Circumstellar dust shells of hot post-AGB stars

Abstract

Parameters of the circumstellar dust shells of 15 hot post-AGB stars have been derived using a radiative transfer code (DUSTY). Combining the optical, near and far-infrared (ISO, IRAS) data of the stars, we have reconstructed their spectral energy distributions (SEDs) and estimated the dust temperatures, mass loss rates, angular radii of the inner boundary of the dust envelopes and the distances to these stars. The mass loss rates (10 -6-10-5 yr-1) are intermediate between stars at the tip of the AGB and the PN phase. We have also studied the ISO spectra of 7 of these stars. Amorphous and crystalline silicate features were observed in IRAS 14331-6435 (Hen3-1013), IRAS 18062+2410 (SAO 85766) and IRAS 22023+5249 (LSIII +5224) indicating oxygen-rich circumstellar dust shells. The presence of unidentified infrared (UIR) band at 7.7 , SiC emission at 11.5 and the "26 " and "main 30 " features in the ISO spectrum of IRAS 17311-4924 (Hen3-1428) suggest that the central star may be carbon-rich. The ISO spectrum of IRAS 17423-1755 (Hen3-1475) shows a broad absorption feature at 3.1 due to C2H2 and/or HCN which is usually detected in the circumstellar shells of carbon-rich stars.