We have determined effective temperature estimates for 165 Miras of the Galaxy using TiO and VO molecular band strengths as temperature indicators. We derived a clear although scattered period-temperature relation, which agrees well with the relation determined by Glass and Feast (1982).
From this period-temperature relation and with the help of a theoretical evolutionary track on the AGB, it is possible to obtain a period-luminosity relation similar to the one determined by Feast et al. (1989) for the Miras of the Large Magellanic Cloud, with the hypothesis that the scatter of the PT relation is due to a distribution in mass. Mass range and absolute bolometric magnitudes are then determined. According to the results of Whitelock et al. (1994), Miras obey to a single period-luminosity relation whatever their metallicity. If this is not the case, the possible effect of metal abundance on mass range determination must be emphasized.
From Lockwood's observed apparent magnitudes, distances are proposed for the sample of Miras. We rely on the trigonometric parallax of R Leo to determine a metallicity correction to the PL relation. We compare the distances with those obtained by Jura et al. (1992, 1993) and Luri et al. (1996b), the latter authors using a completely different method. Our estimates are smaller than the distance of Jura et al. due to different metallicity corrections. However, the very small dispersion in the relation (, ) is remarkable and enhances the necessity to increase the number of good trigonometric parallaxes to improve the distance calibration. The agreement with the second estimates is good for Miras classified as thick disk stars. The discrepancy for halo stars might confirm that Miras of different metallicities may follow different PL relations. Hipparcos will provide good trigonometric parallaxes for some of these Miras, which will enable improvements of the calibration.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998