Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals

A "jump" in the BHB distribution in the V, u-y CMD was recently detected in the GC M13. It is morphologically best characterized as a discontinuity in u, u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs. 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 11,500K, irrespective of [Fe/H], mixing history on the RGB, and other GC parameters. We suggest that the u-jump is a ubiquitous feature, intrinsic to all HB stars hotter than 11,500K. 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 occurs 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 logg-jump are connected on a star-by-star basis. The two are likely different manifestations of the same physical phenomenon. We present a framework which may simultaneously account for the u-jump and the logg-jump. Reviewing spectroscopic data for several field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence that radiative levitation of heavy elements takes place at Teff>11,500 K, dramatically enhancing their abundances in the atmospheres of BHB stars 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.Comment: ApJ, Main Journal, accepted. Contains several changes and update