The Oldest Open Star Clusters Revisited: Their Distances, Reddenings and Ages

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AST 97-31656 Kenneth Janes Dr. Janes is observing five nearby open star clusters with approximate ages greater than 2,000,000,000 years. The goal of the project is to determine the fundamental properties, particularly the ages, of stars in these five open clusters. Perhaps the greatest uncertainties in the ages of star clusters are the uncertainty in the distance scale and the difficulty in determining interstellar reddening. The distance scale (from the nearest stars to the edge of the Universe) depends and builds upon the fundamental determination of stellar parallax. The availability of the new Hipparcos Catalog with its precise parallaxes makes it possible to resolve some of these uncertainties - especially for open clusters. Because there are large numbers of nearby Population I main-sequence stars, many of which have better determined parallaxes than the subdwarfs, and because the standard star color systems are defined by many of those same parallax stars, it is possible to do a better job of tying the old open cluster distance scale to the Hipparcos than can be done with globular clusters. The PI is obtaining new high precision photometry of these five clusters, and also developing improved analysis tools to reduce the data. The five clusters span the age of the Milky Way disk, so they are well matched to the problem. The project depends upon large numbers of observations on moderate aperture telescopes combined with careful and extensive calibrations. Regular photometric monitoring of open cluster stars will make it possible to identify many, if not most, of the non-member stars, as well as variable stars and binaries. Other interesting stellar phenomena might be found and characterized: delta Scuti stars, reflection effects in close (but non-eclipsing) systems, and eclipsing systems with secondary components perhaps as small as Jupiter. ***