Condition for the formation of micron-sized dust grains in dense molecular cloud cores
Abstract
We investigate the condition for the formation of micron-sized grains in dense cores of molecular clouds. This is motivated by the detection of the mid-infrared emission from deep inside a number of dense cores, the so-called `coreshine,' which is thought to come from scattering by micron-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 micron-sized grains: t/tff>3 (5/S) (nH/105 cm-3)-1/4 (where tff is the free-fall time at hydrogen number density nH in the core, and S the enhancement factor to the grain-grain collision cross-section to account for non-compact aggregates). At the typical core density nH=105 cm-3, it takes at least a few free-fall times to form the micron-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.