We study a dark energy scenario in the presence of a tachyon field $\\\\phi$ with potential $V(\\\\phi)$ and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity $\\\\lambda=-M_pV_\\\\phi/V^{3/2}$. If $\\\\lambda$ is a constant, which corresponds to an inverse square potential $V(\\\\phi) \\\\propto \\\\phi^{-2}$, there exists one stable critical point that gives an acceleration of the universe at late times. When $\\\\lambda \\\\to 0$ asymptotically, we can have a viable dark energy scenario in which the system approaches an ``instantaneous\\\'\\\' critical point that dynamically changes with $\\\\lambda$. If $|\\\\lambda|$ approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where $|\\\\lambda|$ is smaller than of order unity

Keywords