The Parkes 64 m telescope has been used to survey a complete sample of nearby red giants (i.e. not members of close binaries or showing strong emission lines of CaII). The sample consisted of all 82 stars in the Bright Star Catalogue within 30 pc having spectral types G, K, M and luminosity classes I, II, III south of the celestial equator.

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).

The Culgoora circular array (CCA) is a 3-km-diameter ring of 96 reflectors operating at 80, 160 and 327 MHz. It has an effective collecting area of ~ 6000 m2 and achieves angular resolutions (full half-power beamwidths) of 3’.70, 1 ‘.85 and 0’.92 at the three operating frequencies. During the interval 1978-1981 we have used the CCA to make 80 and 160 MHz measurements of a comprehensive selection of radio sources which were detected during various complete surveys of clusters of galaxies (see Table 1). We have combined our low-frequency intensity measurements with other available higher-frequency flux data to compute accurate radio spectra. The 160 MHz contour maps for many of the cluster fields were used to find positions and angular sizes for the associated radio sources.

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.

During 1971/72 the radioheliograph at Culgoora, N.S.W. was used to observe 30 bright galaxies at 80 MHz.

There has been considerable speculation in recent years about the evolution of radio galaxies in clusters. The discovery of powerful X-ray emission with an apparently thermal spectrum from a considerable number of clusters has been attributed to a hot (108K) intracluster gas with an electron density of ∼ 10-3 cm -3 at the cluster centre (see e.g. McHardy 1978). Such a gas surrounding a radio galaxy may conceivably retard the expansion or diffusion of the relativistic electrons and thus allow the source to retain its identity for longer intervals than is the case for field galaxies.

A recent analysis of the spectra of about 2000 sources in the Culgoora-3 list (Slee 1977) of radio sources (Slee et al. 1981) has revealed some interesting features which may be the result of source evolution. The Culgoora-3 spectra were obtained by combining the 80 and 160 MHz flux densities from the Culgoora radioheliograph with other published flux data between 10 and 10,000 MHz; this is probably the most accurate set of spectra yet derived for a large number of sources selected for observation in a reasonably consistent manner. In addition to fitting the log S-log v data for each source with a straight line (its slope is the usual definition of spectral index) Slee et al. also fitted second-degree and third-degree polynomials to search for curvature in the spectra.

During the interval May 1968 to March 1969 the 80 MHz radioheliograph at Culgoora has been used in a detailed study of the intensity variations in the pulsars CP 0834, 0950, 1133 and 1919. Some preliminary results for CP 1133 and 1919 have already been reported.

The amplitude scintillations of pulsars and the broadening of their profiles are different manifestations of the scattering of the signal by fine structure in the ionized interstellar gas.

The completion of the Uhuru (Forman et al. 1978) and Ariel V (Cooke et al. 1978) surveys of the sky for X-ray emission has resulted in many proposed identifications with individual galaxies and clusters of galaxies. The X-ray positions are not usually accurate enough to enable a positive identification to be made of the X-ray sources with optical or radio objects, and hence the identification is often based on statistical arguments — viz., the unexpected occurrence of unusual galaxies, radio sources or clusters of galaxies within or near the X-ray error boxes. There is usually no significant information available on the angular size of the X-ray emitter but in two or three cases (e.g. Perseus cluster, Coma cluster and Virgo cluster) the angular resolution is good enough to identify a broad component with dimensions approaching those of the whole cluster. This extended X-ray emission has been ascribed to either inverse Compton scattering of the 3° microwave background by relativistic electrons in the intra-cluster medium or to thermal-bremsstrahlung emission by an optically thin plasma at - 10s K.

The radioheliograph at Culgoora has recently been used to investigate the interesting changes which take place in the amplitudes of the pulses from PSR 0950+08, 1133+16 and 1919+21. The experiments had two main objectives: (i) to investigate the temporal changes in the spectrum near 80 MHz; (ii) to search for periodicities in the pulse amplitudes with periods extending up to one or two minutes.

We present an overview of the survey for radio emission from active stars that has been in progress for the last six years using the observatories at Fleurs, Molonglo, Parkes and Tidbinbilla. The role of complementary optical observations at the Anglo-Australian Observatory, Mount Burnett, Mount Stromlo and Siding Spring Observatories and Mount Tamborine are also outlined. We describe the different types of star that have been included in our survey and discuss some of the problems in making the radio observations.

During routine monitoring of active-chromosphere stars in August 1987 an intense radio flare on the FK Comae star HD 32918 was detected by the Parkes radiotelescope. Other observatories were notified, enabling data to be collected at 843 MHz, 1.4 GHz, 8.4 GHz and optical frequencies.

The flare at radio frequencies was largely similar to a previous event on this star, reaching a peak radio power of 5 < 1012 W Hz−1 at 8.4 GHz. During the flare the spectrum varied as ∼ v1.4 and later flattened as the flare progressed. Compared to the previous flare, circular polarization was higher.

Ca II and Hα optical spectra taken at the AAO 3.9-metre telescope show the star to have a highly active chromosphere and a strong wind, with a terminal velocity of ≈230 km s−1. While much of this activity appears to be long lived, there is good evidence for an increase in the strength of the chromospheric lines during the radio flare event.

