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I. Platais (JHU), V. Kozhurina-Platais (STScI), S.A. Barnes (Florida IT/Yale U.), I.N. Reid (STScI), M. Belfort (U. Pittsburgh), J. Sperauskas (Vilnius U., Lithuania), U. Dzervitis (IoA, Latvia), N.M. Bronnikova (Pulkovo, Russia)
This is a story about the seemingly old (1.4 Gyr) and nearby (d=600pc) open cluster NGC 6882 in Vulpecula. The age, distance, and other basic properties of this cluster were established several decades ago. We have derived new proper motions using 41 plates taken with the Normal Astrograph at Pulkovo Observatory. Our highly precise measurement of proper motions (sigma=0.3 mas/yr) for the first time allows us to separate cleanly cluster stars from the field. Our new cluster membership list, in combination with the CCD BV photometry, the older UBV data, 2MASS JHK photometry, radial velocities, and low resolution spectroscopy indicate that the cluster properties are very different from previous estimates. The derived color-magnitude diagram cannot be fit with a 1.4 Gyr isochrone. Based upon the older UBV data we offer an explanation that NGC 6882 is actually a considerably reddened (E(B-V)~0.6) and young open cluster (~30 Myr). Its distance is definitely larger than 1 kpc although there is difficulty in deriving the precise value due to the effects of differential reddening. Two decades earlier similar conclusions were reached by Dzervitis based upon his photometry in the Vilnius system but using a less certain selection of cluster members. Additionally, our astrometric data indicate that another cluster in the same area, NGC 6885, was incorrectly classified as a star cluster. Most likely it is part of an OB association.
This study is supported in part by NSF grant AST-0321794 to The Johns Hopkins University, NSF grant AST-0338953 to Yale University, and the Taiwan-Baltic collaboration.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.