Dust condensation in the shell of Nova Aql 1993

UBVJHKLM photometry of Nova Aql 1993 has revealed a deep minimum in its visual light curve during the transition period due to the condensation of an optically thick (tau(V) similar or equal to 2.2) dust shell with a mass of similar to 10(-7) M(.). The condensation lasted for about 20 days during which the grains grew to sizes of similar to 0.1 mu m. At maximum infrared light, the bolometric luminosity of Nova Aql 1993, similar to 2.5 x 10(4) L(.), was already almost one order of magnitude lower than its optical luminosity. The mass of the ejected shell is M(shell) similar or equal to 5 x 10(-5) M(.). The rate of infrared-brightness decline in Nova Aql 1993 increased significantly when the optical depth (tau(V)) of the dust shell decreased from similar to 2.2 to similar to 0.8. At this time, the process of grain destruction set in. Over the entire length of observations, the drop in the luminosity of the dust shell was not accompanied by appreciable changes in its color temperature. In the first phase (F-IR similar to t(-0.9)), this was a result of a decrease in the opacity of the optically thick dust shell due to its expansion (tau(dust) similar to t(-2)). In the second phase (F-IR similar to t(-3.4)), the temperatures of grains receding from the star did cease to decrease because of their destruction. By the time of the last observation made similar to 170 days after the outburst, the dust-shell mass had been reduced by a factor of similar to 3.5.