A method for calculating the resultant probability distributions of orbital elements for a small body (a comet, asteroid or meteoroid) after a gravitational encounter with a planet is described. This technique incorporates the frequency of such encounters so that the chance of attaining a certain new orbit per unit time is derived. The use of this technique is then illustrated by considering the effect of Jupiter upon the orbits of near-parabolic comets with perihelia near that planet (q = 5.2 AU) and in the inner solar system (q = 1.0 AU), with prograde (i = 10°) and retrograde (i = 170°) paths. As indicated by previous authors the prograde comets are more easily captured into short-period (P< 20 yr) and intermediate-period (20<P<200 yr) orbits; however, in contradiction to most previous work but in agreement with the results of Stagg and Bailey (submitted to Mon. Not. R. Astron. Soc.) it is found that the comets with smaller perihelia, rather than those with perihelia near Jupiter, have higher capture probabilities. This is apparently due to the fact that a small deflection only is needed to sufficiently decelerate a comet onto a smaller orbit if it makes a near-perpendicular crossing of Jupiter’s path, whereas a larger deflection (to achieve a large orbital change) is needed if the paths are near-parallel. With comparatively modest amounts of computer time this method may be used to calculate the relative capture probabilities as a function of i and q for all values of interest, and is thus a useful precursor to integrations following orbital evolution, since it indicates the most likely avenues whereby shorter-period comets are derived from the near-parabolic flux.

Aperture synthesis is a two-step imaging process in which the final map or image is reconstructed from intermediate measurements. As an alternative to digital computers, optical computers can be used to perform the reconstruction step. Optical reconstruction can be a fast and cheap way to handle the large amount of data now being generated by earth rotation aperture synthesis arrays.

Historically, infrared instrumentation for astronomical research has been developed by individual groups largely for their own enjoyment. The commissioning of the AAO’s infrared photometer-spectrometer (IRPS) in late 1978 provided for the first time in Australia a common-user instrument. Since then it has attracted considerable interest and is now the second most popular instrument in use at the AAT.

Radio emission at centimetre and millimetre wavelengths provides a powerful tool for studying the circumstellar envelopes of evolved stars. These include stars on the asymptotic giant branch (AGB), post-AGB stars and a small number of massive M-type supergiant stars. The AGB stars and M-type supergiants are characterised by extremely high mass-loss rates. The mass loss in such an evolved star is driven by radiation pressure acting on grains which form in the outer stellar atmosphere. The grains are accelerated outwards and transfer momentum to the gas through grain–gas collisions. The outflowing dust and gas thus form an expanding circumstellar envelope through which matter flows from the star to the interstellar medium, at a typical velocity of 15 km s−1. For a recent review of circumstellar mass loss see Chapman, Habing & Killeen (1995).

A simple model for estimating the intrinsic flow direction and speed in the parsec-scale jets associated with extragalactic radio sources is presented. In this model, radio source brightness asymmetries are attributed to the apparent amplification caused when a relativistic jet of radiating material is somewhat aligned with the observers line of sight. The knots of emission commonly seen in parsec-scale radio jets are interpreted as shocks in the relativistic fluid.

Recent observations of the radio-frequency flux spectrum of Jupiter in the frequency range 80-10 000 MHz suggest that the synchrotron component is not independent of frequency as has been generally accepted. Rather, the flux decreases at frequencies below 300 MHz and above 3000 MHz. In this paper we show that extensions and variations of the well-known dipolar model for this emission can account for the modified spectrum.

Since their initial discovery by Hale, the nature of solar magnetic fields has presented us with a number of problems. At one time it was thought that the field consisted of a weak background dipole field of order 1-2 G on which was superimposed the considerably more intense fields associated with active regions and sunspots. However, more recent observational studies by Harvey, Frasier, Stenflo and others have suggested that 90% of the background field appears in the form of intense small-scale fields with intensities of order 103 gauss or greater and which have remarkably similar properties whether they occur in active or quiet regions. In particular, the field intensity appears independent of the total amount of flux present but the appearance of the structure depends critically on the total flux.

We have mapped the molecular clouds of Sgr B2 in the 110 → 101 ortho-transition of C3H2 at 18.3 GHz, using the 70-m NASA telescope at Tidbinbilla (beamwidth 55 arcsec). Three clouds show absorption against the Sgr B2 continuum emission at radial velocities of 50, 65 and 80 km s−1. The 65-km s−1 cloud covers most of the observed area (4 × 6 arcmin in right ascension and declination), has a peak optical depth of 2.7 and a corresponding C3H2 column density of 7.6 × 1015 cm−2. The C3H2 fractional abundance relative to H2 is 1.5 × 10−9. The 80-km s−1 cloud, located north of the Sgr B2 continuum peak, has a peak optical depth of 0.9 and a C3H2 column density of 1.9 × 1015 cm−2. The 50-km s−1 cloud is centred 2 arcmin south of the continuum peak; here the minimum optical depth of 0.5 yields a column density of 5.3 × 1014cm−2.

