Abstract
We argue that impact velocities between dust grains with sizes of less than ∼0.1 μm in molecular cloud cores are dominated by drift arising from ambipolar diffusion. This effect is due to the size dependence of the dust coupling to the magnetic field and the neutral gas. Assuming perfect sticking in collisions up to ≈50 m s−1, we show that this effect causes rapid depletion of small grains, consistent with starlight extinction and IR and microwave emission measurements, both in the core center (n ∼ 106 cm−3) and envelope (n ∼ 104 cm−3). The upper end of the size distribution does not change significantly if only velocities arising from this effect are considered. We consider the impact of an evolved grain-size distribution on the gas temperature, and argue that if the depletion of small dust grains occurs as expected from our model, then the cosmic ray ionization rate must be well below 10−16 s−1 at a number density of 105 cm−3.
Highlights
Knowledge of the dust grain-size distribution is crucial to interpretating observations in molecular clouds
Ossenkopf (1993) made estimates for the contribution of several processes, to the relative velocity between dust grains, and determined turbulent motions to be dominant at densities ≤108 cm−3
Silsbee et al.: Rapid elimination of small dust grains in molecular clouds crossing time at the injection scale and τs is the stopping time of the grain, given by τs where v∗th = 0.92 4kBT/(πmp) is the thermal velocity scale of the gas, ρg = 2.8 mpn is the gas density, and σg = πa2 is the collision cross section between grains and gas particles
Summary
Knowledge of the dust grain-size distribution is crucial to interpretating observations in molecular clouds. Ossenkopf (1993) made estimates for the contribution of several processes (e.g., turbulence, Brownian motion, forces arising from grain asymmetries and gravitational settling), to the relative velocity between dust grains, and determined turbulent motions to be dominant at densities ≤108 cm−3 He ran coagulation simulations for regions with densities between 105 and 109 cm−3. We need to determine the timescale for very small grain removal in prestellar cores, which represent the initial conditions in the process of star and planet formation If this depletion timescale is significantly shorter than the dynamical timescale, the diffusion of magnetic fields (by ambipolar diffusion and the Hall effect) becomes efficient during the protostellar collapse, which greatly promotes the formation of protoplanetary disks. This is noteworthy given evidence (e.g., Fuller & Myers 1992; Pineda et al 2010) that turbulent velocities in dense regions are substantially subsonic
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