AAS 201st Meeting, January, 2003
Session 124. WIYN Open Cluster Study
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[124.04] Tidal Circularization Cutoff Periods in the WOCS Open Clusters NGC 188, NGC 2168 (M35), and NGC 6819

S. Meibom, R. D. Mathieu, K. T. Hole, C. Dolan, S. Barnes (University of Wisconsin - Madison)

Circular orbits prevail among the shortest period binary stars as a consequence of tidal interactions between the stars. The longest period circular orbit in a coeval binary population, known as the "tidal cutoff period", is a measure of the tidal circularization rate integrated over the lifetime of the population. As such, determination of tidal cutoff periods as a function of age provides a critical test for the theories of tidal circularization.

As part of the WIYN Open Cluster Study (WOCS), we have used the WIYN 3.5m telescope and Hydra multiobject spectrograph since 1996 to obtain precise radial velocities for late-type main sequence stars in the open clusters NGC 2168 (age ~ 0.15 Gyr) and NGC188 (age ~7 Gyr). Well defined tidal circularization cutoff periods are found at periods of 10.3 days and 14.9 days for NGC 2168 and NGC 188, respectively. Combined with previously determined tidal circularization cutoff periods of 7.7 days among pre-main-sequence stars, of 8.5 days in the Hyades at an age of ~0.6 Gyr and of 12.4 days in NGC 2682 (M67) at an age of ~4 Gyr, a progression of tidal circularization cutoff period with age is clear for ages greater than ~1 Gyr. That this trend does not continue to younger binary populations suggests that pre-main-sequence tidal circularization processes may set an initial cutoff period at 8-10 days.

We are also nearing completion of a radial-velocity survey of NGC6819 (age ~1.5 Gyr). Our first results also will be reported in this paper.

This work is supported by NSF AST 9731302.

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