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We present a new instability driven by a combination of coagulation andradial drift of dust particles. We refer to this instability as ``coagulationinstability" and regard it as a promising mechanism to concentrate dustparticles and assist planetesimal formation in the very early stages of diskevolution. Because of dust-density dependence of collisional coagulationefficiency, dust particles efficiently (inefficiently) grow in a region ofpositive (negative) dust density perturbations, which lead to a small radialvariation of dust sizes and as a result radial velocity perturbations. Theresultant velocity perturbations lead to dust concentration and amplify dustdensity perturbations. This positive feedback makes a disk unstable. The growthtimescale of coagulation instability is a few tens of orbital periods even whendust-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 dustdepletion. The present instability locally concentrates dust particles even insuch a dust-depleted region. The resulting concentration provides preferablesites for dust-gas instabilities to develop, which leads to furtherconcentration. Dust diffusion and aerodynamical feedback tend to stabilizeshort-wavelength modes, but do not completely suppress the growth ofcoagulation instability. Therefore, coagulation instability is expected to playan important role in setting up the next stage for other instabilities tofurther develop toward planetesimal formation, such as streaming instability orsecular gravitational instability.

Coagulation Instability in Protoplanetary Disks: A Novel Mechanism Connecting Collisional Growth and Hydrodynamical Clumping of Dust Particles