The size distribution of interstellar dust particles as determined from extinction

We have investigate the character of the changes in the size distribution, and illustrate this with two contrasting cases: the diffuse interstellar medium (RV = 3.1), and a dense cloud region (RV = 5.3). For this exploratory investigation we adopted spherical bare silicate and bare graphite particles as in the Mathis, Rumpl, & Nordsieck (MRN) modeling. To extract the size distributions of the two components as objectively as possible we used the Maximum Entropy Method which gives the smoothest solution compatible with the chi squared confidence level on the goodness of fit to the extinction data. Abundance constraints were implemented directly in the method in order that the elements incorporated in the grains did not exceed their cosmically available abundances or contradict depletion data. We found that the silicate and graphite size distributions depart significantly (and robustly) from a simple power law. We also show how the size distribution falls off smoothly beyond a+, perhaps compatible with an exponential cutoff. The implications for the origin and evolution of the grain size distribution are discussed.