Dust shells around carbon Mira variables

The spectral energy distributions and mid-infrared spectra of 44 carbon Mira variables are fitted using a dust radiative transfer model. The pulsation periods of these stars cover the entire range observed for carbon Miras. The luminosities are derived from a period-luminosity relation. Parameters derived are the distance, the temperature of the dust at the inner radius, the dust mass-loss rate and the ratio of silicon carbide to amorphous carbon dust. The total mass-loss rate is derived from a modified relation between the photon momentum transfer rate (L/c) and the momentum transfer rate of the wind (M #upsilon#_#infinity#). Mass-loss rates between 1 x 10"-"8 and 4 x 10"-"5 M_sun yr"-"1 are found. We find good correlations between mass-loss rate and pulsation period (log M=4.08 log P-16.54), and mass-loss rate and luminosity (log M=3.94 log L= 20.79). These relations are not independent as we assumed a P-L-relation. If we had assumed a constant luminosity for all stars there still would be a significant relation between M and P. The dust-to-gas ratio appears to be almost constant up to periods of about 500 days, corresponding to about 7900 L_sun, and then to increase by a factor of 5 towards longer periods and higher luminosities. A comparison is made with radiation-hydrodynamical calculations including dust formation. The mass-loss rates predicted by these models are consistent with those derived in this paper. The main discrepancy is in the predicted expansion velocities for models with luminosities below #propor to#5000 L_sun. The radiation-hydrodynamical calculations predict expansion velocities which are significantly too large. This is related to the fact that these models need to be calculated with a large C/O ratio to get an outflow in the first place. Such a large C/O ratio is contrary to observational evidence. It indicates that a principle physical ingredient in these radiation-hydrodynamical calculations is still missing. Possibly the winds are 'clumpy' which may lead to dust formation on a local scale, or there is an additional outwards directed force, possibly radiation pressure on molecules. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman