  Chiral symmetry restoration at nonzero temperature and quark densities are
investigated in the framework of a linear sigma model with N_f=3 light quark
flavors. After the derivation of the grand potential in mean-field
approximation, the nonstrange and strange condensates, the in-medium masses of
the scalar and pseudoscalar nonets are analyzed in hot and dense medium. The
influence of the axial anomaly on the nonet masses and the isoscalar mixings on
the pseudoscalar \eta-\eta' and scalar \sigma(600)-f_0(1370) complex are
examined. The sensitivity of the chiral phase transition as well as the
existence and location of a critical end point in the phase diagram on the
value of the sigma mass is explored. The chiral critical surface with and
without the influence of the axial U(1)_A anomaly is elaborated as a function
of the pion and kaon masses for several values of the sigma mass.
