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The UV-initiative Hubble Space Telescope Treasury survey of Galactic globularclusters provides a new window into the phenomena that shape the morphologicalfeatures of the horizontal branch (HB). Using this large and homogeneouscatalog of UV and blue photometry, we demonstrate that the HB exhibitsdiscontinuities that are remarkably consistent in color (effectivetemperature). This consistency is apparent even among some of the most massiveclusters hosting multiple distinct sub-populations (such as NGC 2808, omegaCen, and NGC 6715), demonstrating that these phenomena are primarily driven byatmospheric physics that is independent of the underlying populationproperties. However, inconsistencies arise in the metal-rich clusters NGC 6388and NGC 6441, where the discontinuity within the blue HB (BHB) distributionshifts ~1,000 K to 2,000 K hotter. We demonstrate that this shift is likely dueto a large helium enhancement in the BHB stars of these clusters, which in turnaffects the surface convection and evolution of such stars. Our survey alsoincreases the number of Galactic globular clusters known to host blue-hookstars (also known as late hot flashers) from 6 to 23 clusters. These clustersare biased toward the bright end of the globular cluster luminosity function,confirming that blue-hook stars tend to form in the most massive clusters withsignificant self-enrichment.

astrophysical journal/the astrophysical journal

The Hubble Space Telescope UV Legacy Survey of Galactic Globular  Clusters. VII. Implications from the Nearly Universal Nature of Horizontal  Branch Discontinuities