acknowledgements

DS was supported by the German Deutsche Forschungsgemeinschaft, DFG project ``Research Group Laboratory Astrophysics'' (He 1935/17-1), the work of MI was supported by the Max Planck Society. We are grateful to D. Alexander for valuable comments and discussions. For the calculations of the optical properties of multishell spherical particles, we used a code by N.Voshchinnikov (http://www.astro.spbu.ru/staff/ilin2/ilin.html)