No abstract

Near‐infrared, JHKL, photometry of 239 Galactic C‐rich variable stars is presented and discussed. From these and published data, the stars were classified as Mira or non‐Mira variables, and amplitudes and pulsation periods, ranging from 222 to 948 d for the Miras, were determined for most of them. A comparison of the colour and period relations with those of similar stars in the Large Magellanic Cloud indicates minor differences, which may be the consequence of sample selection effects. Apparent bolometric magnitudes were determined by combining the mean JHKL fluxes with mid‐infrared photometry from IRAS and MSX. Then, using the Mira period luminosity relation to set the absolute magnitudes, distances were determined – to greater accuracy than has hitherto been possible for this type of star. Bolometric corrections to the K magnitude were calculated and prescriptions derived for calculating these from various colours. Mass‐loss rates were also calculated and compared to values in the literature. Approximately one‐third of the C‐rich Miras and an unknown fraction of the non‐Miras exhibit apparently random obscuration events that are reminiscent of the phenomena exhibited by the hydrogen‐deficient R Coronae Borealis stars. The underlying cause of this is unclear, but it may be that mass loss, and consequently dust formation, is very easily triggered from these very extended atmospheres.

Published data for large‐amplitude asymptotic giant branch variables in the Large Magellanic Cloud (LMC) are re‐analysed to establish the constants for an infrared (K) period–luminosity relation of the form MK=ρ[log P− 2.38]+δ. A slope of ρ=−3.51 ± 0.20 and a zero‐point of δ=−7.15 ± 0.06 are found for oxygen‐rich Miras (if a distance modulus of 18.39 ± 0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero‐point for these stars using the revised Hipparcos parallaxes together with published very long baseline interferometry (VLBI) parallaxes for OH masers and Miras in globular clusters. These result in a mean zero‐point of δ=−7.25 ± 0.07 for O‐rich Galactic Miras. The zero‐point for Miras in the Galactic bulge is not significantly different from this value. Carbon‐rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period–luminosity relation zero‐points for systems with different metallicity.