Dust in Stellar Winds: A Micro- and Macroscopic View

In the course of the MESS Guaranteed Time Key Program, geometrically thin and overall spherically symmetric detached shells were observed with Herschel/PACS amongst a morphologically diverse sample of (post-)AGB stars. We use 1D dust radiative transfer modelling to determine shell properties such as the dust mass, the corresponding mass-loss rate (MLR) and dust temperature, in order to assess the possible scenarios of shell formation. As an example we present the case of the carbon star U Hya. For the modelling the chosen dust properties are crucial, and we show the key aspects and their effect on the results obtained. We further attempt to establish a physical model of the observed stellar dust envelope by means of a stationary wind code to test whether an abrupt change in the MLR can cause the observed detached shell.