Classification of dark energy models in the (w0, wa) plane

We classify dark energy models in a plane of observables that correspond to the common parameterization of a non-constant equation of state, w(a) = w(0) + w(a) (1-a), where a is the scale factor of the universe. The models fall into four classes and only two of these classes have a region of overlap in the observable plane. We perform a joint analysis of all Type Ia supernova (SNIa) data compiled by the High-Z SN Search Team (HZT) and the Supernova Legacy Survey (SNLS) and find that no class of models is excluded by current SNIa data. However, an analysis of large scale structure, Ly alpha forest and bias constraints from SDSS, the Gold SNIa data and WMAP data indicates that non-phantom barotropic models with a positive sound speed are excluded at the 95% C.L. (c) 2006 Elsevier B.V. All rights reserved.