Other
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991hst..prop.2428m&link_type=abstract
HST Proposal ID #2428
Other
Hst Proposal Id #2428
Scientific paper
We propose to measure the trigonometric parallaxes and proper motions of the three high-proper motion stars which Carney, Latham, and Laird (1988) identify as having the most extreme velocities known in the galactic rest frame. Using these stars, they conclude that the local value of the escape velocity, V(esc), is at least 500 k/s, and this leads them to draw other important conclusions regarding the distribution of mass in the galactic disk. However, their assigned distances, and hence the tangential velocities and V(esc) value, depend on uncertain photometric corrections and reddening estimates. The photometric distances they find are in the range 400-550 pc, so the parallaxes are expected to be of the order of 2 milliarcsec. If these distances are approximately correct, it will be possible to measure them at the 4-sigma level using an FGS on the HST if care is taken with the observations and reductions. It will be of great interest if the parallaxes turn out to be smaller than the estimates of Carney, et al., since this would lead to a higher value for the escape velocity and a larger mass for the galaxy. Alternatively, if the parallaxes are found to be considerably larger than they adopted, either V(esc) is considerably smaller than 500 k/s or these three stars are not the most appropriate for setting a limit on V(esc). NOTE added 16-Apr-1991: Three targets mentioned above changed to two, G166-37 and G233-27, as result of Reassessment in early 1991. What follows is Cycle 1 only.
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