The rate of change of magnetic flux Ф contained within a closed curve C moving with the plasma velocity u, is given by

where B is the magnetic induction and S is a surface bounded by the curve C. Using the hydromagnetic equation,

where η is the magnetic diffusivity, this becomes

In many astrophysical contexts η is small and thus approximately

Since this implies that the flux within any moving closed curve does not change, the field is said to be ‘frozen into’ the plasma and moving with it.

Clusters of galaxies appear to contain much more mass than just that of the galaxies which provide most of their optical luminosity. There are two important ways to investigate the structure of their gravitational potential wells, firstly by an analysis of the radial velocities of the cluster galaxies, and secondly by a study of the X-ray emission from the hot gas which forms part of the intra cluster medium. We present new samples of radial velocity data for three clusters, and discuss some simple types of models of the X-ray sources.

The distribution of faint galaxies in a deep sample, down to a limiting magnitude of B ~ 21.5 in a region of some 30 square degrees of sky at the South Galactic Pole, is investigated. The sample was obtained from objective measurements made with the COSMOS automatic plate-scanning machine. The two-point angular correlation function shows positive correlation on scales out to ~1.5° and thereafter positive correlations on scales ~ 2.5° and ~ 5° indicating the presence of clustering in this sample on linear scales out to ~ 8h-1 Mpc and at ~ 13h-1 and ~ 27h-1 Mpc. On application of an algorithm for the detection of filamentary structure in the distribution of the galaxies, a negative result is obtained.

The alignment of galaxies within clusters and superclusters may be sought either in the major axis of the observed optical distribution or in any radio or X-ray structure from an active nucleus.

It is the usual practice during continuum radio observations at Parkes to observe ‘pointing sources’ concurrently with the program objects.

The galaxy IC4553 (Arp 220) is a peculiar galaxy having a redshift of 0.02. It has been variously described as consisting of a symbiotic pair of galaxies, as having a highly peculiar pair of nuclei, or as consisting of a nucleus bisected by a dust lane. In addition, it exhibits OH maser emission which is both the most intrinsically luminous (by four orders of magnitude) and the most distant yet detected. MERLIN observations of the continuum emission at 18 cm show a core-dominated triple structure, probably indicating jets about 500 pc in extent. The integrated OH maser emission has an identical spatial structure, but when this structure is examined as a function of frequency across the line it is found that each spatial component has its greatest intensity at a different frequency. This is interpreted as amplification of the radio continuum image by the OH masers. The change in spatial structure across the emission line is caused by the velocity structure of the OH gas. Measurements of the velocities of these masers therefore reveal the kinematics of the central part of the galaxy. Optical and infrared observations indicate that at the centre of the galaxy is a Seyfertlike active galactic nucleus, which appears double on optical plates only because of the dense dust lane which bisects the nuclear region. This indicates that IC4553 is an edge-on Seyfertlike galaxy. The OH maser radiation is a result of stimulation of the OH along the line of sight within the galactic plane by the strong far-infrared radiation from the nucleus. No abnormal conditions are required for this process, so IC4553 may not be unique, and we should search for OH maser emission in other edge-on active galaxies.

It is well established (e.g. Slee et al. 1983) that radio galaxies near the centres of rich clusters of galaxies tend to have steeper radio spectra than field radio galaxies. The fact that the sources with the steepest spectra occur in clusters that are highly luminous X-ray emitters has generally been interpreted in terms of the confining influence of a hot (~108 K), relatively dense (10-2 to 10-3 electrons cm-3) intra-cluster gas; the confined relativistic plasma then preferentially loses its high-energy electrons through synchrotron and inverse Compton losses, resulting in a steepening of the radio spectrum. A more detailed review of the evidence for this process is given by Robertson (1983).

Schmidt (1968) and Rowan-Robinson (1968) developed a simple and concise test for the uniformity of the distribution of quasars (QSOs) based on QSO luminosity and occupied volume.

EX Hya was one of the earliest detected eclipsing cataclysmic variables. The 98 min orbital period was first documented as a result of the spectroscopy of Kraft and Krzeminski (1962) and the photometry of Mumford (1964, 1967). However, new properties of this system continue to be discovered and these have required a more complex model than was previously envisaged.

The galactic nuclear bulge is a well defined population, but it is one that is relatively difficult to study in detail because its members never stray into the solar vicinity and they are both distant and obscured. Mould (1982) has summarized much of our knowledge of the masses and composition of stars in the bulge. The red giants resemble those in metal-rich old open clusters and the late M giants are either younger than galactic globular clusters or super-metal-rich or both. Whitford and Rich (1983) have demonstrated the existence of K-giants that are super-metal-rich in iron and Wood and Bessell (1983) have interpreted the properties of a sample of long period variables in the bulge as those of a young, super-metal-rich population.

