1,825 research outputs found
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
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