Can Reflection from Grains Diagnose the Albedo?

By radiation transfer models with a realistic power spectra of the projecteddensity distributions, we show that the optical properties of grains are poorlyconstrained by observations of reflection nebulae. The ISM is known to behierarchically clumped from a variety of observations (molecules, H I,far-infrared). Our models assume the albedo and phase parameter of the dust,the radial optical depth of the sphere averaged over all directions, and randomdistributions of the dust within the sphere. The outputs are the stellarextinction, optical depth, and flux of scattered light as seen from variousviewing angles. Observations provide the extinction and scattered flux from aparticular direction. Hierarchical geometry has a large effect on the flux of scattered lightemerging from a nebula for a particular extinction of the exciting star. Thereis a very large spread in both scattered fluxes and extinctions for anydistribution of dust. Consequently, an observed stellar extinction andscattered flux can be fitted by a wide range of albedos. With hierarchicalgeometry it is not completely safe to determine even relative optical constantsfrom multiwavelength observations of the same reflection nebula. The geometryeffectively changes with wavelength as the opacity of the clumps varies. Limitson the implications of observing the same object in various wavelengths arediscussed briefly. Henry (2002) uses a recipe to determine the scattered flux from a star with agiven extinction. It is claimed to be independent of the geometry. It providesconsiderably more scattering than our models, probably leading to anunderestimate of the grain albedos from the UV Diffuse Galactic Light.Comment: 27 pages, including 7 figures. Accepted by Ap