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.

Polytropic models have played a significant part in the historical development of stellar structure theory and in other related branches of theoretical astronomy such as stellar pulsations and rotation. In modern astronomy polytropic solutions, because of their wide range of density distributions and our ability to introduce them analytically if necessary, now have an acknowledged role in large scale numerical experiments especially during the developmental stages.

A coronal hole is a region of the solar corona characterised by diverging magnetic fields of single polarity and lower-than-average densities (and probably temperatures). It is now generally accepted that coronal holes are the source of high-speed streams in the solar wind (Munro and Withbroe 1972, Kopp and Holzer 1976, Steinolfson and Tandberg-Hanssen 1977, Munro and Jackson 1977; see Pneuman 1980 for references to most of the theoretical and observational papers on coronal holes since their recognition in 1968). We consider an infinitesimal field-aligned flow tube with cross-sectional area A(r) where r is the heliocentric radius (Figure 1), and we adopt the functional form

A(r)/A(r ) = (r/r0)8

where r is the coronal base radius, and 5 is a parameter which measures the divergence of the flow. If s = 2 then the flow is purely spherically symmetric, while if s > 2 the flow is more strongly divergent as is expected to be the case for a coronal hole. The cross-section of a typical coronal hole is shown schematically in Figure 2.

The two important parameters which essentially determine the nature of the convective regime, and consequently the total heat transported by convective processes, across an unstable layer in a star are the Rayleigh and Prandtl numbers defined by (Chandrasekhar 1961)

where ν is the coefficient of kinematic viscosity, x is the coefficient of thermal diffusivity and α, g, d, ΔT are respectively the coefficient of thermal expansion, acceleration due to gravity, depth of the fluid layer and temperature difference across the layer. In addition, the horizontal scale of the convection cell is determined by the horizontal wave number a and for convection to be established R must be greater than the critical value at marginal stability, Rc, which is a function of a, given by

when the free boundary conditions apply.

The radio source PKS 0511-48 is among the hundred strongest southern sources at low frequencies. It was catalogued in the Parkes survey (Ekers 1969) and mapped with the Molonglo Cross at 408 MHz by Schilizzi and McAdam (1975) with a resolution of 2′ .6 arc. It appeared in that work as a complex source, with an integrated flux density of 8.8 Jy. A search for optical identifications was made by Tritton and Schilizzi (1973), but none were found to a level of B ~ 19. We have inspected the field on the ESO B survey film, and find a group of ~ 20 faint galaxies (approximately 19-20 mag).

Our re-examination of the neutral hydrogen gas in the Small Magellan Cloud has led to four important results. Firstly, we find that Hindman’s (1967) total content HI map is a satisfactory representation of the gas in the line of sight. Secondly, we find that the HI gas in the SMC exists in four distinct large masses separated from one another in radial velocity by 20 to 30 km s−1. Thirdly, having made this division of the gas we show that there is good correlation between the radial velocities of HII regions, supergiant stars and HI. Finally, we believe that our observations reveal that the SMC is associated with an extremely large trailing halo of HI gas which forms the major component of the inter-cloud bridge region.

The traditional classification of solutions of the Lane-Emden equation

into E, F and M types is based on the value of the invariant quantity

Ground-based observations of Type III bursts made with spectrographs and spectro-polarimeters, at frequencies above the ionospheric cut-off, reveal that most bursts (excluding storm Type IIIs) have fundamental (F) and harmonic (H) structure (Wild et al. 1959; Dulk and Suzuki 1980). An example of F-H bursts is given by Sheridan (1978). Such bursts are produced by streams of electrons travelling along open magnetic field lines and exciting plasma oscillations which are converted to electro-magnetic waves at both the F and H frequencies of the local plasma frequency in the corona.

In a previous paper (Van der Borght 1979) the techniques developed for studying finite amplitude convection in a compressible medium were applied to an investigation of super-granulation in the Sun. Assuming that the instability is due to He+ recombination, a model was derived which seems to indicate that convection in the upper 8000 km of the Sun may occur as an overtone and not as a fundamental mode. This investigation yielded results which were very close to those obtained from standard mixing-length theory.

We describe a method for reducing displacements in radio source positions caused by ionospheric refraction. Our method is an improvement on that used by Wild et al. (1959), who assumed all frequencies in the 40-70 MHz band came from the same position above the Sun and applied an f-2 correction for ionospheric refraction. Our method retains the dispersion of source position with frequency, which is inherent in the radio source, but allows for the f-2 ionospheric effect. We also discuss ways of estimating the absolute source positions from a knowledge of ionospheric density gradients. A typical example of ionospheric variations on solar records is shown in Figure 1(a). Here we have plotted the observed source positions on an f-2 scale. The measurements refer to a solar storm continuum burst which occurred on 1981 May 9 following an importance 2B flare at 08°N., 38°E., heliographic coordinates. From previous observations of such events we think it highly likely that the true source positions are stationary and displaced with decreasing frequency outwards along a line close to the radial direction above the flare. Figure 1(a) shows that: (a) the 160, 80 and 43 MHz source displacements vary as f-2; (b) there is a systematic slow drift of the source positions towards the south and east which increases with increasing hour angle; (c) superimposed on this steady drift is a quasi-periodic variation in source position with a period ~20 min.

