Astronomy and Astrophysics, volume 330, 505-514 (1998/2-2)
Circumstellar emission from dust envelopes around carbon stars showing the silicon carbide feature.
BLANCO A., BORGHESI A., FONTI S. and OROFINO V.
Abstract (from CDS):
Spectroscopic and photometric data relative to a sample of 55 carbon stars showing the 11.3µm feature have been fitted in the wavelength range between 0.4 and 100µm by means of a radiative transfer model using the laboratory extinction spectra of amorphous carbon and silicon carbide (SiC) grains. The transfer code allows to determine in a self-consistent way the grain equilibrium temperature of the various species at different distances from the central star and gives all the relevant circumstellar parameters which can be very important for the evolutionary study of carbon stars. In order to get meaningful information on the nature and physical properties of the dust grains responsible for the 11.3 µm feature and the underlying continuum, the fitting procedure of the spectra has been applied individually to every single source. For this reason it has been possible to take into account any variation in position and shape of the band from source to source. Our analysis show that all the sources, in addition to the amorphous carbon grains accounting for the continuum emission, need always the presence of α-SiC particles while some of them require also β-SiC. Moreover, the presence of one or both types of SiC particles seems not correlated neither with the total optical thickness nor with any other physical and geometrical parameters of the circumstellar envelope. The best-fit parameters found in this work have been used to calculate the mass-loss rate from the central stars. The clear correlation, that we find between the strength of the SiC feature and the total mass loss-rate, confirms the results already found by other authors for the same kind of sources and derived from the observed CO emission lines.