Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13

Abstract

High-resolution optical spectra of 13 blue horizontal-branch stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (T_(eff) > 12,000 K), helium is depleted by factors of 10-100 below solar, while iron is enhanced to 3 times the solar abundance, 2 orders of magnitude above the canonical metallicity of [Fe/H] ≃-1.5 dex for this globular cluster. Nitrogen, phosphorus, and chromium exhibit even more pronounced enhancements, and other metals are also mildly overabundant, with the exception of magnesium, which stays very near the expected cluster metallicity. These photospheric anomalies are most likely due to diffusion—the gravitational settling of helium and the radiative levitation of the other elements—in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the cluster's horizontal branch and on estimates of its age and distance.

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