HOT HORIZONTAL-BRANCH STARS: THE UBIQUITOUS NATURE OF THE “JUMP” IN STRÖMGREN u, LOW GRAVITIES, AND THE ROLE OF RADIATIVE LEVITATION OF METALS

A “jump” in the blue horizontal-branch (HB) distribution in the V , u − y color-magnitude diagram has recently been detected in the globular cluster (GC) M13 (NGC 6205) by Grundahl, VandenBerg, & Andersen (1998). Such an effect is morphologically best characterized as a discontinuity in the u, u− y locus, with stars in the range 11,500 K . Teff . 20,000 K deviating systematically from (in the sense of appearing brighter and/or hotter than) canonical zero-age HB models. In this article, we present Strömgren u, y photometry of fourteen globular clusters obtained with three different telescopes (ESO Danish, Nordic Optical Telescope, and the Hubble Space Telescope), and demonstrate that the jump in Strömgren u is present in every GC whose HB extends beyond Teff & 11,500 K, irrespective of metallicity, mixing history on the red giant branch (RGB), or any known parameter characterizing our sample of GCs. We thus suggest that the u-jump is a ubiquitous feature, intrinsic to all HB stars hotter than Teff ≃ 11,500 K. We draw a parallel between the ubiquitous nature of the u-jump and the well-known problem of low measured gravities among blue-HB stars in globular clusters and in the field. We note that the “gravity jump” occurs over the same temperature range as the u-jump, and also that it occurs in every metal-poor cluster for which gravities have been determined—again irrespective of metallicity, mixing history on the RGB, or any known parameter characterizing the surveyed GCs. Furthermore, we demonstrate that the u-jump and the gravity-jump are connected on a star-by-star basis. We thus suggest that the two most likely are different manifestations of one and the same physical phenomenon. We present an interpretative framework which may be capable of simultaneously accounting for both the u-jump and the gravity-jump. Reviewing spectroscopic data for several field blue-HB stars, as well as two blue-HB stars in NGC 6752, we find evidence that radiative levitation of elements heavier than carbon and nitrogen takes place at Teff & 11,500 K, dramatically enhancing the abundances of such heavy elements in the atmospheres of blue-HB stars in the “critical” temperature region. We argue that model atmospheres which take diffusion effects into account are badly needed, and will likely lead to better overall agreement between canonical evolutionary theory and the observations for these stars. Subject headings: diffusion — stars: abundances — stars: atmospheres — stars: evolution — stars: horizontal-branch — stars: Population II Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on observations obtained with the Danish 1.5-m telescope at the European Southern Observatory, La Silla, Chile. 3 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 W. Saanich Road, Victoria, BC V8X 4M6, Canada; [email protected], [email protected] 4 University of Victoria, Department of Physics & Astronomy, PO Box 3055, Victoria, BC V8W 3P7, Canada 5 Guest user, Canadian Astronomy Data Centre, which is operated by the Dominion Astrophysical Observatory for the Canadian National Research Council’s Herzberg Institute of Astrophysics 6 NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA; [email protected] 7 Hubble Fellow 8 Visiting Scientist, Universities Space Research Association 9 Raytheon ITSS, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA; [email protected] 10 Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La Palma, La Palma, Spain; [email protected] 1 2 The “Jump” in Strömgren u, Low Gravities, and Levitation