Computer Science
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8389i&link_type=abstract
HST Proposal ID #8389
Computer Science
Hst Proposal Id #8389 Field Stellar Populations
Scientific paper
We propose to re-image the HDF in F606W in order to confirm and better determine the proper motions of four MACHO candidates discovered from the comparison of the original December 1995 HDF frames with WFPC2 F814W frames exposed in December 1997. Four very faint, blue, isolated and unresolved objects are found to have substantial proper motion, 52 +/- 10 mas, 102 +/- 14 mas, 46 +/- 10 mas and 69 +/- 11 mas over the two year baseline. Galactic structure models predict a complete absence of normal stars in the color-magnitude range in which these objects are observed. However, these observations are consistent with recently derived models of old hydrogen atmosphere white dwarfs, whose color, contrary to previous expectations, has been shown to be blue. If so, the stars we have detected have ages in the range 10 Gyr --> 14 Gyr. With a correction for the incompleteness of the sample, the number density of such objects is ~ 0.02 pc^-3, or ~ 0.01 msun pc^-3, assuming for their masses a value typical of white dwarfs. Within the uncertainties arising from the small size of the sample, this mass density is sufficient to account for the entire missing Galactic dynamical mass. The present observations aim to confirm this potentially very important finding.
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