AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 53. Star Clusters Near and Far, Old and Young
Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southeast Exhibit Hall

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[53.09] Blue Horizontal-Branch Stars: The ``Jump'' in Strömgren u, Low Gravities, and Radiative Levitation of Metals

M. Catelan (NASA Goddard Space Flight Center), F. Grundahl (University of Victoria), W.B. Landsman (Raytheon ITSS/NASA Goddard Space Flight Center), P.B. Stetson (Dominion Astrophysical Observatory), M.I. Andersen (Nordic Optical Telescope)

A ``jump'' in the blue horizontal-branch (BHB) distribution in the V, u-V CMD was recently detected in the globular cluster (GC) M13. It is morphologically best characterized as a discontinuity in u, u-y, with stars in the range 11,\!500 < T\rm eff{\rm K} < 20,\!000 deviating systematically from (in the sense of appearing brighter and/or hotter than) canonical zero-age HBs. We present u, y photometry of 14 GCs obtained with 3 different telescopes (Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose HB extends beyond 11,500~K, irrespective of [Fe/H], mixing history on the red giant branch, and other GC parameters. We suggest that the u-jump is a ubiquitous feature, intrinsic to {\em all} HB stars hotter than 11,500~K. We draw a parallel between the ubiquitous nature of the u-jump and the problem of low measured gravities among BHB stars. We note that the ``logg-jump'' occurs over the same temperature range as the u-jump, and that it is present in every metal-poor GC for which gravities have been determined---irrespective of [Fe/H], mixing history on the RGB, or any other GC parameters. Furthermore, the u-jump and the \log\,g-jump are connected on a star-by-star basis. The two are likely different manifestations of the same physical phenomenon.

We suggest a framework which may simultaneously account for the u-jump and the \log\,g-jump. We present strong evidence that radiative levitation of heavy elements {\em does} take place at T\rm eff > 11,\!500~K, {\em dramatically} enhancing their abundances in the atmospheres of BHB stars lying in the ``critical'' temperature region. Model atmospheres taking diffusion effects into account are badly needed, and will likely lead to better overall agreement between canonical evolutionary theory and observations for BHB stars.

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