Interstellar extinction and the intrinsic spectral distribution of variable carbon stars

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 (R-v approximate to 3.1) is shown to provide good fits. The threshold for reddening detection turns to be E(B - V) approximate to 0.02 - 0.03 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 < A(v)/D > approximate to 1.25 +/- 1.1, ranging from 0.37 near 1 approximate to 240 degrees (intercloud) to 2.1 (cloud + intercloud) in two structures correlated with Gould's belt.