Carbon-Rich Mira Variables: Kinematics and Absolute Magnitudes

The kinematics of galactic C-Miras are discussed on the basis of the bolometric magnitudes and radial velocities of Papers I and II of this series. Differential galactic rotation is used to derive a zero-point for the bolometric period-luminosity relation which is in satisfactory agreement with that inferred from the LMC C-Miras. We find for the galactic Miras, Mbol = -2.54logP + 2.06 (+/- 0.24), where the slope is taken from the LMC. The mean velocity dispersion, together with the data of Nordstroem et al. and the Padova models, leads to a mean age for our sample of C-Miras of 1.8 +/- 0.4 Gyr and a mean initial mass of 1.8 +/-0.2 solar masses. Evidence for a variation of velocity dispersion with period is found, indicating a dependence of period on age and initial mass, the longer period stars being younger. We discuss the relation between the O- and C-Miras and also their relative numbers in different systems.Comment: 8 pages, 2 figures, accepted for publication in MNRA

Current Uncertainties in the Use of Cepheids as Distance Indicators

The methods of calibrating the luminosities of galactic Cepheids and determining Cepheid reddenings are considered in some detail. Together with work on NGC4258 this suggests that the calibration presented is valid to about 0.1mag (s.e.) at least for Cepheids with near-solar abundances. Metallicity effects are considered, partly through the use of non-Cepheid moduli of the LMC. To reduce the uncertainty substantially below 0.1mag will require extensive work on metallicity effects. Non-linearities in the period-luminosity and period-colour relations will also need to be considered as will the need to distinguish unambiguously between fundamental and overtone pulsators.Comment: 26 pages, no figs, to appear in "Stellar Candles for the Extragalactic Distance Scale" Lecture Notes in Physics (Springer) ed. D. Alloin and W. Giere