Evolution of the dust envelope of Sakurai's star (V4334 Sgr) in 1997-1999

We have studied the structural evolution of the dust envelope of V4334 Sgr, starting with the onset of its condensation in 1997. A model with complete cloud cover, with the optical depth growing until the end of 1999, gives t he best fit to the photometric data in the optical and IR. The inner radius of the dust layer remained virtually constant, whereas its thickness incre ased due to expansion. The deep minimum in the visual light curve of V4334 Sgr in October 1998 is attributed to the arrival at the dust-grain condensa tion zone of a density discontinuity in the circumstellar envelope. The dis continuity was probably formed early in 1997 due to an increase in the mass -loss rate by a factor of about four, possibly associated with an increase in the luminosity of V4334 Sgr during its transformation into a carbon star . After this luminosity increase, the mass-loss rate was (M) over dot appro ximate to 2 x 10(-6)M./yr. In the summer of 1999, the mass of the dust enve lope was M-dust approximate to 2 x 10(-7) M. (M-gas approximate to 4 x 10(- 6)M.). In the complete-cloud-cover model, the envelope consists of graphite grains with a(gr) = 0.05 mum, to similar to 85% per cent in terms of the n umber of grains. The remaining similar to 15 per cent of the grains have si zes a(gr) = 0.1 and 0.25 mum. To reproduce the small hump in the spectral e nergy distribution of V4334 Sgr near 11 mum, some silicon carbide grains mu st be added to the graphite mixture. Their contribution to the V optical de pth is less than or equal to4%. The first deep minimum in the visual light curve could also be reproduced using a model in which the dust cloud has co ndensed along the line of sight, but a detailed analysis of the resulting c haracteristics of the cloud and envelope indicates that this model is impro bable. (C) 2001 MAIK "Nauka/Interperiodica".