Optical and infrared observations of 23 carbon-rich stars. Modelling of the circumstellar dust shells

A study of circumstellar dust shells (CDS) around carbon stars is presented. This study is based on the analysis of the 1-100 mu m broad-band spectra of 23 objects which have been monitored in the 1-20 mu m region over several years at ESO. The sources in the sample range from Irr/SR variables and optical miras to Extreme Carbon Stars which have only been discovered recently in infrared surveys. A modelling accounting for the photometric variations with phase (with up to 9 spectra per object) has been developed. All the CDSs in the sample can be modelled successfully in spherical geometry with a dust opacity law proportional to lambda(-1.3) and a dust formation temperature similar to 950 K The modelling favors the case of an acceleration of dust, from the site of formation and on a distance of similar to 10(15) Cm, over the case of a uniform expansion. Mass loss rates in the range 0.1-50 10(-6)M(circle dot)yr(-1) are derived. The estimates based on this modelling are consistent with those derived from the CO emission line measurements. In view of the incoming near-infrared surveys (DENIS and 2MASS), an analysis of the location of carbon-rich sources in color diagrams is presented. It is shown that the near-infrared colors are well correlated with the mass loss rate. This property can be used to get an estimate, within a factor 3, of this rate for carbon stars without a priori on the distance.