Collision Times for Grains in Four Environments: When Coagulation Can Reshape the Size Distribution

Coagulation, grains sticking on contact, is the main route to growing the large end of the size distribution, but it needs collisions to happen at all. This paper computes the collision time between tenth-micron grains in four environments, with grain number densities derived from the dust mass and relative velocities taken as stated turbulent values. The collision time is 9.282 million years in diffuse gas, 0.139 million years in a molecular cloud, and 0.028 million years in a dense core, corresponding to 71.82 collisions per grain per ten million years in the molecular case. Coagulation is therefore a molecular-cloud process on cloud lifetimes, negligible in diffuse gas, and fast enough in cores to matter within a single core lifetime, provided sticking is efficient and the velocities stay below the fragmentation threshold. All values are computed, not measured.