Period–Luminosity–Colour distribution and classification of Galactic oxygen–rich LPVs

Abstract

The absolute K magnitudes and kinematic parameters of about 350 oxygen{rich Long{Period Variable stars are calibrated, by means of an up{to{ date maximum{likelihood method, using Hipparcos parallaxes and proper motions together with radial velocities and, as additional data, periods and V K colour indices. Four groups, diering by their kinematics and mean magnitudes, are found. For each of them, we also obtain the distributions of magnitude, period and de-reddened colour of the base population, as well as de-biased period{luminosity{colour relations and their two{dimensional projections. The SRa semiregulars do not seem to constitute a separate class of LPVs. The SRb appear to belong to two populations of dierent ages. In a PL diagram, they constitute two evolutionary sequences towards the Mira stage. The Miras of the disk appear to pulsate on a lower{order mode. The slopes of their de-biased PL and PC relations are found to be very dierent from the ones of the Oxygen Miras of the LMC. This suggests that a signicant number of so{called Miras of the LMC are misclassied. This also suggests that the Miras of the LMC do not constitute a homogeneous group, but include a signicant proportion of metal{decient stars, suggesting a relatively smooth star formation history. As a consequence, one may not trivially transpose the LMC period{luminosity relation from one galaxy to the other 1 . ? Based on data from the Hipparcos astrometry satellite. 1 Appendix B is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html