Dust Grain Size Distributions and Extinction in the Milky Way, LMC, and SMC

摘要

We construct size distributions for carbonaceous and silicate grainpopulations in different regions of the Milky Way, LMC, and SMC. The sizedistributions include sufficient very small carbonaceous grains (includingpolycyclic aromatic hydrocarbon molecules) to account for the observed infraredand microwave emission from the diffuse interstellar medium. Our distributionsreproduce the observed extinction of starlight, which varies depending upon theinterstellar environment through which the light travels. As shown by Cardelli,Clayton & Mathis in 1989, these variations can be roughly parameterized by theratio of visual extinction to reddening, R_V. We adopt a fairly simplefunctional form for the size distribution, characterized by several parameters.We tabulate these parameters for various combinations of values for R_V andb_C, the C abundance in very small grains. We also find size distributions forthe line of sight to HD 210121, and for sightlines in the LMC and SMC. Forseveral size distributions, we evaluate the albedo and scattering asymmetryparameter, and present model extinction curves extending beyond the Lymanlimit.