Condition for the formation of micron-sized dust grains in dense molecular cloud cores

We investigate the condition for the formation of micron-sized grains in dense cores of molecular clouds. This is motivated by the detection of mid-infrared emission from deep inside a number of dense cores, the so-called 'coreshine,' which is thought to come from scattering by micron (mu m)-sized grains. Based on numerical calculations of coagulation starting from the typical grain-size distribution in the diffuse interstellar medium, we obtain a conservative lower limit to the time t to form mu m-sized grains: t/t(ff) > 3(5/S)(n(H)/10(5) cm(-3))(-1/4) (where t(ff) is the free-fall time at hydrogen number density n(H) in the core and S the enhancement factor of the grain-grain collision cross-section to account for non-compact aggregates). At the typical core density n(H) = 10(5) cm(-3), it takes at least a few free-fall times to form the mu m-sized grains responsible for coreshine. The implication is that those dense cores observed in coreshine are relatively long-lived entities in molecular clouds, rather than dynamically transient objects that last for one free-fall time or less.