  There are several indications that an opaque partonic medium is created in
energetic Au+Au collisions \sqrt{s} ~ 100 GeV/nucleon at the Relativistic Heavy
Ion Collider (RHIC). At the extreme densities of ~ 10-100 times normal nuclear
density reached even heavy-flavor hadrons are affected significantly.
Heavy-quark observables are presented from the parton transport model MPC,
focusing on the nuclear suppression pattern, azimuthal anisotropy ("elliptic
flow"), and azimuthal correlations. Comparison with Au+Au data at top RHIC
energy \sqrt{s} = 200 GeV indicates significant heavy quark rescattering,
corresponding to roughly five times higher opacities than estimates based on
leading-order perturbative QCD. We propose measurements of charm-anticharm,
e.g., D-meson azimuthal correlations as a sensitive, independent probe to
corroborate these findings.
