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J. D. Cline, M. W. Castelaz (Pisgah Astronomical Research Institute)
The well-known period-luminosity relationship of Cepheid variables is used for extragalactic distance measurements. Understanding the evolution of Cepheids and their migration off the instability strip is therefore important. Several Cepheids are known to exhibit period increase along with amplitude decrease. The most publicized Cepheid showing these characteristics is a UMi. Alpha UMi is an F7 Ib-II to F8 Ib-II type, V = 2.0 magnitude Cepheid with a period of 3.97 days and amplitude of DV = 0.03 mag. Ferro (1983) describes a rate of period increase and amplitude decrease beginning after 1945. Dinshaw et al. (1989) studied the radial velocity curve of a UMi and concluded that it is evolving to the red edge of the instability strip with pulsation stopping in 1995, and the possible presence of starspots. Fernie, Kamper, & Seager (1993) also suggested a cessation of Cepheid activity by Polaris before 1995. However, in 1998, Kamper & Fernie (1998) note that Polaris was still pulsating, and the decline in amplitude has stopped. They stated that they had no explanation, and that continued monitoring of Polaris is clearly important.
Polaris appears to be close to the red edge of the first-overtone Cepheids and evolving into the area of fundamental pulsators. As the evolution occurs, the amplitude of pulsation of a UMi may increase again (Kamper & Fernie 1998). The behavior of the star in the previous 50 years has been dramatic, and suggests the need for continued monitoring. A systematic study over a period of years will provide a fundamental contribution to our knowledge of stellar structure and physical parameters of the Cepheid a UMi.
We present a new program to observe a UMi 24 hours a days, seven days a week, using a 0.3 m telescope with initial results. The a UMi observation program includes BVRI photometry of a UMi, a comparison, and a check star.
This research is partially supported by a grant from the AAS Small Research Grant Program.
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The author(s) of this abstract have provided an email address for comments about the abstract: dcline@pari.edu
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.