  Heavy quarks are produced through initial hard scatterings and they are
affected by the hot and dense medium created in heavy-ion collisions throughout
its whole evolution. Due to their heavy mass, charm quarks are expected to
thermalize much more slowly than light flavor quarks. The charm quark flow is a
unique tool to study the extent of thermalization of the bulk medium dominated
by light quarks and gluons. At high $p_T$, D meson azimuthal anisotropy is
sensitive to the path length dependence of charm quark energy loss in the
medium, which offers new insights into heavy quark energy loss mechanisms -
gluon radiation vs. collisional processes. We present the STAR measurement of
elliptic flow ($v_2$) of $D^0$ and $D^{\pm}$ mesons in Au+Au collisions at
$\sqrt{s_{NN}}$ = 200 GeV, for a wide transverse momentum range. These results
are obtained from the data taken in the first year of physics running of the
new STAR Heavy Flavor Tracker detector, which greatly improves open heavy
flavor hadron measurements by the topological reconstruction of secondary decay
vertices. The D meson $v_2$ is finite for $p_T > 2 \text{ GeV/c}^2$ and
systematically below the measurement of light particle species at the same
energy. Comparison to a series of model calculations favors scenarios where
charm flows with the medium and is used to infer a range for the charm
diffusion coefficient $2\pi T D_s$.
