Molecular hydrogen in spiral galaxies is distributed in clumps, i.e., molecular clouds, which have mass between 103M⊙ and 106M⊙ and a mass spectrum of n(m) ∝ m−1.6. Molecular clouds with masses greater than 105M⊙, are called giant molecular clouds (GMCs). It is generally accepted that GMCs are formed by the coalescence of molecular clouds through their collision. This process is studied by both numerical simulation and numerical integration. The observation with high resolution identified a great number of CO emission cores in galaxies. Based on this result, the aggregation or clustering formation of GMCs is numerically simulated. In the process of either coalescence or clustering, spiral perturbation plays an important role.

Temporal and spatial characteristics of solar flares are briefly reviewed in this paper. The global, temporal and spatial behaviours of flares are given first. Besides the 154-day periodicity, an 80-day periodicity of occurrence rate of large hard X-ray bursts for the period 1980 February – 1985 December, and the delay of the peak occurrence rate of large flares are pointed out, then the gregariousness of major flares is shown. In the third section, the time process and spatial structure of individual flares are shown and described according to space and ground-based observations. In the last section two problems on flare properties are discussed. (i) Previous classifications of solar flares are based generally on observations in a single spectral region. A new classification of flares based on observations in multi-spectral regions is given. (ii) Energy released in part of a loop seems to be not enough for a whole flare, and a qualitative model in which the energy is supplied by the untwisting of magnetic fields is proposed.

The disk stellar luminosity function has been redetermined by the mean absolute magnitude method, utilising the proper motion data of the LHS Catalog. The derived luminosity function shows a slightly deeper dip than that found by Wielen (1983) in the same magnitude range.

In this paper the author describes the instrumentation for the Infrared Space Observatory to be launched in 1993.

The most favourable methods of observing the phenomenon of gravitational lensing are through high-amplification events and the time delay between the images. These effects provide us with the information to determine the Hubble parameter and the matter distribution in the universe. The image properties due to micro-lensing can be used to find the size and structure of the source.

The recent intensive study of Comet Halley based on in situ measurements, observations carried out with rockets and satellites and supplemented with co-ordinated Earth based observations has not only confirmed pre-Halley results, but also has given new insight into the nature of the nucleus, dust, gas and the interaction of cometary plasma with the solar wind. These observations also have raised many new questions and problems. Several of these aspects will be discussed. For a better understanding of these problems, the planned future missions to comets will also be discussed.

We consider the linear and non-linear evolution of a perturbed X-type neutral point. A semi-analytic treatment is given for the case of small disturbances of the equilibrium field. This problem admits well defined azimuthal modes which allow a formally exact determination of the magnetic annihilation rate. It is shown that the longest lived modes are purely radial and decay of the timescale τ ≈ |ln η| where η defines the resistivity of the coronal plasma. Higher azimuthal modes decay much faster, generally on a fraction (O(m−1)) of the Alfvén timescale for the outer field.

We go on to perform finite amplitude calculations that demonstrate the implosive current build-up that precedes the reconnective phase of the relaxation. In general both linear and non-linear studies support the idea of an initial implosive stage which rapidly releases the bulk of the energy associated with arbitrary field disturbances.

Astronomical observations from space-borne instruments are carried out in India in the areas of infrared, X-ray and gamma-ray astronomy. This paper briefly describes the facilities available in India for conducting experiments in space astronomy using balloons, rockets and satellites. It briefly reviews the important results obtained by Indian astronomers from observations made in India with the balloon, rocket and satellite experiments. The present status of research in different disciplines of space astronomy is discussed.

The relative importance of the stellar sources contributing to the production matrix of the heavy elements up to iron is reviewed. Three main element groups may be distinguished: (a) oxygen and the alpha-process elements; (b) the iron-peak group; (c) helium, carbon and nitrogen. Each of these is produced in stars of a different mass range and in different ways, and it is shown that an examination of metallicity-metallicity relationships can be used to constrain models of the history of star formation, stellar evolution, and the initial mass function in galaxies.

We can conclude that in the case of our local solar neighbourhood the initial Fe/O ratio was set by Type II supernovae, but that Type I½ supernovae were never important. Iron is produced by the Type I deflagration supernovae on a time-scale comparable to the infall time-scale of the gas. Carbon is produced by dredge-up in low-mass stars, but nitrogen is shown to be produced both in the stellar winds of massive stars, and in higher mass stars which give rise to the Type I planetary nebulae.

