Among the currently interesting problems of interpreting the Magellanic Stream as an instance of tidal disruption is that of finding some demonstrable phenomenon in the makeup of the involved material that parallels the better known cases of tidal interaction, as for example NGC4038/39, where the dynamic and star-formation time scales are in agreement (Schweizer 1977). The observational problems that beset the interpretation of the Magellanic Stream are numerous (Mathewson and Schwartz 1976, Kunkel 1979, Bregman 1979), and the marked difference between the composition of the Stream (evidenced purely through HI) and the stellar systems (at least 80 percent stars) is among the outstanding dilemmas remaining. Finding some counterpart in the Magellanic scene comparable to the better recognized instances of tidal interaction may go some way towards clarifying a perplexing situation.

It is becoming increasingly clear that no (or only one or two) binaries occur among the evolved stars in globular clusters. Therefore, if binaries exist at all in these systems, they must be found on or near the main sequence. I have chosen 6 clusters to search for faint eclipsing binaries by the following criteria:

1. (1) the apparent visual distance modulus (Harris 1976) (m-M)V ≤ 14.5 mag;

2. (2) the Peterson and King (1975) concentration class c ≤ 1.5, so that the search can be conducted near or at the cluster center where binaries would most likely be found; and

3. (3) the galactic latitude is sufficiently large to avoid problems of extreme contamination by field stars.

An observing programme was started in the summer of 1977 to measure radial velocities of red giants in open clusters with the spectrovelocimeter “Coravel” (Baranne et al. 1979) attached to the 1 m telescope of the Geneva Observatory installed at the O.H.P. (France). 750 measurements for 160 stars in 30 open clusters north of −20°, with ages ranging from those of NGC 752 to Pleidades, have been obtained. The limiting magnitude is B = 12.5.

In order to study the development of the binding-energy distribution of hard binaries in a star cluster, solutions of the appropriate kinetic equation have been obtained, using the three-body encounter rates calculated by Heggie (1975). The binaries in a homogeneous, time-independent stellar medium are considered first. We calculate an analytical solution, of self-similar form, that can be applied to very hard binaries. Integrated forward in time from an initial state containing no hard binaries, a numerical solution of the kinetic equation rapidly approaches the equilibrium ‘Saha’ form of the energy distribution at small energies, while at high energies the numerical solution behaves like our analytical self-similar solution. The fluctuations in the distribution, due to the stochastic nature of binary creation and evolution, are analyzed. We calculate the rate of exchange encounters when stars of different masses are present; these rates are then combined with the other Heggie rates to find the binary distribution in a multi-mass environment. Next, the creation rate as a function of energy is obtained for the binaries that form in two-body, tidally dissipative encounters. This rate is combined with the three-body encounter rates to calculate how the energy distribution of the tidal binaries evolves. Finally, the binding-energy distribution of hard binaries is obtained in a more realistic inhomogeneous, time-dependent cluster model. Although the evolution of the cluster is governed primarily by the effects of two-body relaxation, the influence of the binaries on the cluster is considered as well. A detailed report of this study has been submitted to The Astronomical Journal.

The high quantum efficiency, linear response, large dynamic range and large number of picture elements make the electronographic camera the most suitable of the presently available detector systems for the study of the colors and magnitudes of stars in the clusters associated with the Magellanic Clouds. Owing to the remoteness of these objects and the consequent small angular separation of the stars within them, the best method of determining electronographic magnitudes of the cluster stars is still that of measuring the star image profiles on one-dimensional scans made using a Joyce Loebl microdensitometer. While this technique does not utilize all of the information contained in the star image, it provides better correction for overlapping star images and variable background than is achieved with two dimensional methods that simulate observation through a focal-plane diaphragm. By this method, it has been possible to measure a star of B =23.8 on a 180 min exposure in 2.0 arcsec seeing taken with a Spectracon image converter on the 1.5 m Tololo reflector. The probable error of this measurement is ±0.12 mag, while the error due to photon statistics is ±0.06 mag. Thus, using electronographic cameras on larger telescopes and in good seeing, it should be possible to study the stellar content of the Magellanic Cloud clusters down to about the luminosity of the Sun. Further, current studies indicate that if a sufficient number of electronographic magnitudes are determined in a cluster, these can then be used to set up calibration curves as a function of radial distance from the cluster center, permitting photographic photometry of a large number of stars to be carried out, free of the usual limitations imposed by the effects of the unresolved background light of the cluster on the photographic measures. Using this technique, cluster observations can be carried out effectively using a small-field electronographic camera such as the Spectracon, which may be much smaller and less expensive than the large-field electronographic cameras, and thus more practical to use at distant observing sites.

