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