  We analyze quantum fluctuation effects at the onset of incommensurate $2k_F$
charge- or spin-density wave order in two-dimensional metals, for a model where
the ordering wave vector $\boldsymbol{Q}$ connects a single pair of hot spots
on the Fermi surface with a vanishing Fermi surface curvature. The tangential
momentum dependence of the bare dispersion near the hot spots is proportional
to $|k_t|^\alpha$ with $\alpha > 2$. We first compute the order parameter
susceptibility and the fermion self-energy in random phase approximation (RPA).
Logarithmic divergences are subsequently treated by a renormalization group
analysis. The coupling between the order parameter fluctuations and the
fermions vanishes logarithmically in the low-energy limit. As a consequence,
the logarithmic divergences found in RPA do not sum up to anomalous power laws.
Instead, only logarithmic corrections to Fermi liquid behavior are obtained. In
particular, the quasiparticle weight and the Fermi velocity vanish
logarithmically at the hot spots.
