Dust grain-size distributions and extinction in the Milky Way, large magellanic cloud, and small magellanic cloud

We construct size distributions for carbonaceous and silicate grain populat ions in different regions of the Milky Way, LMC, and SMC. The size distribu tions include sufficient very small carbonaceous grains (including polycycl ic aromatic hydrocarbon molecules) to account for the observed infrared and microwave emission from the diffuse interstellar medium. Our distributions reproduce the observed extinction of starlight, which varies depending on the interstellar environment through which the light travels. As shown by C ardelli, Clayton, and Mathis in 1989, these variations can be roughly param eterized by the ratio of visual extinction to reddening R-V. We adopt a fai rly simple functional form for the size distribution, characterized by seve ral parameters. We tabulate these parameters for various combinations of va lues for R-V and b(C), the C abundance in very small grains. We also find s ize distributions for the line of sight to HD 210121 and for sight lines in the LMC and SMC. For several size distributions, we evaluate the albedo an d scattering asymmetry parameter and present model extinction curves extend ing beyond the Lyman limit.