The Second Parameter Effect in Metal-Rich Globular Clusters: A Snapshot Study of NGC 6388

Details The Second Parameter Effect in Metal-Rich Globular Clusters: A Snapshot Study of NGC 6388 The Second Parameter Effect in Metal-Rich Globular Clusters: A Snapshot Study of NGC 6388

Jul 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003hst..prop.9821p&link_type=abstract

HST Proposal ID #9821

Physics

Hst Proposal Id #9821 Stellar Populations

Scientific paper

While it has long been known that at least one parameter besides the metallicity, [Fe/H], determines the horizontal branch {HB} morphology of Galactic globular clusters {GCs}, our ignorance as to the nature of this second parameter {or parameters} has been a major stumbling block in understanding the formation history and age of the GC system. The hot HB populations recently discovered by HST in the metal-rich GCs NGC 6388 and NGC 6441 provide a unique opportunity for unraveling this second-parameter effect. Many different theories have been proposed to explain the pronounced upward slope of the HBs in these GCs, including stellar rotation, metallicity spread, and a dwarf galaxy origin. We propose to test these theories by obtaining B, V, I time-series photometry of the RR Lyrae variables in the core of NGC 6388 in order to determine whether, as predicted, the pulsation periods are unusually long due to a high HB luminosity. If confirmed, this would argue against age or mass loss as the second parameter in NGC 6388 and, more generally, would have implications for the use of RR Lyrae stars as standard candles for determining GC distances and ages. Light curves will also be obtained for the crowded Population II Cepheids near the core of NGC 6388, the most metal-rich GC, along with NGC 6441, known to contain such stars. We waive proprietary rights to any data obtained.

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