2.1 Escape Speed for a Simple Spherical Model

We can tie in local observations of high velocity stars with the notion of the escape speed (cf. Binney & Tremaine 1987). For a simple spherical model we have :

(1)

The corresponding escape speed is

(2)

Substituting V_c = 220 km s^-1, r = R = 8.5 kpc and an estimate for the escape speed of 500 km s^-1, we find r_* = 4.9 R 41 kpc, and M (r = r_*) = 4.6 10¹¹ M. Note that Olling and Merrifield (1998) advocate lower values for the distance of the Sun to the Galactic Center and for the rotation velocity at the distance of the Sun. Recent estimates of the escape speed from HIPPARCOS data are discussed in Meillon et al. (1997), and fall within a range of 400 to 550 km s^-1.