The study of globular cluster systems is very important. Because these systems are so old, their investigation is a form of archeo-astronomy whereby a slow and, at times, tedious gathering of fragmentary data laboriously uncovers the skeleton of early galactic structures. Our theories and our imagination can then try to put some flesh on these bones in attempts to visualize what these early inhabitants of the Universe might have looked like.

Rapid improvements in instrumentation over the past few years have made the spectroscopic study of individual globular cluster giants feasable. Three years ago I began a program of high dispersion abundance analyses of such stars to provide a calibration for the many photometric systems used to rank globular clusters in metallicity. The results for four clusters (M92, M15, M13, and M3) of low and intermediate metallicity have already appeared (Cohen, 1978, 1979), and additional detailed analyses of stars in M5 and M13 (Pilachowski, Wallerstein and Leep, 1979) will soon be available. Ignoring the elements C, N, and O, to which we shall return later, these detailed abundance analyses yielded few great surprises; perhaps the metallicity scale that had previously been used was too high by about 0.2 dex, and also it became clear that M3 was a very metal poor cluster. However, the calibration of the metal rich globulars beyond the simple ranking level of Mould, Struthman, and McElroy (1979) had not been attempted.

This paper reviews some recent theoretical results on the evolution of cores in globular clusters, including the effects of relaxation, binaries, and a central black hole.

Globular clusters in galaxies still challenge astronomers to find a satisfactory explanation for a number of their properties, namely: (i) their very formation; (ii) the origin of their heavy elements; (iii) the trend of their metallicity with distance from the centre of the parent galaxy; (iv) the apparent correlation of their average metallicity with the mass of the galaxy with which they are associated; and, (v) the need of a “second parameter” (age? helium? [CNO/Fe]?) to explain the variety of morphologies of their colour-magnitude diagrams. Dwarf spheroidal galaxies pose similar problems, to an even more puzzling degree. It is clear that an understanding of the above topics is practically the same as the understanding of the very early evolution of galaxies. Any model for the formation and early evolution of galaxies should necessarily be tested whether or not the predicted properties of the associated globular cluster family are in agreement with the observational constraints. Conversely, any attempt to account for the characteristics of globular clusters would be blind without the support of detailed protogalactic models.

NGC 6144 is situated in the general area behind the nebulosity in Scorpio-Ophiuchus between its counterpart M4 and the bright red giant star Antares. The color magnitude diagram (CMD) is characterized by a steep giant branch, a group of exclusively blue horizontal branch stars, and a well defined instability gap within the colour range ~0.4 < B-V < 0.8. The CMD has the typical features of metal poor clusters, similar to M3, M15, and M92. From the analysis of five circular areas equal to the one measured in the cluster, it is shown that absorption increases towards the NE direction. The deduced reddening of E(B-V)=0.40 places the object at 15.8 kpc from the Sun, 4.3 kpc from the galactic plane and 7.8 kpc from the galactic centre, assuming Ro=9 kpc. The inferred values of δV~3.1 and S~5, corroborate the tendency of metal deficiency suggested by the integrated colours, which render Q=-0.41.

We present photographic photometry for 1135 stars in the globular cluster NGC 6397, which, at a distance of 2.4 kpc, is most likely the second nearest globular to the Sun. The Racine wedge with the CTIO Yale 1 m telescope (Δm=3. 60 mag), the CTIO 4 m telescope (Δm=6. 83 mag) and the ESO 3.6 m telescope (Δm=3. 87 mag) was used to extend the photoelectric calibration from V≃16.1 to V≃20.7. The main sequence turnoff at V=16.7 and B-V=0.52 with respectively Mv =4.30 and (B-V)o =0.36 yields (m-M)v=12.40 and E(B-V)=0.16. Using the models of Iben and Rood (1970) and the isochrones of Demarque and McClure (1977), we deduce the cluster's age to be 17 × 109 years. This makes this object the oldest of the nine globular clusters with age determination and gives a lower limit to the age of the universe, rendering Ho ≤ 57 km sec−1 Mpc−1 if qo ≥ 0 is assumed. The large age spread of 6 billion years between NGC 6397 and 47 Tuc (the youngest counterpart with age data) indicates both that the protogalaxy underwent a slow collapse phase and that the abundances in globular clusters are lower for the oldest. The fact that the galactocentric distances for these clusters have the narrow range of 6 <R < 13 kpc makes it highly important to secure age data for extremely metal poor globulars far out in the halo.

