High velocity stars

A number of high-velocity stars near the Sun have measured velocities^7.1 up to $v_\star\approx 500{\hbox{\rm km s$^{-1}$}}$. Their existence provides us with a probe of the galaxy's mass, if we assume that these stars are still bound to the MW: it means that the escape speed^7.2 at the solar circle, $v_e<v_\star$.