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

Abstract

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 mid-infrared emission from deep inside a number of dense cores, the so-called ‘coreshine,’ which is thought to come from scattering by micron (μm)-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 μm-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 of 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 μm-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.