OLD OPEN CLUSTERS AS PROBES OF STELLAR AND GALACTIC DISK EVOLUTION

Abstract

ABSTRACT Open clusters have traditionally been used to investigate the chemical evolution of the Galactic disk, using the assumption that they reflect the chemical abundance of the interstellar medium (ISM). This assumption was tested by analyzing two samples of ~20 old open cluster spectra, one of evolved, red-horizontal-branch stars in the ~10-Gyr-old NGC 6791, and the second of unevolved G-dwarfs in the younger, ~4.5-Gyr-old M 67. Unlike many globular clusters, the band and line strengths of CN, CH, Ca, and Fe show neither a bimodal distribution, nor a CN-to-CH anticorrelation. However, the data for the evolved stars of NGC 6791 do provide evidence for a dispersion in CN band strengths. If the explanation for this is that stellar evolution after ~4.5 Gyr produces CN inhomogeneities, the CN-band strengths of very old, evolved stars are questionable tracers of the initial chemical abundance of the ISM. Stellar clusters are often used to study stellar evolution because such stars are coeval and equidistant. The results are then extrapolated to disk stars with little proof that the old disk stars originated in clusters. An essential link may be moving groups, stars that are not in close physical proximity but have similar kinematics and [Fe/H]. The age of eight HR 1614 Moving Group dwarfs was investigated, and it was determined that all eight are at least 3 Gyr old. This supports the scenario that many of the disk field stars formed in open clusters which subsequently were disrupted. Theories about Galactic disk evolution can be evaluated by comparing their predictions to the old open cluster system. The "inside-out" type of theory predicts that the ISM in the inner part of the Galactic disk is older and more chemically evolved than that of the outer part. Two samples of old open clusters, one from the inner and one from the outer disk, had significantly different mean [Fe/H] values, but similar mean ages. The former is in accordance with inside-out theories: the latter is not. The system of old open clusters appears to have extended to a larger radial distance ~5-7 Gyr ago, also not consistent with inside-out theories. These results can not be attributed to the preferential destruction of inner-disk open clusters.