Grain destruction in the shell of Nova Aql 1993

Calculations of models for the dust shell of Nova Aql 1993 show that the period of its isothermal expansion, i.e., the period during which the shape of the energy distribution in its spectrum remains unchanged, is related to perceptible destruction of dust particles. In order for the computed light curves to coincide with the observed ones, the destruction of grains must be complex in nature, incorporating their fragmentation, in which the total mass of the dust shell is preserved, and destruction as a result of bombardment of their surface by particles of the gaseous phase (sputtering), in which the grain size, i.e., their total mass, decreases, but their total number is preserved. It seems quite reasonable to suppose that both these processes proceed simultaneously in clumpy nova shells with a large scatter of internal velocities. Within similar to 110 days after the condensation, the originally optically thick (tau(v) similar or equal to 2.5) dust shell of Nova Aql 1993 becomes essentially transparent (tau(v) similar or equal to 0.04). By this time, the destruction of grains had led to a similar to 25-fold increase in their number and similar to 7-fold decrease in their total mass. The grain size is reduced by a factor less than or equal to 6. It is noted that during the expansion of an optically thick dust shell, the partial stabilization of its color properties may be caused by optical effects arising from a decrease in its opacity, with the grain temperature at its inner boundary actually decreasing.