  Classification of dark energy models in the plane of w and w', where w is the
dark energy equation of state and w' its time-derivative in units of the Hubble
time, has been studied in the literature. We take the current SN Ia, CMB and
BAO data, invoke a widely used parametrization of the dark energy equation of
state, and obtain the constraints on the w -w' plane. We find that dark energy
models including the cosmological constant, phantom, non-phantom barotropic
fluids, and monotonic up-rolling quintessence are ruled out at the 68.3%
confidence level based on the current observational data. Down-rolling
quintessence, including the thawing and the freezing models, is consistent with
the current observations. All the above-mentioned models are still consistent
with the data at the 95.4% confidence level.