A catalogue of 38 supernova remnants (SNRs) identified in the Large and Small Magellanic Clouds from X-ray, optical and radio observations has recently been presented by Mills et al. (1984). One important consideration is the completeness of this catalogue and of the Galactic catalogues with which it is compared. These are currently being investigated.

We present broadband photoelectric light curves for the RS CVn type star PZ Telescopium for 1980, 1982 and 1983. The photometric period is about 0.943 days. The V light curve shows radical changes in form and range over a few months, and may be continuously variable. B and V data were obtained in 1982 and 1983. In 1982 no (B-V) change with phase was detected. However, in the first part of the 1983 observing season, a (BV) change of around 0.02 magnitude was found. Also at this time, maximum light was some 0.05 magnitude above that measured previously. Our preliminary spectroscopic data obtained in 1983 indicate that PZ Tel is a double lined binary whose components are of approximately equal luminosities, but this is yet to be confirmed. We suggest that the photometric variations are due to the presence of large cooler starspots on the photosphere of one or both components, as seems to be the case for related systems. The rapid changes in the observed light curve imply equally rapid changes in the distribution of the starspots, and make this an interesting object for further study.

The mechanism by which planetary nebula (PN) shells are ejected is still subject to considerable uncertainty. It is generally assumed that the precursors of these objects are low mass (M< 5 M⊙) asymptotic giant branch (AGB) stars, and that the nucleus of a planetary nebula (NPN) is undergoing a final gravitational contraction to the white dwarf state. The shell consists of some or all of the remaining unburnt (though not necessarily uncontaminated), hydrogen-rich material out of which the star was originally formed.

Carbon, nitrogen and oxygen are of interest for stellar and galactic evolution for four main reasons. Firstly, they comprise most of the mass of elements heavier than helium, so their abundance reflects the bulk of chemical enrichment. Secondly, all of the oxygen, much of the carbon and perhaps some of the nitrogen is believed to be produced in shorter-lived stars more massive than those responsible for the Fe production (Tinsley 1979), so their abundance relative to Fe in very metal deficient objects should provide key information for modelling the chemical history of galaxies. Thirdly, because C and O comprise the bulk of the metals in stellar material (Fe:C:N:0 = 1:12:2.5:21 in the Sun) it is their abundance as well as that of iron, which is needed to compute evolutionary tracks for different metallicities. Finally, the O abundance will indicate whether CNO material has been mixed to the surface.

We have carried out a survey of the 115 GHz CO emission (J = 1→0 transition) towards the Carina nebula (NGC 3372) using the Epping 4 m radio telescope of the CSIRO Division of Radiophysics. Of all the molecules detected in the interstellar medium CO is perhaps the best tracer of molecular clouds. It is an abundant molecule (second only to H2) and its line emission at microwave frequencies occurs at relatively low excitation.

There is a difference of opinion as to whether W33, an HII-region/molecular-cloud complex of size about 15′ arc centred near l = 12°.8, b = -0°.2 is a single object or a chance line-of-sight alignment of two complexes at completely different distances.

The Molonglo Observatory synthesis telescope (MOST) of the University of Sydney (Mills 1981) produces maps of the 843 MHz continuum emission from fields of width 23′, 46′ or 70′ arc. The telescope comprises two co-linear east-west cylindrical paraboloids each 2186λ in length and separated by a gap of 43λ. For each paraboloid a phasing network (Durdin et al. 1984) generates a comb of 64 contiguous fan beams. Mapping is accomplished in real time during a 12-h observation by overlaying, in the map plane, the instantaneous cross-correlations of corresponding beams. The synthesized point-source response (beam) produced by this method has a width of 43″ (E-W) by 43″ cosec δ (N-S).

A sensitive search has been made for OH maser emission from a sample of 16 symbiotic stars. This sample has been selected on the basis of infrared optical depth and variability, so that the stars within it have circumstellar shells similar to those seen in the well-known OH/IR and OH/Mira stars. There were no significant detections, except for one unassociated background source, and we conclude that the presence of a hot binary companion inhibits any possible OH maser action.

In its normal synthesis mode of operation, the Molonglo Observatory Synthesis Telescope (MOST) tracks a region of sky for a period of 12 hours with 64 real-time fan beams having high sensitivity at 843 MHz (Mills 1981). It thus provides an excellent opportunity to monitor the sky at the same time for transient radio events. During a 12 hour synthesis observation the fan beams rotate 180° on the field. Thus any sources producing occasional radio transients can be located by analysing the positions of the beams on which the events are recorded. Futhermore, by rejecting events which occur simultaneously on non-adjacent beams, local terrestrial sources of impulsive interference may be eliminated. This technique for recognizing extra-terrestrial sources was of considerable value in the first Molonglo pulsar search when only two beams were used.