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

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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


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 [FORMULA] 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 [FORMULA] in good agreement with the slope found for LMC variables by Reid et al. (1995). The LMC distance modulus then derived is [FORMULA] 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

Send offprint requests to: J. Bergeat

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998