GRAIN DESTRUCTION IN THE SHELL OF NOVA-AQL-1993

Calculations of models for the dust shell of Nova Aql 1993 show that t he period of its isothermal expansion, i.e., the period during which t he 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 des truction of grains must be complex in nature, incorporating their frag mentation, 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., thei r total mass, decreases, but their total number is preserved. It seems quite reasonable to suppose that both these processes proceed simulta neously in clumpy nova shells with a large scatter of internal velocit ies. Within similar to 110 days after the condensation, the originally optically thick (tau(v) similar or equal to 2.5) dust shell of Nova A ql 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 2 5-fold increase in their number and similar to 7-fold decrease in thei r 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 dus t shell, the partial stabilization of its color properties may be caus ed by optical effects arising from a decrease in its opacity, with the grain temperature at its inner boundary actually decreasing.