Infrared spectroscopy of Nova Cassiopeiae 1993 - IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared (IR) spectral energy distribution (SED) with the dusty code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of 2:1 by number (similar to 1.26:1 by mass) and a relatively flat size distribution. The 9.7- and 18-mu m silicate features are consistent with freshly condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions similar to 0.06 mu m; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova unidentified infrared (UIR) features.