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

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2. The absolute magnitudes

The near-infrared absolute magnitudes [FORMULA] are often used in PL-diagrams. Amplitudes smaller than observed in the visible and low corrections for interstellar extinction are also good reasons to make this choice. In addition, it has been argued that the possible effect of metallicity would be smaller on the [FORMULA] relation than it is on its [FORMULA] counterpart (Groenewegen & Whitelock 1996). Here, we restrict ourselves to the former one. The apparent K-magnitudes corrected for interstellar extinction were taken from the data-base of Knapik & Bergeat (1997). The average of maximum and minimum magnitudes can be used for Miras (Feast et al. 1989). For semiregular variables (SRs), mean values were estimated from the available data, the error on the distance modulus prevailing in this latter case. The data on 115 stars to be used hereafter, is given in Table 1 (available only in electronic form at the CDS). The entries are the numbers in Stephenson's catalogue (1989), the period in days, the absolute magnitude [FORMULA] and y as defined in Sect. 4, the estimated true parallax [FORMULA] in mas and the difference [FORMULA] with the observed parallax. They were derived from a model of the parallax distribution of HIPPARCOS carbon stars (Knapik et al. 1997). The effect of the space distribution (part of the LK effect) is thus statistically corrected for. Even if the trigonometric parallaxes of the HIPPARCOS catalogue (ESA,1997) are distributed according to a normal gaussian law, this is not the case of the absolute magnitudes or distances (e.g. Smith & Eichhorn 1996). Due to this remaining bias, the y-quantity of Eq. (1) in Sect. 4 is used instead.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998