The PL relation of galactic carbon LPVs. The distance modulus to LMC

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 (M_K,log P) 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 M_K=(-3.99+/-0.13)log P+(2.07+/-0.15), in good agreement with the slope found for LMC variables by Reid et al. (1995). The LMC distance modulus then derived is mu =18.50+/-0.17. 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. This research has made use of the Simbad database operated at CDS, Strasbourg, France.