Astron. Astrophys. 332, L53-L56 (1998)

Letter to the Editor

The PL relation of galactic carbon LPVs ^{* ** ***}

The distance modulus to LMC

J. Bergeat , A. Knapik and B. Rutily

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 11 July 1997 / Accepted 6 February 1998

Abstract

We present a period-luminosity (PL) diagram of 115 galactic carbon-rich long period variables (LPVs) observed by the HIPPARCOS satellite, in the form of the relation. Our plot is compared to the diagram of carbon variables observed in the Large Magellanic Cloud (LMC). Both diagrams are found very similar and three samples are delineated: long period variables close to the PL relation of Feast et al. (1989), short period-overluminous variables and a few underluminous LPVs, respectively Samples 1, 2 and 3. The used data were deduced from expectations of true parallaxes (Knapik et al. 1997) which are statistically free of the Lutz-Kelker effect. The remaining bias due to the non-gaussian distribution of absolute magnitudes is avoided: a non-linear parametric method is applied in Sect. 4 to the analysis of the PL relation for Sample 1 (72 LPVs). We obtain in good agreement with the slope found for LMC variables by Reid et al. (1995). The LMC distance modulus then derived is A well-defined upper limit (ul) for long period stars in Sample 1 is found, with similar slopes in both the Galaxy (-4.85) and LMC (-4.72). No correction for metallicity was applied to the results.

Key words: stars: AGB and post-AGB stars: carbon stars: fundamental parameters galaxies: distances

* This research has made use of the Simbad database operated at CDS, Strasbourg, France.
** Based on data from the ESA HIPPARCOS astrometry satellite
*** Table 1 is only available in electronic form at the CDS via anonymous ftp 130.79.128.5

Send offprint requests to: J. Bergeat

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Contents

• 1. Introduction
• 2. The absolute magnitudes
• 3. The vs. log P diagram
  • 3.1. The three samples
  • 3.2. Underluminous variables (UVs)
• 4. The PL relation and LMC distance modulus
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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