Probe the QCD phase boundary with elliptic flow in relativistic heavy ion collisions at STAR

We present measurement of elliptic flow, $v_2$, for charged and identified particles at midrapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 - 39 GeV. We compare the inclusive charged hadron $v_2$ to those from transport model calculations, such as UrQMD model, AMPT default model and AMPT string-melting model. We discuss the energy dependence of the difference in $v_2$ between particles and anti-particles. The $v_2$ of $\phi$ meson is observed to be systematically lower than other particles in Au+Au collisions at $\sqrt{s_{NN}}$ = 11.5 GeV.