MID-INFRARED AND FAR-INFRARED PROPERTIES OF DYNAMICAL MODELS OF CARBON-RICH LONG-PERIOD VARIABLES

We have calculated spectral energy distributions and synthetic IRAS co lours of carbon-rich long-period variables. On top of dynamical models for the stellar atmosphere and circumstellar envelope which consisten tly treat the time-dependent hydrodynamics, the formation of amorphous carbon grains and grey radiative transfer, frequency-dependent radiat ive transfer calculations have been carried out in the range between 1 mu m and 290 mu m. Calculating the IRAS colours we find that the mode ls lie in regions I(stars without circumstellar shells) and VII(variab le stars with carbon-rich circumstellar shells) of the IRAS two colour diagram in accordance with observational results for carbon stars. Th ey form an almost linear sequence (near the black body line) reflectin g the different mass loss rates. We compare our results to empirical f ormulae which link the mass loss rate to the observed flux at 60 mu m. Furthermore, we have also investigated qualitatively the effects of d ust with alpha-SiC grains and of detached shells. The SiC-contribution causes mainly a blue-shift of about 0.2 mag in the [12]-[25] colour. Adding a detached shell results in higher [25]-[60] values partly shif ting the models into region VIa (non-variable stars with relatively co ld dust at large distances) of the IRAS two colour diagram as expected from observations.