Period-Luminosity-Colour distribution and classification of galactic oxygen-rich LPVs

The kinematic and Period-Luminosity-Colour distribution of O-rich Long-Period Variable (LPV) stars of the solar neighbourhood is interpreted in terms of pulsation modes, masses and metallicities. It is first shown that, because of input physics imperfections, the periods and mean colours derived from the existing linear and nonlinear nonadiabatic models must significantly depart from the actual behaviour of the stars. As a consequence systematic corrections have to be applied, as a first approximation, to our linear model grid. These free parameters, as well as the mixing length, are calibrated on the LPVs of the LMC and of some globular clusters, assuming a mean mass of $1 M_\odot$ for the LMC Mira-like stars. Then, the masses and metallicities corresponding to the four kinematic/photometric populations of local LPVs are evaluated. The possibility of a varying mixing-length parameter is discussed and taken into account. Stars of the old disk appear pulsating in the fundamental mode: one group, mainly composed of Miras, has mean mass $ \simeq 0.9 M_\odot$ and mean metallicity $ \simeq 0.02$, both strongly increasing with the period; a second group, slightly older and mainly composed of SRb's, has $ \simeq 0.9 M_\odot$ and $ \ga 0.03$ . Stars of the thin disk appear pulsating in the first and second overtones, with $ \approx 1.05 M_\odot$, $ > 0.75 M_\odot$ and $ \ge 0.04$ . Stars of the extended disk/halo appear pulsating in the fundamental mode, with $ \simeq 1.1 M_\odot$ and $ \simeq 0.01$. The mixing-length parameter probably decreases along the AGB by no more than 15% per magnitude. The large, positive period corrections (more than 30% for the fundamental and 8% for the first overtone) that have to be applied to the LNA models used in this study do not seem to be explained by imperfect sub-photospheric physics alone, especially when nonlinear effects are taken into account. The origin of the extra period increase (at least 15% for the fundamental mode) may be the stellar wind, which was neglected by all pulsation codes up to now