarisation and intensity measurements of the quiescent (i.e. non-flaring) emission indicate that the emission is gyrosynchrotron emission from mildly relativistic electrons spiralling in a magnetic field. A three-dimensional dipole magnetic field model for the stellar field is presented and the quiescent gyrosynchrotron emission from such a model is calculated and compared with observations. The model can account for many phenomenological features of quiescent emission. Quantitative comparisons of model results with observations indicate that the electron distribution in the emission region may be a magnetic mirroring distribution.

An acousto-optical spectrograph (AOS) with a high spectral resolution of 28 kHz has been developed for spectral line observations of objects, such as dark clouds, where the line-widths are low. It has been mounted on the 4-m-diameter millimetre-wave telescope at CSIRO Division of Radiophysics at Epping, NSW and successfully tested using 115 GHz Co observations of a sample of southern dark clouds. At 115 GHz the velocity resolution is 0.07 km s-1.

We present the results of an examination of low-resolution IUE spectra of Galactic planetary nebulae in an effort to determine the occurence of stellar winds in their central stars. Our analysis confirms the results obtained in previous studies.

The spectrum of a symbiotic star consists of an M-type absorption spectrum, a B-type shell spectrum and nebula emission lines, the relative contributions of these three components varying with time. The light curves of the symbiotic stars vary with a semi-regular period typically 200-800 days while larger eruptions occur on a timescale of ~ 3.5 years. Some suggestions which have been advanced to explain the combination spectrum, variability and eruptive behaviour of the symbiotic stars are:

1. (a) the symbiotic stars are binaries consisting of a hot and cool component.

2. (b) the symbiotic stars consist of a single hot star surrounded by a large optically thick envelope giving the appearance of a hot continuum with the absorption spectrum of a cool star superimposed on it.

3. (c) the symbiotic stars are single stars surrounded by a shock wave heated chromosphere.

   Although some of the symbiotic stars are undoubtedly binaries (for example, T Coronae Borealis), observatienal evidence suggests that others may be explained by hypothesis (c) above. The calculations described below provide an explanation of the symbiotic stars in conjunction with hypothesis (c).

A recent study has indicated that a substantial fraction of young pulsars born in supernovae have travelled outside the boundaries of their corresponding shell supernova remnants. A simple model suggests that this should not be the case, implying either that some postulated pulsar/remnant associations are false, or that pulsars are particularly difficult to detect until they have emerged from their remnants.