CURRENT METEOR PRODUCTION OF COMET P SWIFT-TUTTLE 1992T/

Two wide-field Schmidt images of the dust tail of comet P/Swift-Tuttle 1992t are analysed by means of the inverse Monte-Carlo model of dust tails (Fulle 1989). The model allows to infer the dust environment on a limited time interval (from five months to a month before perihelion ) and on a time-dependent size interval (this fact being due to the in trinsic complex shape of syndynes): from 0.1 mm to 10 mm five months b efore perihelion, and from 5 mu m to 500 mu m a month before. Only a d ust production involving all of the sunward hemisphere can account for the tail shape, in agreement with the wide thrust angles of dust ejec tion from the nucleus active spots and cracks found by Sekanina (1981) . The dust ejection velocity from the inner coma is almost size-indepe ndent for the considered size range, and increases from 25 +/- 5 m s(- 1) to 50 +/- 5 m s(-1). For the considered time interval, correspondin g to heliocentric distances decreasing from 2.5 AU to 1 AU, the mass l oss rate is about constant at (5 +/- 3)10(3) kg s(-1): we point out th at this result refers to the time-dependent size interval constrained by the syndynes, which considers large grains at the beginning of the time interval, and smaller grains at the end. If the mass loss rate is dominated by large grains, our result is compatible with a mass loss rate proportional to the inverse square of the heliocentric distance. Our results on the size distribution suggest that this is the case, be cause the power index of the time-averaged distribution is -3.3 +/- 0. 2 for dust diameters between 5 mu m and 3 mm. Therefore the dust produ ction is dominated by millimeter-sized grains, really Perseids at thei r birth. For pre-perihelion times, the lower limit of meteor productio n of P/Swift-Tuttle 1992t is 7 10(10) kg, (90 +/- 10)% of which was in jected into the Perseid stream. Therefore, a few thousands of passages similar to the 1992 one would account for the total estimated mass of the Perseid stream (3 10(14) kg, Hughes and McBride 1989). We stress that all of these results are limited to the size and time ranges ment ioned above: no extrapolation out of the mentioned limits is possible with sufficient accuracy. Moreover, we adopt standard values of the du st bulk density and scattering efficiency: in the paper we discuss the ir influence on the quoted results.