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pre - planetary nebulae ( ppns ) , objects in transition between the agb and planetary nebula ( pn ) evolutionary phases , hold the key to our understanding of the relationship between these late evolutionary stages of low and intermediate mass ( @xmath11 ) stars . the hydrodynamic interaction of one or more fast , collimated post - agb outflows , believed to occur during the ppn phase , with the dense , massive , slowly - expanding spherical agb wind is believed to be responsible for shaping planetary nebulae ( sahai & trauger 1998 , sahai 2002 ) . imaging surveys with the hubble space telescope have been crucial in revealing a wide variety of aspherical morphologies in ppns ( e.g. sahai 2004 ; ueta et al . 2000 ) , however the bulk of the circumstellar mass , which often resides in a spherical , dusty component surrounding the central aspherical nebula , is not well - probed in these data . thus a crucial ingredient for theoretical studies of such interactions , namely the mass and density of the ambient circumstellar medium ( e.g. , lee & sahai 2003 , 2004 ) , is lacking for a majority of the ppns discovered in hst surveys . although attempts have been made to estimate the mass from submillimeter data ( e.g. gledhill et al . 2002 ) , these are compromised by the simplifying assumptions about the dust temperature . the mineralogy of circumstellar dust shells around ppns became possible with the wavelength coverage and spectral resolution offered by the infrared space observatory ( iso ) ( see e.g. hrivnak et al . 2000 ; molster et al . 2002a , b , c ; gauba & parthasarathy 2004 and references therein ) . in order to understand the grain temperatures , mass - loss history and morphology responsible for the formation of these minerals in the circumstellar environment of ppns , detailed modelling of the spectral energy distributions ( seds ) of a few ppns ( e.g. , hd161796 , hoogzaad et al . 2002 ; iras 16342 - 3814 , dijkstra et al . 2003a ; iras 16279 - 4757 , matsuura et al . 2004 ; gauba & parthasarathy 2004 ) have been undertaken .    in this paper , we examine the composition and distribution of dust in the ppn , iras19475 + 3119 ( hereafter i19475 ) and model its sed from the optical to the far - infrared to set constraints on the physical properties of its dust shell ( mass , temperature , size ) . i19475 is listed as a f3ia star in the case - hamburg luminous stars survey ( stock et al . high resolution optical spectra of the star have confirmed its post - agb nature ( klochkova et al . 2002 ; sivarani et al . 2001 ; arellano ferro et al . 2001 ) . a point - symmetric nebula with an extent of 4.9@xmath12 3.4  has been seen in j - band polarized flux images ( gledhill et al . 2001 ) . co j=10 and 21 emission typical of agb and post - agb objects was detected from the circumstellar envelope of the star ( loup et al . hrivnak & bieging ( 2005 ) noticed asymmetry and possible structure in the co j=21 and 43 lines . recent imaging of i19475 with the hubble space telescope at optical wavelengths reveals a dusty quadrupolar nebula and spherical halo , surrounding the central star ( sahai 2004 ; sahai et al . 2006 ) . the rest of the paper is organized as follows . in  2 , we provide observational details of the iso data , in  3 we describe the iso spectra , and the re - construction of i19475 s spectral energy distribution ( sed ) from these as well as broad - band photometric data , in  4 we describe our modelling of the sed using a spherical dust radiative transfer code , in  5 we discuss our models , and in  6 , we present our conclusions . infrared space observatory ( iso ) observations of i19475 were extracted from the iso data archive . these include spectroscopic observations made with the short wavelength spectrometer ( sws ) and the long wavelength spectrometer ( lws ) and spectrophotometric observations made with the imaging photopolarimeter ( isophot ) onboard iso . an off - source measurement made with the lws in order to estimate the background was also extracted from the archive . a log of the observations is given in table 1 . iso sws spectra have a wavelength coverage of 2.38@xmath1345.2 @xmath8 m . our spectra were obtained in the low resolution mode ( aot 01 ) of the sws instrument ( de graauw et al . 1996 ) with a 33@xmath12 20aperture . each sws spectrum contains 12 subspectra , that each consist of two scans , one in the direction of decreasing wavelength ( ` up ' scan ) and the other in the direction of increasing wavelength ( ` down ' scan ) . there are small regions of overlap in wavelength between the sub - spectra . each sub - spectrum is recorded by 12 independent detectors . lws spectra extend from 43@xmath13197@xmath8 m . the lws observations were obtained in lws01 mode , covering the full spectral range at a resolution ( @xmath14 ) of  @xmath10 200 . the lws circular field of view had a diameter of 84 . the iso lws instrument and its calibration are described in clegg et al . ( 1996 ) and in swinyard et al . ( 1996 ) respectively . isophot observations were carried out using the spectrophotometer subsystem pht - s . pht - s consists of two low - resolution grating spectrometers covering the wavelength ranges 2.5@xmath134.9 @xmath8 m ( pht - ss ) and 5.8@xmath1311.6 @xmath8 m ( pht - sl ) and having a common entrance aperture of 24@xmath12 24 . each channel has a linear 64-element array of si : ga detectors . the spectral band width ( fwhm ) of a single detector is 0.0383 @xmath8 m for pht - ss and 0.0918 @xmath8 m for pht - sl , resulting in a mean @xmath14 of about 95 for both channels . a more detailed description of pht - s has been given by klaas et al . 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we present the spectral energy distribution ( sed ) of the pre - planetary nebula , iras 19475 + 3119 ( i19475 ) , 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 i19475 using a 1-d radiative transfer code , and find that a shell with inner and outer radii of 8.8@xmath0 and 4.4 @xmath1 cm , and dust temperatures ranging from about 94k to 46k provide the best fit . 
 the mass of this shell is @xmath21[34 @xmath3 g@xmath4/@xmath5m)][@xmath6/200 ] , where @xmath7(100@xmath8 m ) is the 100@xmath8 m  dust mass absorption coefficient ( per unit dust mass ) , and @xmath6 is the gas - to - dust ratio . in agreement with results from optical imaging and millimeter - wave observations of co emission of i19475 
 , our model fits support an r@xmath9 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 which results in the observed shapes of ppns and pns . 
 we find that the observed jcmt flux at sub - millimeter wavelengths ( 850 ) is a factor @xmath102 larger than our model flux , suggesting the presence of large dust grains in the dust shell of i19475 which are not accounted for by our adopted standard mrn grain size distribution .