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A. Knapik and
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 () is shown to provide good fits. The threshold for reddening detection turns to be 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 , ranging from 0.37 near (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
Send offprint requests to: J. Bergeat
Online publication: June 30, 1998