Blue Horizontal-Branch Stars: The ``Jump'' in Stromgren u, Low Gravities, and Radiative Levitation of Metals
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_effK < 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 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 does take place at T_eff > 11,500 K, 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.
American Astronomical Society Meeting Abstracts #194
- Pub Date:
- May 1999