Statistics
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11633r&link_type=abstract
HST Proposal ID #11633. Cycle 17
Statistics
Scientific paper
We propose to obtain second epoch imaging of the globular cluster NGC 6397, which has the deepest ACS/WFC dataset {126 orbits in Cycle 13} ever obtained in a globular cluster. These additional 14 orbits would enable the construction of a proper motion - cleaned white dwarf cooling sequence reaching fainter than the observed truncation point of the white dwarf luminosity function; a byproduct will be absolute proper motions {relative to the extragalactic reference frame} of the rich spheroid field population. Our data shows the long sought ``blue hook'', a feature in the WD cooling sequence predicted in theoretical white dwarf models; the improved photometry and statistics afforded by the second epoch observations are needed to confirm the blue hook and to test other aspects of white dwarf cooling models that contribute to a 0.5 Gyr or greater uncertainty in the age. The proposed observations will provide formal constraints on the age and formation timescale of NGC 6397 that will be smaller than 1 Gyr and place the cluster's formation epoch relative to the end of reionization. The final proper motion-purified white dwarf sequence will be a powerful constraint for white dwarf cooling models and atmospheres, the basis for white dwarf-based globular cluster age determinations, and also constrain the fraction of binary white dwarfs.;
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