Active Galactic Nuclei (AGN) occur with widely differing characteristics of luminosity, variability and non-thermal emission. Although it is generally accepted that all such objects are powered by the accretion of matter to a super-massive black hole, no convincing model has been proposed which accounts for the diversity of observed phenomena. The model described here is based upon the hypothesis that shear flows in a misaligned accretion disc around a Kerr black hole lead to the growth of dynamical instabilities which disrupt the disc near the Bardeen-Petterson radius. Resulting clouds of disc material are fragmented by collisions and ablated by radiation from the compact core to produce a hot Parker wind which is optically thick to electron scattering. Within this wind, many fragments of disc material are entrained and photoionised to produce the broad emission lines, while some are accreted to power the system. Usually the wind impedes jet flows, in which case the object is radio quiet. When the disc and black hole are only slightly misaligned, however, the wind is equatorially biased and collimated axial jets may form. Such objects will be radio loud, and may exhibit superluminal phenomena if the jet is oriented close to the line of sight.

This paper describes a preliminary series of observations of the Sun made at a frequency of 80 MHz with the 3 km radioheliograph of the Culgoora Observatory. The instrument records, at one-second intervals, pictures of the solar image in the form of 60 (E-W) × 48 (N-S) points, each separated in angle by half the Rayleigh limit (2’ arc in the zenith). At the time of the present observations the instrument was incomplete in three main respects : (a) the facilities for recording opposite senses of circular polarization were not available; (b) the automatic image compensation for zenith-angle foreshortening was not available—hence the optical disk of the Sun appears elliptical; and (c) the phase and amplitude calibration procedures had not been fully established, resulting in a higher sidelobe level than that specified in the design—the effects are sometimes evident in the pictures as spoke-like brightenings.

At a previous meeting of the Society I well remember Paul Wild saying, in a humorous vein of course, that until recently he had always thought that an H-R diagram was a plot of H against R. It occurs to me that the same kind of vagueness, although perhaps not to the same degree, might be present with astronomers not directly concerned with the theory of stellar atmospheres in regard to ‘L.T.E.’ and ‘Non-L.T.E.’ analysis of spectral lines. In the present paper, I would like to point out some of the important features of non-L.T.E. analysis, to indicate how the basic theory may be developed, and to show what effect this has on the theoretically produced absorption lines. I will make particular reference to the solar atmosphere and here we take the ‘model atmosphere’ approach where all the physical parameters such as gas pressure, electron pressure, temperature etc., are all specified as functions of optical depth.

Several catalogues of radio sources at 408 MHz have been prepared from observations made with the Molonglo cross telescope. These include (in chronological order) ‘The Molonglo Radio Source Catalogue 1’, the MC1, by Davies et al. (1973), the MC2 and MC3 by Sutton et al. (1974), MC4 by Clarke et al. (1976), ‘The Molonglo Deep Sky Survey of Radio Sources’ by Robertson (1977a,b,d) and ‘The Molonglo Reference Catalogue of Radio Sources’ (MRC) by Large et al. (1981). The catalogues MC1-4 cover selected areas of sky to a flux density limit of ~0.2 Jy to 0.3 Jy. The MRC includes extragalactic radio sources between δ = +18° and δ = −85° and is essentially complete at S408 = 1.00 Jy with raany sources to ˜0.7 Jy. The Wyllie (1969a,b) scale of flux density is used throughout.

Studies of coronal transients observed in white-light (Gosling et al., 1976) have shown that fast-moving events (≤ 400 km s-1) are closely associated with flares and with type II and IV radio bursts while slow-moving events are not. We now report the first detection of the radio counterpart of a slow-moving transient. The event of 1974 January 21 is shown to be visible on maps of the quiet Sun made at a frequency of 80 MHz.

During the past twelve years five series of observations have been made of the polarization of Jupiter’s radio emission at a wavelength of 11 cm. This data shows characteristics which have been stable over a period of years as well as some unexplained variations. The observations were made during one complete orbital period of Jupiter and hence were obtained over the full range of values of D E, the angle between Jupiter’s rotational axis and the plane of the sky. These are summarized in Table 1. The 1967 observations have been reported previously (Komesaroff and McCulloch 1967) and the 1963 data is from Roberts and Komesaroff (1965).

In an early paper Sterne investigated the pulsational properties of three stellar models with specified density distributions. He assumed a solution for the relative amplitude of the displacement to be of the formand on substitution into Eddington’s equation for small radial adiabatic oscillations of a starobtained a two-term recurrence relation to solve for the an in the case of each model.

The University of Wollongong operates an 18-inch telescope located on the escarpment behind Wollongong. The greatest contribution from a telescope of this size is gained by photoelectric observations which larger telescopes often do not undertake because they are time consuming. In addition such observations require ‘photometric weather’. The sky in Wollongong is ‘photometric’ often enough to support a viable programme. However, it is vital that the telescope (and its instruments) works quickly and efficiently to utilise the fine periods.