2175 Å and 3.4 micron absorption bands and carbon depletion in the diffuse interstellar medium

Using a theoretical model previously developed to study the UV extinction produced by small carbon particles, we present results that show that the observed strength, profile and central wavelength of the interstellar 2175 Angstrom absorption band can be reproduced with a two-component mixture of these particles. These consist of small dehydrogenated coronene molecules together with a size distribution of larger particles having a range of hydrogenation. Observed values of tau (2175 Angstrom)/A(upsilon) and tau (3.4 mu m)/A(upsilon) can be obtained if n(C)/n(H) similar to 80 ppm is contained in this mixture. The 2175 Angstrom peak in interstellar spectra can be simulated as the superposition of a central peak having gamma = 0.9 mu m(-1) and lambda(0)(-1) = 4.58 mu m(-1) arising from dehydrogenated coronene and a somewhat broader peak (gamma similar to 1.25 mu m(-1), lambda(0)(-1) similar to 4.60 mu m(-1)) due to larger particles. Other solutions, incorporating as little carbon as n(C)/n(H) = 28 ppm are also possible. Photoluminescence from these particles is also discussed in relation to the extended red emission spectrum.