Mathematics – Logic
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10110v&link_type=abstract
HST Proposal ID #10110
Mathematics
Logic
Hst Proposal Id #10110 Cold Stars
Scientific paper
The ages of the oldest stars are a key constraint on the evolution of our Galaxy, the history of star formation, and cosmological models. These ages are usually determined from globular clusters. However, it is alternatively possible to determine ages of extreme Population II subgiants in the solar neighborhood based on trigonometric parallaxes, without any recourse to clusters. This approach completely avoids the vexing issues of cluster distances, reddenings, and chemical compositions. There are 3 known nearby, extremely metal-deficient Pop II subgiants with Hipparcos parallax errors of 6-11% which are available for such age determinations. At present, based on the latest isochrones, the derived ages of these stars {HD 84937, HD 132475, and HD 140283} are all close to 14 Gyr, uncomfortably close to or higher than current estimates of the age of the universe. However, the errors in the Hipparcos parallaxes imply uncertainties of at least 2 Gyr in the ages of the 3 stars. We propose to measure parallaxes of these three Pop II subgiants using HST's Fine Guidance Sensor 1R. We expect to reduce the Hipparcos parallax error bars by factors of 5-6, providing the most stringent test yet of current theoretical stellar models of Pop II stars and pushing the age uncertainties to below 0.5 Gyr. These data will also provide a major new constraint on the distance scale of globular clusters, with wide implications for stellar evolution and the calibration of Pop II standard candles.
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