The Hubble Space Telescope Uv Legacy Survey of Galactic Globular Clusters. Vii. Implications From the Nearly Universal Nature of Horizontal Branch Discontinuities*

Abstract

The UV-initiative Hubble Space Telescope Treasury survey of Galactic globular clusters provides a new window into the phenomena that shape the morphological features of the horizontal branch (HB). Using this large and homogeneous catalog of UV and blue photometry, we demonstrate that the HB exhibits discontinuities that are remarkably consistent in color (effective temperature). This consistency is apparent even among some of the most massive clusters hosting multiple distinct sub-populations (such as NGC 2808, ω Cen, and NGC 6715), demonstrating that these phenomena are primarily driven by atmospheric physics that is independent of the underlying population properties. However, inconsistencies arise in the metal-rich clusters NGC6388 and NGC6441, where the discontinuity within the blue HB (BHB) distribution shifts ∼1000-2000 K hotter. We demonstrate that this shift is likely due to a large helium enhancement in the BHB stars of these clusters, which in turn affects the surface convection and evolution of such stars. Our survey also increases the number of Galactic globular clusters known to host blue-hook stars (also known as late hot flashers) from 6 to 23 clusters. These clusters are biased toward the bright end of the globular cluster luminosity function, confirming that blue-hook stars tend to form in the most massive clusters with significant self-enrichment.

A Universal Transition in Atmospheric Diffusion for Hot Subdwarfs Near 18,000 K∗

“…However, the Balmer jump in these red stars is not significantly stronger than that in the Figure 1. WFC3 photometry (black points) of HB stars in ω Cen (Bellini et al 2017a), shown as a color-color diagram that makes it easy to discern the G-jump and M-jump (labeled; as in Brown et al 2016). The ordinate is a color index (C) that is useful for discerning changes in the stellar atmosphere (in the hot stars here but also the cooler stars studied in the multiple populations phenomenon).…”

“…For our investigation, we obtained STIS near-UV (G230L) and blue (G430L) spectroscopy of six stars (colored symbols matching those in Figure 2) that straddle the M-jump, with wavelength coverage that includes the three photometric bands employed in this color-color diagram (F275W, F336W, and F438W). If the photometry here is aligned to a theoretical zero-age HB locus (see Brown et al 2016), the gray band representing the M-jump spans ∼15,000-18,000 K in effective temperature. Decl.…”

“…That said, even with the cluster's distinct subpopulations, the cluster's composite population is better constrained than that of an assorted collection of field subdwarfs. More importantly, Brown et al (2016) demonstrated that the G-jump and M-jump are the products of stellar atmosphere transitions instead of population distinctions; the transition responsible for the G-jump can be shifted to a hotter effective temperature at extreme He abundance, but the M-jump is constant even in the presence of such He enhancements. For these reasons, the peculiar populations of ωCen are not a concern for this study.…”

“…The G-jump is associated with the onset of radiative levitation for metals and the gravitational settling of helium in the atmospheres of subdwarfs hotter than the G-jump (Moehler et al 1999(Moehler et al , 2000Behr 2003;Pace et al 2006). Brown et al (2016) explained that the hotter G-jump in NGC6388 and NGC6441 is likely associated with helium enhancement because it shifts the onset of surface convection to higher effective temperatures. They also found that the M-jump is at a consistent temperature for all 15 of the survey clusters where it is populated, including the clusters NGC6388 and NGC6441.…”

“…Brown et al (2016) used this dataset to explore the "Grundahl jump" (hereafter G-jump; Grundahl et al 1998Grundahl et al , 1999 and "Momany jump" (hereafter M-jump; Momany et al 2002Momany et al , 2004) that appear as discontinuities within the HB distribution near effective temperatures of ∼12,000 and ∼18,000 K (Figure 1). In the color-color plane of Figure 1, the stars between the G-jump and M-jump appear to deviate from the expected location of the theoretical zero-age HB (see Figures6 and 7 in Brown et al 2016). They found that the G-jump falls at a consistent temperature for 31 of the 33 survey clusters where it is populated; the only exceptions are NGC6388 and NGC6441, two metal-rich clusters greatly enhanced in helium.…”