Dust Grain–Size Distributions and Extinction in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud

Abstract

We construct size distributions for carbonaceous and silicate grain\npopulations in different regions of the Milky Way, LMC, and SMC. The size\ndistributions include sufficient very small carbonaceous grains (including\npolycyclic aromatic hydrocarbon molecules) to account for the observed infrared\nand microwave emission from the diffuse interstellar medium. Our distributions\nreproduce the observed extinction of starlight, which varies depending upon the\ninterstellar environment through which the light travels. As shown by Cardelli,\nClayton & Mathis in 1989, these variations can be roughly parameterized by the\nratio of visual extinction to reddening, R_V. We adopt a fairly simple\nfunctional form for the size distribution, characterized by several parameters.\nWe tabulate these parameters for various combinations of values for R_V and\nb_C, the C abundance in very small grains. We also find size distributions for\nthe line of sight to HD 210121, and for sightlines in the LMC and SMC. For\nseveral size distributions, we evaluate the albedo and scattering asymmetry\nparameter, and present model extinction curves extending beyond the Lyman\nlimit.\n