AAS 196th Meeting, June 2000
Session 41. Globular Clusters
Display, Wednesday, June 7, 2000, 10:00am-7:00pm, Empire Hall South

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[41.10] Two Post-AGB A-F Stars in the Globular Cluster NGC 5986

H. E. Bond, D. R. Alves, C. Onken (STScI)

The little-studied southern Galactic globular cluster NGC 5986 is remarkable in containing two very luminous A-F-type stars. Membership of the two stars in the cluster is confirmed by their radial velocities and by Strömgren photometry showing very low surface gravities. These stars are candidate objects lying on post-asymptotic-giant-branch (PAGB) tracks.

We present new CCD BV\/ photometry of the cluster, based on frames from the CTIO 0.9-m telescope. The cluster's color-magnitude diagram (CMD) reveals a blue horizontal branch. The cluster also contains about half a dozen RR Lyrae variables, for which we have obtained light curves. Our data lead to new estimates of the cluster's reddening and distance.

The two PAGB stars both have nearly the same absolute magnitude, near MV=-3.4, demonstrating that A-F type PAGB stars are excellent candidates for a Population~II standard candle. Neither star shows significant variability, showing that both lie to the blue of the instability strip in the CMD. It is, however, remarkable to find two PAGB stars in the same globular cluster, given the extremely short lifetimes of such stars. It is alternatively possible that they have evolved from the blue end of the horizontal branch, and are related to Population~II Cepheids.

Supported by NASA grant NAG5-6821.

The author(s) of this abstract have provided an email address for comments about the abstract: bond@stsci.edu

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