Ian Thompson

The distances and ages of globular clusters -- spherical systems consisting of upwards of 100,000 stars in our galaxy -- are key stepping-stones on the path to understanding the age and scale of the universe. Globular clusters are among the oldest components of our galaxy. Previous measurements of the distances to them have compared the characteristics of different types of stars found in our solar neighborhood (for which we can estimate distances) with the same types of stars found in the clusters. For various reasons, measurements of the globular cluster distances suffer from systematic errors and remain the limiting factor in determining the ages of these objects.

Ian Thompson, with colleagues at the Observatories and the University of Warsaw, has a different approach to this distance problem. Observations of detached eclipsing binary stars allow a geometric measurement of distances. These systems are composed of two separated stars that orbit about each other, with the orbital plane in the line of sight to the stars. As one star passes in front of the other, the total light is modulated. Analysis of the shapes of these eclipses permits a measurement of the relative sizes of the stars and their separation. After additional observations are made of the variations of the radial velocities of the two stars, the absolute dimensions of the system can be derived from a simple application of Kepler's laws of gravity. Once the absolute sizes of the individual stars in the binary are known, the distance to the system can be measured by comparing the apparent brightness of the stars to their surface brightness as estimated from the infrared colors.

Thompson is searching for these exceedingly rare stars using the Swope 1-m telescope to monitor a selection of nearby southern globular clusters. Many nights of continuous monitoring are needed to detect the eclipses and then measure their orbital periods. Detailed measurements of the shape of the light curves are made with the du Pont 2.5-m telescope. When the Magellan telescopes are operational, the radial velocity curves will be measured with the echelle spectrograph. Although only one or two eclipsing binaries are expected to be detected in each cluster, these will be sufficient to overhaul our imperfect knowledge of their distances and ages. This research, and the collaboration with George Preston, Andrew McWilliam, and Steve Shectman in the search for extremely metal-poor stars, will show the new way in which the smaller telescopes at Las Campanas will be used in the upcoming Magellan era. Resources will be devoted to extensive surveys for interesting objects that can be studied in greater detail with the twin telescopes.

Fig. 10. This image shows light curves of a detached eclipsing binary in the globular cluster Omega Centauri, as measured in the blue, visual, and near-infrared. The eclipses occur at phases 0.0 and 0.5, and the different depths of the eclipses indicate that the two stars in the binary are not equally bright.