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Session 54 - Star Clusters, Kinematics and the Galactic Center.
Display session, Wednesday, June 10
Atlas Ballroom,

[54.06] On the Identification of New Low-mass Members of the Alpha Persei Cluster and Rotation Periods of Open Cluster Stars

C. F. Prosser (NOAO)

A review is given of the photometric/spectroscopic examination of possible optical counterparts to more than 200 ROSAT X-ray sources in the region of the \alpha Persei open cluster resulting from the analysis of two X-ray datasets: (a) a raster survey (Prosser amp; Randich 1997 [Center for Astrophysics (CfA) Preprint 4537]) and (b) three 22-25 ksec ROSAT PSPC pointings (Prosser, Randich amp; Simon 1997 [CfA Preprint 4538]). CCD photometry is employed to obtain magnitudes and colors for stars/objects close to the X-ray positions, with additional echelle and low-dispersion H\alpha spectra provided for some stars.

In the raster survey, of 73 X-ray sources not matched to catalogued stars, \sim 40 have an optical counterpart with photometry acceptable for cluster membership and \sim 20 of these also have radial velocities consistent with membership. In the PSPC pointings, for almost 60 X-ray sources a photometric candidate member is identified, some of which can be excluded from membership on the basis of discrepant radial velocity or X-ray characteristics. On the order of 30 new members or likely members have been identified from the PSPC pointings based on available data. We discuss the X-ray properties of these potential new members and why they may not have been identified in earlier membership surveys.

Additionally, photometric rotation periods of open cluster stars have been reported by Prosser amp; Grankin 1997 [CfA Preprint 4539]. Several members of the \alpha Persei cluster have been monitored and the corresponding relation between coronal X-ray activity and rotation period derived. The relation among mid-G/K type members illustrates both the previously noticed downturn in L_X/L_bol at high rotation rates and the sharp decrease in coronal activity at long rotation periods as seen among Pleiades stars. Intensive observation of one slowly rotating G-type member of IC4665 has enabled a period determination of 8-10 days to be made and illustrates the need for (and limitations of) high quality observations.

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Program listing for Wednesday