Optical and infrared observations of 23 carbon-rich stars. Modelling of the circumstellar dust shells *
T. Le Bertre
Received 2 December 1996 / Accepted 9 January 1997
A study of circumstellar dust shells (CDS) around carbon stars is presented. This study is based on the analysis of the 1-100 µm broad-band spectra of 23 objects which have been monitored in the 1-20 µm 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 and a dust formation temperature 950 K. The modelling favors the case of an acceleration of dust, from the site of formation and on a distance of 1015 cm, over the case of a uniform expansion. Mass loss rates in the range 0.1-50 10-6 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.
Key words: radiative transfer stars: carbon (stars:) circumstellar matter stars: mass loss stars: AGB and post AGB infrared: stars
Send offprint requests to: T. Le Bertre
Tables 2, 3 and 4 are only available in electronic form at the CDS via anonymous ftp 18.104.22.168.
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998