Analytic formulas for frequency and size dependence of absorption and scattering efficiencies of astronomical polycyclic aromatic hydrocarbons

In a series of two recent papers, the frequency and size distribution dependence of extinction spectra for astronomical silicate and graphite grains was analyzed by us in the context of MRN type interstellar dust models. These grains were taken to be homogeneous spheres following the power law (a−3.5) size distribution which is very much in use. The analytic formulas were obtained for the graphite and silicate grains in wavelength range 1000 Å-22,500 Å and their utility was demonstrated. In this paper of the series, we present analytic formulas for the scattering and absorption spectrum of another important constituent of interstellar dust models, namely, the polycyclic aromatic hydrocarbons (PAHs). Relative contribution of the PAHs to extinction vis a vis carbonaceous classical grains has been examined.

► Wavelength range 1000Å≤λ≤22,500Å, PAH size range 5Å≤a≤50Åa is radius. ► Formulas can be used for computing extinction spectra of some bulk PAHs material. ► Slight difference in extinction behavior of ionic and neutral PAHs. ► Range of λ over which absorption efficiency differ depend on the PAH radius. ► Comparing efficiency of equal volume of graphite ⊥,∥ & PAH shows its importance in model.