CIRCUMSTELLAR DUST SHELLS AROUND LONG-PERIOD VARIABLES .2. THEORETICAL LIGHTCURVES OF C-STARS

Theoretical lightcurves of long-period and Mira variables are presente d. Based on dynamical models of circumstellar dust shells (CDS), which include time-dependent hydrodynamics and a detailed treatment of the processes of formation, growth and evaporation of carbon grains, frequ ency-dependent radiative transfer calculations have been carried out w hich yield the synthetic infrared lightcurves of these dust shell mode ls. It turns out that the discrete structure of the CDS due to the per iodic formation of new dust layers decisively influences the shape of the lightcurves by producing e.g. intermediate extrema. This is demons trated in detail for one model. Furthermore, dust shell models are pre sented whose lightcurves exhibit the superposition of two different os cillations: The oscillation with the period of the interior pulsation is superposed by an oscillation with a longer period which is produced by the dynamical structure of the CDS. Similar features are present i n observed lightcurves, which indicates that our, nevertheless prelimi nary, modelling approach shows promise for interpreting the optical ap pearance of Miras and long-period variables.