Can Composite Fluffy Dust Particles Solve the Interstellar Carbon Crisis?

Interstellar dust models are facing a "carbon crisis", so called becauserecent observations suggest that the abundance of carbon available for dust inthe interstellar medium is less than half of the amount required to be tied upin graphite grains in order to explain the interstellar extinction curve. This paper presents an detailed assessment of a newly-proposed dust model(Mathis 1996), in which the majority of the interstellar carbon is contained incomposite and fluffy dust (CFD) grains. Per unit mass, these grains producemore UV extinction, and can therefore account for the interstellar extinctioncurve with about half the carbon required in traditional dust models. The results of our analysis show that the CFD model falls short in solvingthe carbon crisis, in providing a fit to the UV-optical interstellar extinctioncurve. It also predicts a far-infrared emissivity in excess of that observedwith the COBE/DIRBE and FIRAS instruments from the diffuse interstellar medium.The failure of the new model highlights the interrelationships between thevarious dust properties and their observational consequences, and the need tosatisfy them all simultaneously in any comprehensive interstellar dust model. In light of these problems, the paper examines other possible solutions tothe carbon crisis.Comment: 12 pages 7 figures, LateX aaspp4 style. submitted to the Astrophysical Journa