A New Method of Determining Reddening: Results for Distant Open Clusters

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Session 110 -- Open Clusters
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[110.03] A New Method of Determining Reddening: Results for Distant Open Clusters

L.N. Hong, E.D. Friel, K.V. Tran (MMO)

We have developed a technique for calibrating intrinsic color from measurements of the equivalent widths of the Balmer lines (H$\beta$,H$\gamma$,H$\delta$) in moderate resolution spectra of early type stars. These calibrations are used to determine reddening, E(B-V) or E(V-I), for stars in the field of open clusters for which there are no previous reddening determinations, including some of the oldest and most distant open clusters known. The observations were made with the multiobject spectrograph Hydra at the KPNO 4meter as part of a program to determine metallicities and radial velocities for evolved stars in these clusters, and included as many early type stars and potential blue straggler members in the field of each cluster as possible.

Stars in the clusters M 67, NGC 2420, and NGC 7789 which covered the range of intrinsic colors $-0.2< (B-V)_{0} <$ 0.9 or 0.0$< (V-I)_{0} <$0.8 were used as calibrating standards. We estimate that we can determine intrinsic colors and reddening to $\sim$ 0.02 mag over the range of intrinsic colors 0.1$< (B-V)_{0} <$0.4 or 0.1$< (V-I)_{0} <$0.5. Using this calibration, the intrinsic colors for early type stars in the field of the clusters NGC 188, 2158, 6791, Be 17, 31, 32, and King 5 and 11 were determined from Balmer line equivalent widths, and estimates for mean reddening to the clusters were derived. These reddening estimates are of particular importance in the determination of cluster abundances and of ages by main-sequence isochrone fitting for these oldest open clusters.

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