The Second-Parameter Effect in Metal- Rich, Globular Clusters: A Snapshot Study of NGC 6441

Details The Second-Parameter Effect in Metal- Rich, Globular Clusters: A Snapshot Study of NGC 6441 The Second-Parameter Effect in Metal- Rich, Globular Clusters: A Snapshot Study of NGC 6441

Jul 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8251s&link_type=abstract

HST Proposal ID #8251

Computer Science

Hst Proposal Id #8251 Stellar Populations In Clusters

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 newly- discovered hot HB populations in the metal-rich GCs NGC 6388 and NGC 6441 provide an unique opportunity for unraveling this second-parameter effect. Sweigart & Catelan {1998} have presented second-parameter models which predict unusually bright HBs for these GCs. We propose to test these models by obtaining B, V, I time-series photometry of the RR Lyrae variables in the core of NGC 6441 in order to determine whether, as predicted, the pulsation periods are unusually long as implied by a bright HB luminosity. If confirmed, this would argue against age or mass loss as the second parameter in NGC 6441 and, more generally, wo uld have implications for the use of RR Lyrae stars as standard candles for determining GC distances and ages. A possible new class of bright RR Lyrae-like variables will also be investigated. The interplay between stellar evolution and cluster dynamics will be studied by comparing the radial distribution of the RR Lyrae stars to those of blue and red HB stars. We waive proprietary rights to any data obtained.

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