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Astron. Astrophys. 324, 1059-1070 (1997)

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

T. Le Bertre

DEMIRM, Observatoire de Paris, 61 av. de l'Observatoire, F-75014 Paris, France

Received 2 December 1996 / Accepted 9 January 1997

Abstract

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 [FORMULA] and a dust formation temperature [FORMULA] 950 K. The modelling favors the case of an acceleration of dust, from the site of formation and on a distance of [FORMULA] 1015 cm, over the case of a uniform expansion. Mass loss rates in the range 0.1-50 10-6 [FORMULA] 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

* Based on observations obtained at ESO, La Silla, Chile.

Send offprint requests to: T. Le Bertre

Tables 2, 3 and 4 are only available in electronic form at the CDS via anonymous ftp 130.79.128.5.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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