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Session 107 - Open Clusters.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[107.04] Determining Cluster Reddenings: A New Method

N. A. Miller, L. N. Hong (Maria Mitchell Obs.), E. D. Friel (MMO/CTIO), K. A. Janes (Boston U.)

We have developed a technique for determining the reddening to open clusters by using the equivalent width of the Balmer line, H\beta, to determine the intrinsic color of early-type stars in the clusters' fields. Our technique attempts to quantify spectral classification in spectra of moderate resolution using the temperature sensitivity of the H\beta line. We also use the strength of secondary indicators like Mgb (5170 Åto help distinguish spectral type for stars near A0, where the H-line strength is double-valued with respect to color.

Members of the well-studied open cluster M67 were used to develop the calibration. The moderate resolution spectra used in this project were taken with the multi-object spectrographs at the CTIO and KPNO 4-meter telescopes. The calibrating cluster M67 could be observed from both sites, allowing the two data sets to be consistently combined. The two observations of M67 also resulted in a large number of calibrating stars, giving a well-defined relationship between (H\beta) strength and intrinsic color for stars of known luminosity class.

The calibration has been applied to obtain estimates of intrinsic color and thus reddening to individual stars in the fields of a number of open clusters, and the distribution of reddening with distance then constrains the reddening along the line of sight to the clusters. For clusters whose parameters are known, the technique gives excellent agreement with previously published estimates of reddening. The method also provides reddening estimates to a number of open clusters that lack estimates of reddening by traditional methods, such as King 5 and 11, Be 17, 20, 31, 32, and 39, To 2, Pi 2, Cr 261. These clusters include the oldest and most distant open clusters known, and improved estimates of their reddening are crucial for accurate determinations of cluster age and metallicity.

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