THE PLANETARY-NEBULA BD-DEGREES-3639 - THE INFRARED-SPECTRUM DURING POST-AGB STELLAR EVOLUTION(30)

We present a radiative-transfer calculation which reproduces the infra red spectrum of the planetary nebula BD + 30 degrees 3639. We calculat e the transfer process through absorption and scattering in a spherica lly symmetric, multigrain dust shell. The emission of transiently heat ed particles is taken into account, as well as polycyclic aromatic hyd rocarbons (PAHs). We obtain an acceptable fit to most of the spectrum, including the PAH infrared bands. At submillimetre wavelengths the ob served emission is greater than the model predicts, indicating that la rge dust conglomerates ('fluffy grains') may be needed as an additiona l constituent. The fit favours a distance of greater than or equal to 2 kpc, which implies that BD + 30 degrees 3639 has evolved from a mass ive progenitor of several solar masses. A low dust-to-gas mass ratio i s found in the ionized region. The calculations yield an original mass -loss rate of 2x10(-4) M. yr(-1) on the asymptotic giant branch (AGB). Using this mass-loss rate, we calculate how the infrared spectrum has evolved during the post-AGB evolution. We show, in particular, the ev olution of the IRAS colours during the post-AGB evolution.