Supermetallicity and stellar evolution

The rich globular cluster systems of the Milky Way and the Magellanic Clouds have provided a set of stellar-population templates spanning from very low to Solar metallicity. However, only the open cluster NGC 6791 is metal rich at +0.4 dex. The Galactic bulge is a complex population with a range in abundance with even some age spread possible; see e.g. Zoccali et al. (2003). At high metallicity, we observe the RGB to become dominated by M giants; metal-line and TiO opacity in those stars can cause the V magnitude of the RGB tip to be as faint as the horizonal branch. The I band is also affected; the first giant-branch tip appears to curve downward or descend. The AGB becomes populated with Miras and OH/IR stars; these are numerous in the bulge, and the early work of Blanco found large numbers of M giants toward the Galactic Center. The progression to cool, luminous giant branches with Mira and OH/IR stars is the dominant effect observed at high metallicity.

The helium-burning stars are generally confined to the red clump, with a blue horizontal-branch extension seen only in old, metal-poor populations - at least in the classical view of stellar populations. However, blue EHB populations are observed in NGC 6388 and 6441. These are not super-metal-rich clusters, but are metal-rich enough that such blue HBs should not exist (Rich et al. 1997). Blue HB stars are also found in NGC 6791 (Peterson & Green 1998). In red elliptical galaxies with no sign of star formation, a hot component (now known as the UVX) has been detected since the early days of satellite astronomy (Code 1969) and remains unexplained (O'Connell 1999). Burstein et al. (1988) in an influential paper found that ellipticals and bulges exhibit a correlation between the UVX and the metallicity-sensitive Mg2 index; this has not been confirmed in a larger sample of GALEX-selected quiescent early-type galaxies from the SDSS (Rich et al. 2005) and there remains only a weak correlation in a larger sample of nearby elliptical galaxies (Rich et al., work in preparation). While UV light is present in many early-type galaxies, its cause remains mysterious. A full review is beyond the scope of this paper, but additional factors (subpopulations with enhanced helium abundance) might be at play.

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