The Dust Envelope of the Preplanetary Nebula IRAS 19475+3119

Abstract

We present the spectral energy distribution (SED) of the preplanetary nebula, IRAS 19475+3119 (hereafter I 19475), from the optical to the far-infrared. We identify emission features due to crystalline silicates in the ISO SWS spectra of the star. We have fitted the SED of I 19475 using a one-dimensional radiative transfer code and find that a shell with inner and outer radii of 8.8×10¹⁶ and 4.4×10¹⁷ cm and dust temperatures ranging from about 94 to 46 K provides the best fit. The mass of this shell is >~1[34 cm² g^-1/κ(100 μm)](δ/200) M_solar, where κ(100 μm) is the 100 μm dust mass absorption coefficient (per unit dust mass) and δ is the gas-to-dust ratio. In agreement with results from optical imaging and millimeter-wave observations of CO emission of I 19475, our model fits support an r^-3 density law for its dust shell, with important implications for the interaction process between the fast collimated post-AGB winds and the dense AGB envelopes that results in the observed shapes of PPNs and PNs. We find that the observed JCMT flux at submillimeter wavelengths (850 μm) is a factor ~2 larger than our model flux, suggesting the presence of large dust grains in the dust shell of I 19475 that are not accounted for by our adopted standard MRN grain size distribution.