Growing dust grains in protoplanetary discs - III. Vertical settling

We aim to derive a simple analytic model to understand the essential properties of vertically settling growing dust grains in laminar protoplanetary discs. Separating the vertical dynamics from the motion in the disc mid-plane, we integrate the equations of motion for both a linear and an exponential grain growth rate. Numerical integrations are performed for more complex growth models. We find that the settling efficiency depends on the value of the dimensionless parameter gamma, which characterizes the relative efficiency of grain growth with respect to the gas drag. Since gamma is expected to be of the same order as the initial dust-to-gas ratio in the disc (similar or equal to 10(-2)), grain growth enhances the energy dissipation of the dust particles and improves the settling efficiency in protoplanetary discs. This behaviour is mostly independent of the growth model considered as well as of the radial drift of the particles.