DUST CONDENSATION IN THE SHELL OF NOVA-AQ L-1993

UBVJHKLM photometry of Nova Aql 1993 has revealed a deep minimum in it s visual light curve during the transition period due to the condensat ion 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 abo ut 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 19 93, similar to 2.5 x 10(4) L(.), was already almost one order of magni tude 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 o ptical 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 c olor temperature. In the first phase (F-IR similar to t(-0.9)), this w as a result of a decrease in the opacity of the optically thick dust s hell 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 f rom the star did cease to decrease because of their destruction. By th e time of the last observation made similar to 170 days after the outb urst, the dust-shell mass had been reduced by a factor of similar to 3 .5.