CONSTRAINTS ON THE PROPERTIES OF THE 2175-ANGSTROM INTERSTELLAR FEATURE CARRIER

Constraints on the possible shape and clustering, as well as optical p roperties, of grains responsible for the 2175 Angstrom interstellar ex tinction feature (interstellar UV bump) are discussed. These constrain ts are based on the observation that the peak position of the interste llar UV feature is very stable (variations less than or similar to 1%) , that the large variations in width (less than or similar to 25%) are uncorrelated with the peak position except for the widest bumps, and that the shape of the feature is described extremely well by a Drude p rofile. The UV extinction of small graphite grains is computed for var ious clustering models involving Rayleigh spheres. It is shown that co mpact clusters qualitatively satisfy the above observational constrain ts, except that the peak position falls at the wrong wavelength. As an alternative to graphite to model the optical properties of the inters tellar UV feature carrier, a single-lorentz oscillator model is consid ered, in conjunction with a clustering model based on clusters of sphe res. Intrinsic changes in the peak position and width are attributed t o variations in chemical composition of the grains, impacting upon the parameters of the Lorentz oscillator Further broadening is attributed to clustering. These models are shown to satisfy the above observatio nal constraints. Furthermore, the correlated shift of peak position wi th increased width, observed for the widest interstellar UV features, is reproduced. Models involving a second Lorentz oscillator to reprodu ce the FUV rise are also considered. The impact of this extra Lorentz oscillator on the peak position, width, and shape of the bump is inves tigated. Synthetic extinction curves are generated to model actual one s exhibiting a wide range of FUV curvatures. Physical mechanisms which might be of relevance to explain the variations of these optical prop erties are discussed.