Establishing fundamental properties for Evolved Stars in Open Clusters

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It is possible to learn a lot about stars from the company they keep. Nowhere are such connections more obvious than in those beautiful stellar ensembles we call open clusters, groups of hundreds to thousands of stars sharing a common age, distance, and original chemical makeup. What makes open clusters of immense value is their ability to serve as mileposts indicating distance, luminosity, and evolutionary status for rare classes of objects that are found within them. For many years that has included Cepheid variables, pulsating stars about as hot as the Sun but orders of magnitude larger, that have evolved from stars roughly 3 to 30 times more massive than our Sun and that have consumed all of the hydrogen fuel available in their hot, dense cores. In 1975, lists of cluster Cepheids contained only about 10 objects, but that number currently exceeds 50, many newer additions arising from work by the applicant. Likewise, the number of planetary nebulae currently recognized to belong to open clusters is zero, or one at most, but that number might be 6 or more with a bit of focused effort. Planetary nebulae represent a stage of evolution beyond the Cepheid interlude, and they have a distance scale that is known much less reliably than that of Cepheids, even before Cepheids were recognized to be members of open clusters. Other exotic objects found in open clusters include massive, pulsating M supergiants, standard candles in their own right much like Cepheids, but also the last stage of evolution prior to their explosion as Type II supernovae. The common thread linking these various objects is that they represent advanced stages in stellar evolution, where our knowledge base is less secure. Perhaps we can learn more about such stages of stellar evolution from the examples located in open clusters.