THE INFRARED-SPECTRUM OF THE OPTICALLY THIN DUST SHELL OF V705-CASSIOPEIAE (NOVA-CASSIOPEIAE 1993)

We report 1.25 to 18 mu m infrared photometry spectroscopy of the opti cally thin dust shell of V705 Cassiopeiae (Nova Cas 1993) between 330 and 418 days after the outburst. The measurements show that the dust s hell, which had been optically thick until at least day approximate to 131, now shows the spectral signatures of optically thin astrophysica l silicate grains at 10 and 20 mu m and hydrocarbons at 3.2-3.4 and 11 .3 mu m. The 1-8 mu m continuum which is due to carbon dust is still p resent, although it no longer has a blackbody spectral energy distribu tion. We estimate mass of silicate grains required to produce the obse rved visual extinction and conclude that the condensible elements in t he silicate grains may be overabundant with respect to hydrogen.