The Rotation of Blue Horizontal Branch Stars

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Astronomers use the "H-R Diagram" (surface temperature vs. luminosity) of stellar clusters to infer the evolutionary properties of stars of a common age and chemical abundance. Over a decade ago it was noticed that the distribution of stars along the "horizontal branch" of globular clusters depends in part on chemical abundance and in part on something else. What this "something else" might be constitutes the "second parameter problem" in the study of stellar clusters. In earlier NSF-supported work the Principal Investigator (PI) has found, to the surprise of most astronomers, that horizontal branch stars rotate rather rapidly. The PI will examine the rotation rates of horizontal stars in eight globular clusters to determine whether rotation is the missing parameter. In addition, the PI's earlier work led theoretical astronomers to suggest that the rotation rates of cores of intermediate mass stars may continue to remain high during the stars' lifetimes while the rates at their surfaces decelerate from the influence of magnetic fields in their coronae. The new work is expected to test this possibility and to offer one of the few ways of inferring the interior rotation rates of stars currently on the "main sequence." These interior rates are otherwise inaccessible to our direct measurement.