On the Dust-to-Gas Ratio and Large Particles in the Interstellar Medium

A comparison of mass (or size) distributions of interstellar dust particles in various environments (from diffuse to dense), as determined from interstellar extinction and linear polarization curves, shows that for dense (dark) clouds with higher RV and λmax than typical of the diffuse interstellar medium the relative number of small particles is greatly diminished, whereas the shape of the mass distribution at the large size end (where much of the mass is) is much the same. This appears to be consistent with evolution of interstellar dust particles in dense clouds by aggregation involving small particles; there is only modest evolution of the larger particles. There is some evidence for dark cloud lines of sight with lower than average extinction per unit mass of gas (e.g., AV/NH is one measure). With specific application to the ρ Oph line of sight, we show quantitatively that this demands the existence of particles much larger than usually contemplated (10 μm). Large particles have low extinction efficiencies per unit volume in the optical and fairly neutral extinction with wavelength and so can lock up a lot of mass fairly inconspicuously. Producing such particles requires much more aggressive aggregation, with large particles sticking to one another. While our analysis is based on specific grain models, an instructive Kramers-Kronig treatment illustrated by these models suggests that our conclusions are robust.