Report on the latest developments in the modelling of Solar granules and in particular, the influence of various viscosity laws on the theoretical predictions, such as the velocity field, intensity modulation, degree of overshooting into upper stable layers and so on.

Observations have been made of the interstellar D-line absorption of Nal towards a loose grouping of OB stars (containing the association Ara OB1 and probably the Hα emission regions RCW 107 and 108) centred at 1 = 337°.5 and b= -0°.5. The individual absorption spectra contain several velocity components which show well-defined groupings at –37, –18 and 0 km s-1. The corresponding median optical depths are 2, 7 and 6. The evidence indicates that both the –37 and –18 km s-1 Nal clouds are located in the Carina-Sagittarius spiral arm. Several HII regions are located at similar longitudes, but most are close to zero latitude and are more distant than the arm.

The association contains the galactic cluster NGC 6193. The results suggest that the turbulence seen optically towards this cluster has been caused by the interaction between two cloud groups moving towards each other at velocities of 10 to 15 km s-1.

Time dependent solutions of the nonlinear modal equations for cellular convection in a fluid layer heated below have demonstrated the existence of a nonlinear bifurcation which leads to a stable regime with reduced heat flux and vertical velocities. This new state is brought about by the growth, to a significant level, of the vertical component of vorticity after an initial quasi-steady state has been established. The growth rate mechanism has been investigated analytically and compared with the numerical results. These vorticity modified solutions exhibit favourable features Which could be established in the solar convection zone.

A particle method for the simulation of the global evolution of cold gas in galactic potentials is described. It incorporates an efficient artificial viscosity, based on the smoothed particle hydrodynamics approach, which conserves both linear and angular momenta and in the continuum limit is equivalent to a Navier-Stokes viscosity in a compressible gas. Parameters of this viscosity have been calibrated in such a way as to make a gas disk, embedded in an axisymmetric galactic potential and originally inclined to its equatorial plane, settle to the preferred plane on the time scale characterizing the differential precession. After a careful analysis of the previously published settling times we find that our results are consistent with Steiman-Cameron’s (1984) lower limits, and we argue that Simonson’s (1982) time scales have been seriously underestimated. We also demonstrate that two dimensional methods based on rigid rings do not model the evolution of a differentially precessing settling gas disk realistically and cannot be used to study the morphology of gas distribution.

Roche (1850) made the first study of the stability of binary systems. The problem he studied, and which now bears his name, is concerned with the stability of a liquid star in a synchronous, circular orbit about a point mass. Roche’s calculations predicted that if the separation between the liquid star and the point mass is S2.5rs’/3, where r is the unperturbed radius of the liquid star and s the ratio of the mass of the point to the mass of the liquid star, then the liquid star would be unstable. This result was generalised to two liquid stars orbiting each other by Darwin (1910, p. 436). Using the tensor virial method Chandrasekhar (1969) has given a uniform analysis of the classical result. The classical results have been extended by Hachisu and Eriguchi (1984a, b) who looked for the onset of instability in polytropes in binary orbits.

The Fleurs synthesis telescope, which provides 20 arcsec resolution at 1.4 GHz, was used to map the continuum emission in NGC 4945 and the Circinus galaxy. Both objects have prominent small-diameter radio nuclei, containing 50% to 75% of the total intensity, superimposed on extended emission associated with the outer regions of the galaxies. The scale of the nuclei, together with the large velocity widths of the associated spectral-line profiles, are not unlike those encountered in the central region of the Galaxy.

The five major members of the Sculptor Group of galaxies and NGC 45 have been observed with the Molonglo observatory synthesis telescope. NGC 247 and NGC 300 were not detected and upper limits to their 843 MHz flux densities are given. Radio emission from NGC 7793 is discussed with particular attention to its morphology, radial dependence compared with that of the blue light and possible coincidences between HII regions and 843 MHz peak flux densities.

We have discovered a radiation levitation mechanism which, under certain circumstances, can remove the last vestiges of hydrogen-rich material from an incipient planetary nebula nucleus if its mass exceeds a critical value of about 0.85M⊙. This process may be responsible for the production of helium-rich planetary nuclei, and their progeny the DB white dwarfs.

A total of 18 radio sources selected on the basis of steep low-frequency radio spectra have been searched for the presence of millisecond pulsars using the Molonglo Observatory synthesis telescope. The search covered pulsar periods down to 2 ms with a limiting sensitivity of approximately 10 mJy. No pulsars were detected.

