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 } \lesssim T _ { eff } \lesssim 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 T _ { eff } \gtrsim 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 T _ { eff } \simeq 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 T _ { eff } \gtrsim 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 .