The Modern Laplacian Theory (Prentice 1978) of the origin of the solar system assumes a non-dissipative model of supersonic turbulence and the existence of stable rings left behind during the contraction of the proto-Sun. We show by numerical simulation that the turbulence is highly dissipative and the rings are unstable. As a result of the instability the rings spread and interact with the proto-Sun. The rings therefore cannot form in the way Prentice has proposed.

III Zw 35 is a pair of galaxies characterised by intense OH maser emission, and powerful far-infrared and radio continuum. We have made a detailed study of the galaxy pair based on optical, infrared and radio observations. The brighter northern component is identified as a LINER or Seyfert galaxy and contains an active nuclear region from which radio continuum, OH maser and thermal dust emission are detected. We propose that the northern component has a compact active nucleus deeply embedded in an obscured region of diameter ~ 210 pc within which enhanced star-formation occurs. The lower luminosity, southern component is of low mass and is undergoing starburst activity over an extended region of diameter ~ 5.5 kpc. The origin of the starburst and non-thermal activity appears to be an interaction between the two components.

During 1990 we surveyed the southern sky using a multi-beam receiver at frequencies of 4850 and 843 MHz. The half-power beamwidths were 4 and 25 arcmin respectively. The finished surveys cover the declination range between +10 and −90 degrees declination, essentially complete in right ascension, an area of 7.30 steradians. Preliminary analysis of the 4850 MHz data indicates that we will achieve a five sigma flux density limit of about 30 mJy. We estimate that we will find between 80 000 and 90 000 new sources above this limit. This is a revised version of the paper presented at the Regional Meeting by the first four authors; the surveys now have been completed.

The first QSO with a redshift z > 4 was found using a combination of UK Schmidt Telescope (UKST) plates and the APM automatic plate measuring machine (Warren et al. 1987a). By continuing to make use of UKST survey plates plus the APM facility we have added substantially to the number of known QSOs with z > 4. A brief description of the general survey technique is presented together with a preliminary discussion of some of the results obtained so far.

The multi-object spectroscopic facility FOCAP at the Anglo-Australian Telescope has been used to obtain spectra centred at the Ca II IR triplet of 14 stars in the field of the Sextans dwarf spheroidal (dSph) galaxy. This satellite of our own Galaxy was recently discovered by Irwin et al. (1990) from APM measures of UK Schmidt Telescope photographic plates.

An accretion disc becomes warped when subjected to a torque which is misaligned with the disc plane. Such torques may be caused by Lense-Thirring precession near a spinning compact object, or the quadrupole field of a binary star. Here the flow in an adiabatic warped disc is modelled as a two-dimensional shear layer with linear velocity profile and free surface boundary conditions, and is investigated by means of a linear stability analysis.

The flow is found to be unstable whenever it contains a critical layer, i.e., a level at which the shear velocity is equal to the phase velocity. The instability occurs over a broad wavenumber range and has a typical dimensionless growth rate ≈ 0.1 for both the compressible and incompressible cases. These waves grow with a time-scale of about one orbital period, and are likely to have a major effect on the disc viscosity.

This paper presents the current status of a Ph.D. project undertaken to search for extended-scale bursts of star formation (> 1 kpc) in irregular and collisionally produced ring galaxies, principally in the southern half of the sky. Results of recent 8.4 GHz radio continuum observations and UBVRI CCD imaging of some of the program galaxies are presented.

This paper discusses several aspects of radio emission and star formation in dwarf galaxies.

A high-resolution image of the strong extragalactic source 1343–601 has been made with the Molonglo Observatory Synthesis Telescope (MOST) at 843 MHz. The source is identified with a 10 mag galaxy (Mv = −24) which is heavily obscured through our Galaxy. A recent redshift (z = 0.01215) gives a distance of 73 Mpc and a corresponding radio luminosity of log P843 = 24.9.

The MOST image shows a core linked by a double jet to asymmetric lobes which are surrounded by a confined balloon (400 × 200 kpc2) of low-brightness plasma. This has a complex network of ridges or flows which lead out into the wider cluster medium. If similar balloons exist around other double sources, they could provide the depolarising medium required by the Laing-Garrington effect.

Separate observations show the core is compact with strong X-ray emission.

Surface photometry of the elliptical galaxy NGC 1700 is presented and discussed in terms of providing evidence for warping of the outer isophotes in elliptical galaxies.