A photoelectric sequence has been set up in the Oosterhoff Type II cluster NGC 4590 (M68) and UBV photographic photometry has been secured for all the uncrowded stars using the Radcliffe 1.9 m telescope. A series of 40 plates in each of UBV have enabled light and colour curves to be obtained for all the uncrowded RR Lyrae variables. For most of these, periods were found very similar to those found by Osterhoff. One star, however, showed variable amplitude from night to night and great difficulty was encountered in fitting a single period. Subsequent p.e. photometry with the 1.0 m SAAO reflector has indicated that this RR Lyrae variable is pulsating in at least two modes. The mean colour is that of the transition from fundamental to first overtone pulsators, as shown by the other ab and c type variables in the cluster. The two periods found are 0.526 and 0.392 days with amplitudes 0.39 and 0.47 mag indicating a ratio P1/Po ~ 0.745. Further observations are planned to give an accurate value for this ratio and to obtain simultaneous radial velocities to give mass and radius information.

The assumption that individual globular clusters formed from chemically homogeneous protosystems, which seems to be valid for some clusters, is clearly not valid in others, e.g., ω Cen and M22 (see Freeman and Rodgers 1975, Hesser, Hartwick and McClure 1976, Butler, Dickens and Epps 1978 and references quoted therein). One indicator of chemical inhomogeneity is the width of the cluster giant branch in the C-M diagram, because the B-V colors for the cool giants are sensitive to heavy element abundance. NGC 3201 and M 5 show narrow giant branches (Lee 1977, Arp 1962) and so are not expected to show large star-to-star heavy element differences.

It is now well established that there is a wide range of surface chemical composition among the evolved stars of ω Centauri, and that this causes exceptional scatter on the red giant branch in the V, (B-V) C-M diagram. However, different atomic and molecular species vary in different ways and it is not clear how much of the variation should be attributed to mixing during evolution and how much if any is primordial.

Yerkes refractor plates of M3, M5, M13, M15, and M92 have yielded proper motions for a few hundred stars in each cluster with typical standard errors cent−1. Membership probabilities have been derived, leading to color-magnitude diagrams essentially free of field star contamination.

One of the two classes of models that have been proposed for the production of X-rays in globular clusters involves accretion of gas by a more or less massive (10 M⊙–1000 M⊙) black hole at cluster centre (Cowie, Ostriker and Stark 1978 - COS). The accretion rates are not large (~1017 g s−1), and we may enquire how such rates compare with those for accretion on cluster stars in general.

This talk will briefly summarize some recent results based on infrared observations of giant stars in globular clusters. The program is being carried out jointly with Eric Persson and Judith Cohen of the Hale Observatories.

The diffusion process of helium was studied in evolutionary star models. A typical horizontal-branch (HB) star (M=0.53 M⊙, Y=0.3, Z=0.001) was evolved up to the beginning of the asymptotic giant-branch phase. Helium was shown to diffuse inwards in the atmosphere and subatmosphere in a rather short interval of time. Therefore the suggested efficiency of diffusion in explaining the helium deficiency in old B- and HB-stars in globular clusters received a significant support. In models where the surface convection is not efficient enough to restore the original helium content in the outermost stellar layers, stars in subsequent evolutionary stages (such as blue and white dwarfs) should still be deficient of helium.

This is a preliminary report on a project in which I have been comparing observed and theoretical giant branch luminosity functions for a relatively large number of globular clusters. I believe that I have found some novel evidence to support significant He differences among globulars, especially for the inner part of the galaxy, where He appears to be increasing toward the galactic center.

Spectrum scans at 10 A resolution have been obtained for four likely clusters in the field of the giant elliptical galaxy NGC 4486 (M87), using the IPCS at the 4-metre Anglo-Australian Telescope. A wavelength range of 3500–7000 A was covered, with the objects having brightnesses from V=19.5 to V=19.8 mag. Preliminary analysis suggests that one of these objects is comparable to the metal-richest known Galactic globular clusters, while the other three seem to be even more metal-rich. Alternative explanations, such as gross differences in the stellar mix within the M87 clusters compared to those in our own Galaxy, may account for the observed spectroscopic differences; quantitative analysis is just beginning.