The Equation of State of Tracker Fields
  We derive the equation of state of tracker fields, which are typical examples
of freezing quintessence (quintessence with the equation of state approaching
toward -1), taking into account of the late-time departure from the tracker
solution due to the nonzero density parameter of dark energy $\Omp$. We
calculate the equation of state as a function of $\Omp$ for constant
$\Gamma=VV"/(V')^2$ (during matter era) models. The derived equation of state
contains a single parameter, $w_{(0)}$, which parametrizes the equation of
state during the matter-dominated epoch. We derive observational constraints on
$w_{(0)}$ and find that observational data are consistent with the cosmological
constant: $-1.11< \wzero< -0.96 (1 \sigma)$.