An HI survey of the Large Magellanic Cloud (LMC) has been reanalyzed to find the transverse velocity of the LMC, and derive an upper limit of 4.5 × 1011 M⊙ for the mass of our Galaxy out to 50 kpc. A rotation curve is derived for the LMC from the HI data giving a best mass estimate of (4.0±0.4)×109 M⊙. Velocity observations of 97 planetary nebulae (PN) in the Large Cloud are used to compare the old and young components. Our results are found to be at odds with an earlier sample of 9 old clusters, which is interpreted as being due to the low number of objects in that sample. The w-component of velocity dispersion of the PN population is 35 km s-1 and that of the HI 10 km s-1. If this difference is a result of stellar diffusion then the average age of the PN population is 1.3 × 109 yr, implying a precursor mass of 1.8 M⊙ and a remnant mass of 0.63 M⊙.

As part of a program to investigate southern extragalactic radio sources, the Molonglo Reference Catalogue source 1452-517 has been observed with, the MOST and FST. It was found to have a compact core and broad lobes giving an angular size of around 20 arcmin. Despite the low galactic latitude (b = 6°) a probable identification has been made with a magnitude 18 galaxy indicating a physical size of over 1 Mpc.

Observations of the galactic background radio emission at a number of frequencies between 2.7 MHz and 1.07 MHz were made in association with the plasma depletion experiments of Spacelab-2 to test the concept of making low frequency radio observations through an artificial ionospheric window. Following the Shuttle OMS burn at a time when foF2 was 1.99 MHz, a decrease in the maximum ionospheric electron density of approximately 30% occurred. The first observations of the radio emission at high galactic latitudes with good angular resolution (25 degrees) were obtained during this event.

As part of an extensive southern survey of interstellar NH3 with the Parkes 64-m radio telescope (with a beamwidth of 81 arcsec), the (1, 1), (2, 2), and (3, 3) transitions have been observed towards the galactic centre molecular cloud G1.6-0.025. The cloud has an overall size of 10 arcmin, and contains several concentrations with differing velocities. It has several features also observed in other galactic centre clouds, e.g. high optical depths and kinetic temperatures above 50 K.

The fast-approaching return of Halley's comet in early 1986 has awakened fresh interest in comets and their role in the origin and evolution of the solar system. Some 2 years ago I suggested (Prentice 1983a, referred to as P83a) that the Sun may possess at its centre a comet-like core of mass about 1%M⊙ which had accreted from a swarm of grains and cometesimals which once occupied the interstellar cloud fragment from which the Sun had formed. The existence of such a central chemical inhomogeneity leads to a new class of evolved solar models which have a neutrino capture rate of 2.1 SNU, in line with the Davis experiment (Cleveland et al. 1981).

At optical wavelengths NGC 4945 is a nondescript edge-on spiral galaxy, much of which is obscured by dense dust clouds. However, at longer wavelengths a prominent nucleus is exposed which possesses both Seyfert and star-burst characteristics. Microwave observations highlight dense molecular clouds located near the nucleus. The large velocity range of these clouds lacks an adequate interpretation.

We have detected OH maser radiation from the comet Giacobini-Zinner using the Parkes radio telescope. The emission was detected in two transitions of the ground state of OH, at an intensity consistent with the predicted OH molecular production rate. We also searched unsuccessfully for OH emission from comet Halley, from which we are able to place a significant upper limit on the intensity of OH emission.

Propagation of 150 μs radio pulses (f= 1.91 MHz) along a magnetospheric field-aligned ionization duct have been observed, under conditions of low ionospheric critical frequency. The echoes, presumably reflected from the ionosphere at the magnetic conjugate point, were received at the transmitting site near Hobart. Positively identified echoes were observed on six separate occasions during periods of moderate solar activity, with delay times of approximately 260 ms, representing a value L = 2.95.

Two hundred and seventeen HI profiles at positions approximately 1 ° apart in the bridge region between the Small and Large Magellanic Clouds have been observed with a 15’ arc beam. Diagrams of all the profiles, lists of column densities and average radial velocities are given, together with details of the extensive Gaussian analysis needed to account for the components.

It is shown that the bridge region is most complex. (a) Two radial velocity groups, +214 and +238 km s-1, represent the actual HI bridge between the two galaxies, (b) Three other components, at mean radial velocities of +155, +177 and +195 km s-1 are seen to be integral parts of the SMC, stretching east to R. A. ~ 04h. (c) A further three components in groups at mean radial velocities +253, +272 and +293 km s-1 appear to be extensions of HI from the main body of the LMC.

Nine sets of five closely spaced observations in the lower Magellanic Stream and in the bridge region at high sensitivity supply further information about the region.