CONTENTS

Course: VARIABLE STARS IN GLOBULAR CLUSTERS

Prof. Michael W. Feast
University of Cape Town SOUTH AFRICA

"HIPPARCOS" AND THE NEW DISTANCE SCALE

Hipparchus of Nicaea (II Century BC) catalogued 850 stars. The precision of his measurements allowed him to discover the retrograde motion of the celestial sphere known as "precession of the equinoxes". In August 1989 the European Space Agency launched an astrometric satellite christened with the rather forced acronym HIPPARCOS (from HIgh Precision PARallax COllecting Satellite) in homage to the Greek astronomer. The new HIPPARCOS Catalogue contains the positions, parallaxes and proper motions of 120000 stars, with a hitherto unprecedented degree of precision that correct previous values by at least 10 per cent. Professor Michael W. Feast of the University of Cape Town (South Africa) presented results on the distance scale obtained from the data of this satellite at the III meeting of the Spanish Astronomical Society. According to this investigator, thanks to the parallaxes of variable stars obtained with HIPPARCOS, it has been possible to estimate the distances to globular clusters with greater accuracy than ever before. These distance calculations are of vital importance for establishing the ages of globular clusters - they are as old as their respective galaxies - and, in turn, the age of the Universe. Accuracy is of the utmost importance in this case: ‘a change in distance of 10 per cent changes the age by 20 per cent’, observes Feast.

"Globular clusters have been of immense importance for our understanding of the Universe. Attempts to understand the first colour-magnitude diagrams of globular clusters led to the currently accepted theory of stellar evolution. In this theory stars stay most of their lives on the main sequence and then involve through the following sequence; giant branch, horizontal branch, AGB, post-AGB, planetary nebula and white dwarf. These early c-m diagrams changed our whole concept of stellar evolution. Furthermore the realisation that globular cluster stars were both old and metal-poor was a major indicator that most elements other than hydrogen and helium have been manufactured inside stars through the life-time of the Universe rather than all being produced in an initial big-bang as was at one time suspected."

"Detailed studies of globular clusters both observationally and theoretically are of the utmost importance for our estimation of the age of the utmost importance for our estimation of the age of the oldest stars. These ages obviously set a lower limit to the age of the Universe. The quest to determine the age of globular clusters has driven observers to obtain photometry and spectroscopy of cluster stars of ever increasing accuracy. In the course of this work a great deal has been, and still is being, learned about he processes of stellar evolution, the dredging up of nuclear-processed material form the interior of the star to the surface etc."

"Hipparcos parallaxes of nearby subdwarf stars can be compared with the subdwarf main-sequence in globular clusters to provide much better estimates of cluster distances than was possible previously. These distances are of vital importance for several reasons. For instance when comparing observations of a cluster distance. The derived age is very sensitive to the adopted distance. A change in distance of 10 per cent changes the age by about 20 per cent. It seems likely that the uncertainties in the post-Hipparcos ages of clusters will be due more to uncertainties in the theory of stellar evolution than uncertainties in the distances."

"A particularly intringuing problem is the following. There appears (at least at first sight) to be a discrepancy between the absolute magnitudes of RR Lyrae variables in globular clusters and those in the general field of the galactic halo. The luminosities of RR Lyraes in clusters are derived via the cluster distances which are based on Hipparcos parallaxes of nearby subdwarfs. The luminosities of RR Lyrae variables in the general halo field are derived from a statistical parallax analysis. It is not yet certain that this difference is real. If it is, it will have important implications of the use of RR Lyrae stars as distance indicators to nearby galaxies. It will also indicate that the field and cluster RR Lyraes (of the same overall metallicity) are different from each other in some way, perhaps in the relative abundances of different elements in their atmosphere."

Michael W. Feast was born in England and obtained a PhD in physics from Imperial College (London). He then spent two years as a post-doctoral fellow working with G. Herzberg at the National Research Council of Canada in Ottawa. All his research work up to that time had been in laboratory studies of molecular spectra.
In 1952 he moved to the Radcliffe Observatory Pretoria, South Africa. This Observatory had at that time the largest telescope in the southern hemisphere (1.9m). The work there was mainly spectroscopy and centred on studies of the structure of our Galaxy and the Magellanic Clouds as well as astrophysical studies of objects of particular interest including the globular cluster 47 Tuc which was quickly shown to be unusual and is now the prototype of metal-rich globular clusters.
In 1974 Feast moved to Cape Town where the old Royal Observatory became the headquarters of the South African Astronomical Observatory (SAAO) and the 1.9m reflector was moved to the SAAO station at Sutherland. He was director of the SAAO from 1976 to 1992 and from 1983 an honorary professor in the University of Cape Town where he continues his research.
Much of his recent work has been on the use of variable stars to establish distance scales and in studies of galactic structure, the Magellanic Clouds and stellar evolution.