We present preliminary results from a programme designed to produce deep images of radio source fields drawn from the Parkes 2700 MHz and Molongolo 408 MHz catalogues using the charge-coupled-device (CCD) camera system built at the Institute of Astronomy, Cambridge. The programme is directed at a search both for faint extensions and nebulosity around radio QSOs and BL Lac objects and for faint objects in otherwise empty radio source fields; a detailed examination of the morphology of selected radio galaxies is also included.

All studies of circulation in stars have been based on a linear perturbation analysis (e.g. Sung 1975). This analysis establishes criteria for the onset of circulation and, if the perturbations remain weak, allows the circulation pattern to be determined. (For a survey see Tassoul 1978).

A type of metre-wavelength continuum is observed in close association with solar flares and Type II radio bursts. This continuum is stationary in position, often intense, of a long duration (occasionally lasting well over an hour) and highly polarized (Robinson and Smerd 1975; Robinson 1978). It is thought to be produced by the conversion of Langmuir waves to electromagnetic waves near the local plasma frequency. Because of its close association with Type II bursts, the class has been designated as Type II related flare continuum, or FCII. In this paper I examine a particularly clear example of such an event, with emphasis on the relation between the Type II and the start of the continuum.

The HR 1614 moving group of stars (Eggen 1978a) is one of a number of such groups identified by Eggen from amongst the disk population of the Galaxy (see e.g. Eggen 1969). The group was selected from a sample of 44 field stars south of + 30° declination, with right ascensions mainly > 12h and <2h, and with motions similar to that of the star HR 1614, which has a velocity of (U, V) = (−1, −58) km s−1, where U and V are the components of space motion in the direction of the galactic anticentre and galactic rotation respectively, measured relative to the Sun. The colours of these 44 stars, when plotted in a (b-y, R-I) diagram, were such that 26 of them had an excess blanketing of ~ 0.03 in b-y, measured with respect to the Hyades group. This excess blanketing was consequently chosen to be a criterion for group membership.

A Type II solar radio burst is a relatively narrow-bandwidth metre-wavelength emission which drifts outward in the solar corona at a velocity between 500 and 1500 km s-1. It was first described by Wild and McCready (1950) and since then it has been the subject of numerous investigations (see e.g. McLean 1974; Nelson and Robinson 1975).

Carbon stars have always been amongst the most intriguing of stars which show grossly non-solar abundances. All such stars raise the questions:

(a) at what stage of stellar evolution does this peculiar abundance phase occur?

(b) do all stars pass through this stage?

(c) is the phase significant in the overall picture of galactic nucleosynthesis?

When two stars are in orbit about each other, tidal forces cause an exchange of energy and angular momentum between the orbital motion and the fluid motion of each star about its own centre of mass. The orbit is progressively altered, and one or both stars may be catastrophically disrupted.

Duley (1974) has shown that, at the temperatures usually associated with interstellar gas clouds, we would expect CNO grains to be present. During gravitational collapse these grains migrate to the centre of the gas cloud, leading to an enhancement of the heavy-element abundance in the core (Prentice 1976, 1978). It was Krautschneider (1977) who verified such a scenario, by considering the dynamical collapse of gas and grain clouds. If such an initial radial abundance inhomogeneity existed, Prentice (1976a) showed that this configuration may well survive the later convective mixing phase and thus approach the zero-age main-sequence (ZAMS) with a small (-v 3% by mass) metal enhanced core.

The statistical theory of strong three-body interactions (Monaghan 1976a, b, Nash and Monaghan 1978) is based on the assumption that the motion of the system in phase space is ergodic, and that the energy and angular momentum are the only isolating integrals. The a priori phase space is infinite because the system is not confined by walls, but this is only a formal difficulty since the numerical calculations show that the motion takes place in a finite region of ordinary space and this fact can be incorporated in the statistical description. The probability density of the system in phase space is essentially Gibb’s Micro-Canonical ensemble.

Many of the important events in the life of a star occur, or are thought to occur, during the red giant or supergiant phase of evolution. For example, in heavy and intermediate mass stars supernova explosions terminate normal evolutionary processes while in lower mass stars the stellar envelope is entirely removed giving rise to planetary nebulae and, subsequently, white dwarfs. Theoretical calculations suggest that before the onset of these rather drastic events, a significant amount of nucleosynthesis occurs, giving rise to enhanced surface abundances of He, C, N and s-process elements (e.g., Iben and Truran 1978; Renzini and Voli 1981); loss of the envelope material by stellar winds, planetary nebula ejection and supernova explosions produce overall galactic enrichment in these elements.