  In the color-magnitude diagrams (CMDs) of globular clusters, when the locus
of stars on the horizontal branch (HB) extends to hot temperatures,
discontinuities are observed at colors corresponding to ~12,000 K and ~18,000
K. The former is the "Grundahl jump" that is associated with the onset of
radiative levitation in the atmospheres of hot subdwarfs. The latter is the
"Momany jump" that has remained unexplained. Using the Space Telescope Imaging
Spectrograph on the Hubble Space Telescope, we have obtained ultraviolet and
blue spectroscopy of six hot subdwarfs straddling the Momany jump in the
massive globular cluster omega Cen. By comparison to model atmospheres and
synthetic spectra, we find that the feature is due primarily to a decrease in
atmospheric Fe for stars hotter than the feature, amplified by the temperature
dependence of the Fe absorption at these effective temperatures.
