Session 103 - Cool Stars.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

## [103.07] The Onset of Chromospheric Activity in Open Cluster Stars

B. L. Rachford, R. W. Canterna (University of Wyoming)

We have analyzed published UBV photometry of the main sequences of a sample of nearby open clusters, and find a significant low number density area (gap") in the distribution at B-V=0.30. This corresponds to a previously observed gap in field stars near B-V=0.26. These B-V values correspond to spectral type F0, which is near the expected boundary between radiative atmospheres and convective atmospheres. Preliminary results are given of a survey of chromospheric activity in late A- and early F-type stars in open clusters using the He I \lambda5876 (D_3) absorption feature. These results indicate that the observed gap is at least an indirect consequence of the onset of convective energy transport.

This research was supported in part by the Wyoming Planetary and Space Science Center, NASA Grant #NGT-40050.