STRUCTURE AND EVOLUTION OF THE DUST SHELL AROUND THE POSTRGB CANDIDATE 89-HER

Results from photometric IR observations of 89 Her, conducted between 1987 and 1993, are presented. The IR excess in the wavelength range 1 - 1300 Cum can be approximated by the emission from a dust shell forme d on a time scale of similar to 3 x 10(4) years, when the mass-loss ra te of this binary star reduced: M(t) proportional to t(0.2) (where the time is measured from now to the past). The mass of gas in the shell is estimated to be similar to 2 x 10(-2) M., and its optical thickness in the visible is similar to 0.4. In the suggested model, the calcula ted brightness variations in the star are consistent with observations both in the optical and in the infrared. The component of the binary system 89 Her, classified as a F2Ib star, appears to be a post-AGB can didate with a low luminosity (similar to 10(3) L.), rather than a post -AGE candidate.