Main Sequence Turnoff Ages For Second Parameter Clusters in M33

Details Main Sequence Turnoff Ages For Second Parameter Clusters in M33 Main Sequence Turnoff Ages For Second Parameter Clusters in M33

Jul 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003hst..prop.9873s&link_type=abstract

HST Proposal ID #9873

Physics

Hst Proposal Id #9873 Stellar Populations

Scientific paper

In cycle 5, we were granted 40 orbits to study the early formation history of M33 by investigating the nature of the "second parameter" phenomenon among its globular star clusters. Discovered among the globular clusters of the Milky Way more than 30 years ago, the "second parameter" effect describes the degeneracy in the behavior of horizontal branch {HB} morphology with metal abundance. This degeneracy implies the existence of a second parameter, which, in addition to metal abundance, influences the morphology of the HB. We constructed {V, V-I} color-magnitude diagrams for 10 M33 halo globular clusters. From these diagrams, we measured the cluster metallicities and HB morphologies. Surprisingly, 8 of the 10 clusters display extremely red horizontal branches, with most of the HB stars lying near or on top of the red giant branch, yet their metal abundances are in the range -1.6 <= [Fe/H] <= -1.0. A likely explanation is that the halo clusters in M33 are several Gyr younger than those in the Milky Way. To test this hypothesis, we propose to obtain main sequence turnoff photometry for two of our M33 clusters with similar metallicities but vastly differing HBs - a so-called `second parameter pair.' This will help to answer the question of whether age is the second parameter among the M33 halo clusters and provide an important clue to the overall nature of the second parameter effect.

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