Role O f High - Altitude Tracers In The Measurement O f The Galactic Disk Potential

The role of high-altitude (Z > 200~~) tracer,stars in determining the Galactic disk potential is systematically analyzed. It is shown that such tracers cannot, by themselves, be used to measure the missing mass in the disk. Up to 60% of the disk mass may be in unobserved components with only a 20% discrepancy between the measured high-altitude disk potential and what could be expected from the observed components alone. In addition, a sample of-1000 stars is necessary to measure the disk potential to even 20% l-a accuracy. However, high-altitude tracers may be used to constrain the high-velocity dispersion components of the unobserved matter. Consequently, while high-altitude tracer observations are important, they should not be given undue emphasis in tracer observation programs.