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

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

Abstract

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

* 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 130.79.128.5

Send offprint requests to: J. Bergeat

SIMBAD Objects

Contents

• 1. Introduction
• 2. The selection of multicolour photometry
  • 2.1. The short wavelength range
  • 2.2. The red and near infrared photometry
  • 2.3. The far infrared photometry
• 3. A new classification scheme for intrinsic spectral distributions
  • 3.1. A galactic latitude versus colour index diagram
  • 3.2. An optimized colour-colour diagram
  • 3.3. A grid of unreddened spectral energy distributions
• 4. The new pair method for carbon stars
  • 4.1. The evaluation of the extinction
  • 4.2. A study of the residues
  • 4.3. The proportionality factors of dereddened spectral distributions
• 5. The results
  • 5.1. Unreddened carbon variables
  • 5.2. A comparison of obtained extinctions to values from the field method
  • 5.3. The galactic distribution of reddening material
• 6. Conclusion
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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