CIRCUMSTELLAR DUST SHELLS AROUND LONG-PERIOD VARIABLES .4. BRIGHTNESSPROFILES AND SPATIAL SPECTRA OF C-STARS

Theoretical brightness profiles and spatial spectra of dynamical model s for circumstellar dust shells of long-period variables are presented . Based on dynamical models of circumstellar dust shells, which includ e time-dependent hydrodynamics and a detailed treatment of the process es of formation, growth and evaporation of carbon grains, frequency-de pendent radiative transfer calculations have been carried out which yi eld the synthetic brightness distributions and spatial spectra of thes e dust shell models. The brightness profiles show a pronounced time-de pendent internal structure on a spatial scale of a few stellar radii, which reflects the discrete shell-like distribution of dust across the circumstellar shell. Internal structures are also present in the synt hetic spatial spectra (or visibilities) calculated from these intensit y profiles. Similar structures, which are present in the observed brig htness profiles and spatial spectra of several long-period variable st ars can be physically explained by the formation of discrete dust laye rs resulting from the model calculations presented here. We also find temporal variations of the synthetic spatial spectra which are in quan titative agreement with respective observations. Our model calculation s indicate, that the instants of time, when the minimum and maximum va lue of the visibility is reached depend on wavelength.