Computer Science
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7503m&link_type=abstract
HST Proposal ID #7503
Computer Science
Hst Proposal Id #7503 Stellar Populations
Scientific paper
We propose to obtain the third and final epoch for our measurement of differential proper motions in the core of 47 Tuc using deep U {F300W} and V {F555W} WFPC2 images over two years {1^st epoch: Oct. 1995; 2^nd: Cycle 6 scheduled}. The resulting motions will have a 1-Sigma uncertainty of 0.23 mas/yr - corresponds to a 5-Sigma detection of all stars with velocities greater than 22 km/s. The choice of F300W will allow stars to be measured over the whole color-magnitude diagram, from the red-giant branch to well down the main sequence. Such a complete census will provide unique constraints on relaxation processes, collision and ejection rates, and the velocity distribution, as a function of the stellar mass. Binary stars play an essential role during the late phases of the dynamical evolution of a globular cluster. They transfer energy to passing stars, and so can strongly influence the cluster evolution. Hard binaries are known to exist in cluster cores, e.g., in th e form of millisecond pulsars. Th e presence of hard binaries may also be revealed by searching for the by-products of close encounters: high-velocity stars. Two such stars were serendipitously discovered in the core of 47 Tuc by Meylan et al. {1991}, and similar stars have since been detected by Pryor et al. {1994}. This represents the limit of the radial velocity data from the ground. If more progress is to be made in the search for high-velocity stars in 47 Tuc, it must be made by obtaining proper motions, a task for which only HST is suitable.
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