We present some preliminary results of an optical and radio study of the very active RS CVn binary HD 127535. Photometric measurements show the presence of a large amplitude wave which exhibits marked changes in shape and range on time scales as short as a few months. This photometric variation is almost certainly due to large cool starspots on the cooler, more luminous component. As part of a survey of southern active-chromosphere stars with the Parkes radio telescope, HD 127535 has been observed at 5, 8.4 and 22 GHz. No detection was made at 5 GHz, possibly because of confusion due to the angular proximity of the star to the galatic plane. However, it is one of the strongest sources detected in the 8.4 GHz survey, and is one of only two stars detected at 22 GHz. Photometry obtained two cycles before the 8.4 GHz observations suggest a possible correlation between the radio emission and the photometric wave, i.e. spot visibility, but more data are needed.

Properties of the microwave emission from HR1099 are examined in an attempt to determine whether the emission arises as gyro-synchrotron radiation from mildly relativistic electrons trapped in magnetic fields above starspots on the active K subgiant component. It is shown that radio curves do not exhibit a systematic variation in phase with the rotation rate, as one might expect for emission from a source situated above a long-lived starspot. However, there is some evidence that the radio flaring occurs at two preferred longitude zones. Whether these zones agree with starspot locations remains to be determined by light curve modelling. What we can say with confidence is that the measured spectral index of the microwave emission does not fit a simple gyro-synchrotron source model, such as that proposed to explain the observed reversal with frequency of the sense of circular polarization.

The single G8V active chromosphere star HD36705 (AB Dor) was observed at 8.4 GHz with the Parkes 64 m telescope during three observing sessions involving a total of 21 days in the interval 1985 December to 1986 February. Subsequent photometric observations were made of the star with the 0.25 m and 0.45 m telescopes of the Monash Observatory in 1986 March-April. Two strong radio flares, each lasting three days, were detected; they yielded peak radio powers of P8.4≈4×109 W Hz-1, comparable with the microwave power emitted by the RS CVn binaries. Significant circular polarization of 13% left-hand was measured on only one of the six active days. The 8.4 GHz flux density showed smooth variation over an interval of several hours, consistent with the flare source being partly occulted by the stellar disk as the star rotated. When all the radio data was phase-binned using the known rotation period of 0.514 day we found two radio maxima corresponding to radio sources at stellar longitudes ~180° apart. The subsequent photometric data showed intensity variations that were consistent with the starspots at the same approximate longitudes. We thus interpret our radio curve as showing the presence of comparatively small (<0.5 D*) radio sources in the corona above the star spots. The upper limit to source diameter gives a peak brightness temperature ≥2×l010 K, which can be achieved by gyro-synchrotron emission only if the source is optically thick and the electrons, with average energy ~ 2 MeV, have a hard energy spectrum. The observed radiation can be due only to very high harmonics of the gyro-frequency, leading to an estimate for the magnetic field strength of ~30G.

This paper presents the results of a multiwavelength observational study of the active young F-type star HR 1817. The star was monitored at 4.80 and 8.64 GHz over 2 × 12 h allocations with the Australia Telescope Compact Array on 8 and 9 December, 2000. The Anglo-Australian Telescope was used for simultaneous optical spectropolarimetry during a 2 h period on 9 December.

The low levels of observed radio emission have characteristics that are similar to those seen in other active stars, and a gyrosynchrotron mechanism is proposed to explain them; this is supported by the relatively low fractions of circular polarisation measured in HR 1817.

Comparison of the emissions from 4.80 and 8.64 GHz shows a very strong cross-correlation peak, indicative of a common origin, although the shift of this peak indicates that 8.64 GHz variations tend to precede those at 4.80 GHz by, typically, ˜20 min.

The optical spectropolarimetry reveals polarisation signals characteristic of surface magnetic fields, with profile changes indicating a complex dynamo-type magnetic topology is present on the star. This result makes HR 1817 the star with the earliest spectral type on which dynamo magnetic fields have been detected directly up to now.

The Australia Telescope and Anglo-Australian Telescope were used in May 2000 to record the radio and optical emissions from the dMe flare star Proxima Centauri. Eight bright optical flares over a two-day interval resulted in no detectable excess short-term radio emission at 1.38 and 2.50 GHz. However, a slowly declining 1.38 GHz emission over the two-day interval was nearly 100% right circular polarised and was restricted to a relatively narrow bandwidth with total intensity (I) and circular polarisation (V) varying significantly over the 104 MHz receiver bandwidth. These are the first observations to show that highly-polarised narrowband flare star emission can persist for several days. This signature is attributed to sources of coherent radio emission in the star's corona. Similarities with various solar radio emissions are discussed; however, it is not possible with the existing observations to distinguish between fundamental plasma emission and electron–cyclotron maser emission as the responsible mechanism.

The Algol type eclipsing binary KZ Pav has been observed over one complete orbit (0.95 days) with theAustraliaTelescope CompactArray. Contemporaneous optical photometric data from the University of Southern Queensland’s Observatory at Mount Kent were also collected and have been included in this multi-wavelength study.

Preliminary indications are that the low levels of emission observed share similarities to those of some RS CVn binaries. The optical data show phase-linked effects which can be related to the mass transfer process of Algols. The possibility that the radio emission may be related to this process is considered.