Collaborative Research: Fundamental Stellar Physics from Large-sample Globular Cluster Photometry

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AST 0098696SandquistIn a collaborative research project, Dr. Eric Sandquist at San Diego State University and Dr. Michael Bolte at the University of California at Santa Cruz will use photometric observations of large numbers (tens of thousands) of evolved low-mass stars in globular clusters to probe the physical conditions found inside the stars. Because the observable properties of a star evolve quickly once it finishes consuming its core hydrogen and because a star's lifetime is primarily determined by its mass, stars of very slightly different masses can be in very different phases of their lives. By simply counting the numbers of stars present as a function of luminosity and temperature, these researchers can estimate how quickly stars of approximately 80% of the mass of the Sun change under the influence of internal processes.The investigators will pursue a number of definite tests of stellar physics that can be conducted with these kinds of observations. In several metal-poor clusters, a surplus of stars has been observed in the subgiant evolutionary phase, which is an indication that there may be an unusually efficient means of transporting energy away from the cores of those stars. The red giant bump has long been known to test how deeply convective motions penetrate into giant branch stars, and the investigators will test mixing processes that this feature and counts of red giant stars at all luminosities can provide. They also describe several new diagnostics that can be applied to the helium fusion phases of a star's life ( the horizontal branch and asymptotic giant branch). The asymptotic giant branch in particular has been difficult to study because of its short duration. The diagnostics provide practical methods of testing whether helium-burning stars evolve in ways predicted by theory. Finally, they describe the constraints that can be placed on the dynamical evolution of entire clusters by observations of blue straggler stars -a population that appears to be created by collisions of stars or the evolution of binary stars.The study will involve the analysis of CCD data already collected from a number of heavily-populated globular clusters, followed by thorough study of the incompleteness of the photometry -- a computationally intensive step that has been responsible for the small number of large-sample studies to date. To derive robust star counts, what is needed is detailed understanding of the probability that cluster stars were lost in the light of other stars or in noise as a function of stellar brightness and distance from the cluster center. Photometric studies of this kind are very important stepping stones toward new tests of our understanding of stellar physics and stellar populations.***