AAS 207th Meeting, 8-12 January 2006
Session 11 Binary Models and Parameters
Poster, Monday, 9:20am-7:00pm, January 9, 2006, Exhibit Hall

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[11.05] The Secret Lives of Cepheids II: More surprises as We Extend the Period/Luminosity Range of our Sample

S. Engle, E. Guinan (Villanova University), W. Heintz (Swarthmore College), J. Carton, A. Hernandez (Villanova University)

We report on further results of an ongoing study of the brightness and radial velocity observations of Classical Cepheids over time. The older photometry and spectroscopy are combined with recent observations to determine possible changes in the pulsation periods, mean luminosities, light and radial velocity amplitudes with time. In some cases, over 100 years of observations have been collected and analyzed in a uniform way. The Cepheids selected in our initial program were those already known to have significant period changes. However, our second batch of program stars also includes Cepheids with no known changes in their periods. Surprisingly, the majority of these Cepheids also display what could be a change in their light amplitude over time.

Our pilot study of Cepheid evolutionary changes included Polaris (\alpha UMi), SZ Tau, \delta Cep and SV Vul which, with a period of ~45d, is one of the longest period Classical Cepheids known. Our newest studies include SU Cas because this star is one of the shortest period Classical Cepheids (~1.95d) known. SU Cas displays no change in period yet may be undergoing changes in its light amplitude as characterized by archival and recently obtained (by us) photometry. SU Cas will join the several other Classical Cepheids in our study as we extend our sample to the largest possible range in both periods and luminosities.

Studying the changes of the light and radial velocity curves as well as the pulsation periods for these stars will provide important clues on what may be discernible (real-time) evolutionary changes for Classical Cepheids. The period changes are especially sensitive to variations in the internal structure of the stars from evolutionary effects. Therefore, it is very interesting to see if any relationship between period, light amplitude, and brightness changes exists. It is also interesting to see how a star of seemingly `constant' period, such as SU Cas, has evolved over time. The results for SU Cas and other program stars will be presented and discussed, as well as additional recent observations of some stars in the pilot program.

This research is supported in part by NSF/RUI Grants AST-0507542 and AST-0507536 which we gratefully acknowledge.

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