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Observations show that the time of onset of dust formation in classical novaedepends strongly on their speed class, with dust typically taking longer toform in slower novae. Using empirical relationships between speed class,luminosity and ejection velocity, it can be shown that dust formation timescaleis expected to be essentially independent of speed class. However, following anova outburst the spectrum of the central hot source evolves, with anincreasing proportion of the radiation being emitted short-ward of the Lymanlimit. The rate at which the spectrum evolves also depends on the speed class.We have therefore refined the simple model by assuming photons at energieshigher than the Lyman limit are absorbed by neutral hydrogen gas internal tothe dust formation sites, therefore preventing these photons reaching thenucleation sites. With this refinement the dust formation timescale istheoretically dependent on speed class and the results of our theoreticalmodification agree well with the observational data. We consider two types ofcarbon-based dust, graphite and amorphous carbon, with both types producingsimilar relationships. Our results can be used to predict when dust will formin a nova of a given speed class and hence when observations should optimallybe taken to detect the onset of dust formation.

Rapid Dust Formation in Novae: The Speed Class - Formation Timescale Correlation Explained