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Astron. Astrophys. 321, 236-244 (1997)

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Interstellar extinction and the intrinsic spectral distribution of variable carbon stars * **

A. Knapik and J. Bergeat

Centre de Recherche Astronomique de Lyon (UMR 5574 du CNRS), Observatoire de Lyon, 9 avenue Charles André, F-69561 St-Genis-Laval Cedex, France

Received 15 July 1996 / Accepted 10 October 1996


We present a new method of evaluation of the extinction by interstellar dust on cool carbon variables. These late-type stars show no marked relationship between spectral classification (the R, N- and C-types) and photometric colour indices. The pair method is thus ruled out, at least in the form currently in use for early-type or intermediate stars.

Our method makes use of the whole spectral energy distributions from UV to IR. A sample of 60 unreddened carbon variables is delineated and new colour-colour diagrams are proposed where the reddening vector is nearly perpendicular to their narrow intrinsic locus. Six photometric groups (or boxes : CV1 to 6) are derived among unreddened stars. They show a continuous range of spectral energy distributions from "bluer" to "redder", and mean colour indices are obtained. A pair method is described where each presumably reddened star is compared to these mean unreddened stars, a given extinction law being assumed.

As an illustration, the results are shown for a sample of 133 well-documented stars. The mean extinction law usually adopted for the diffuse interstellar medium ([FORMULA]) is shown to provide good fits. The threshold for reddening detection turns to be [FORMULA] A good correlation is observed when the derived colour excesses are compared to values from maps in the literature. The mean rate of visual extinction amounts to [FORMULA], ranging from 0.37 near [FORMULA] (intercloud) to 2.1 (cloud + intercloud) in two structures correlated with Gould's belt.

Key words: stars: AGB and post-AGB – stars: carbon – dust, extinction

* This research has made use of the Simbad database operated at CDS, Strasbourg, France.
** Table 3 is only available in electronic form at the CDS via anonymous ftp

Send offprint requests to: J. Bergeat

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998