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Session 14 - Milky Way Galaxy.
Display session, Monday, January 15
North Banquet Hall, Convention Center
POS--mode observations with the HST FGS system have been obtained of the two--high--proper motion stars Ross 50 (G166--37) and Wolf 624 (G16--25) over a 25--month period. These stars are thought to have space velocities near the escape speed of the galaxy, but the velocities are not well known because of the large uncertainties in the spectroscopic/photometric distances used to derive the tangential motions. The FGS astrometry yielded positions from a single measure with a typical precision of 2 mas. This allowed proper motions and trignometric parallaxes to be determined. R50 was observed at six epochs near parallax factor maxima and yielded a parallax of 4 \pm 0.6 mas implying a speed in the local rest frame of 498 \pm 82 k/s. Only four epochs of observations were scheduled for W624, and the decreased baseline made our parallax determination inconclusive. The parallax is less than 3.7 mas and may be as small as 2 mas which equates to the extremely large rest--frame velocity of 800 km/s. Several requirements and limitations of FGS astrometry that have been learned in the course of this study will also be presented.
