THE ORIGIN OF THE BLUE EMISSION DURING PROFOUND VISUAL DIMMINGS OF FGSGE

A model is constructed for the dust shell of FG Sge in the course of i ts condensation that reproduces the characteristic features of the sta r's photometric behavior: during its first profound dimming in 1992, i n particular, the substantial decrease in the B-V color index (a bluin g of the star) in a certain interval of this decline. In this interval , the shell consisted of optically thick dust clouds which covered rou ghly half of the sphere around the central star and which produced the observed infrared excess. Their time-varying optical depth over the e ntire period of brightness decline in the star was determined by dust grains of radius of a(gr) greater than or equal to 0.1 mu m. About hal f of the direct starlight was scattered by an optically thin [tau(V) a pproximate to 0.2] dust structure, which could be both part of the con densing shell and the dust shell that had previously existed around FG Sge. The size of the grains that mainly contributed to its optical de pth was a(gr) similar or equal to 0.05 mu m. The dust-shell parameters after the completion of the condensation and the disappearance of the star's bluing are the following: tau(V) approximate to 4, R-in approx imate to 90R(), M-env approximate to 2.7 x 10(-8) M-., and T-gr = 10( 3) K.