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Stars on the asymptotic giant branch (AGB) lose considerable amounts ofmatter through their dust-driven stellar winds. A number of such sources havebeen imaged by Herschel/PACS, revealing a diverse sample of differentmorphological types. Among them are a few examples which show geometricallythin, spherically symmetric shells which can be used to probe the mass losshistory of their host stars. We aim to determine the physical properties of thedust envelope around the two carbon stars U Hya and W Ori. With themuch-improved spatial constraints from the new far-infrared maps, our primarygoal is to measure the dust masses contained in the shells and see how they fitthe proposed scenarios of shell formation. We calculated the radiative transferof the circumstellar dust envelope using the 1D code More of DUSTY (MoD).Adopting a parametrised density profile, we obtained a best-fit model in termsof the photometric and spectroscopic data, as well as a radial intensityprofile based on Herschel/PACS data. For the case of U Hya, we also computed agrid of circumstellar envelopes by means of a stationary wind code and comparethe results of the two modelling approaches. The Herschel/PACS maps show U Hyasurrounded by a detached shell of $114''\ (0.12\,\mathrm{pc})$ in radius,confirming the observations from previous space missions. The dust massescalculated for the shell by the two approaches are consistent with respect tothe adopted dust grain properties. In addition, around W Ori, we detect for thefirst time a weak spherically symmetric structure with a radius of $92''\(0.17\,\mathrm{pc})$ and a dust mass of$(3.5\pm0.3)\times10^{-6}\,\mathrm{M_\odot}$.

Extended view on the dust shells around two carbon stars