  We present a new instability driven by a combination of coagulation and
radial drift of dust particles. We refer to this instability as ``coagulation
instability" and regard it as a promising mechanism to concentrate dust
particles and assist planetesimal formation in the very early stages of disk
evolution. Because of dust-density dependence of collisional coagulation
efficiency, dust particles efficiently (inefficiently) grow in a region of
positive (negative) dust density perturbations, which lead to a small radial
variation of dust sizes and as a result radial velocity perturbations. The
resultant velocity perturbations lead to dust concentration and amplify dust
density perturbations. This positive feedback makes a disk unstable. The growth
timescale of coagulation instability is a few tens of orbital periods even when
dust-to-gas mass ratio is of the order of $10^{-3}$. In a protoplanetary disk,
radial drift and coagulation of dust particles tend to result in dust
depletion. The present instability locally concentrates dust particles even in
such a dust-depleted region. The resulting concentration provides preferable
sites for dust-gas instabilities to develop, which leads to further
concentration. Dust diffusion and aerodynamical feedback tend to stabilize
short-wavelength modes, but do not completely suppress the growth of
coagulation instability. Therefore, coagulation instability is expected to play
an important role in setting up the next stage for other instabilities to
further develop toward planetesimal formation, such as streaming instability or
secular gravitational instability.
