Mira variables are useful distance indicators , due to their high luminosities and well-defined period-luminosity relation .
We select 1863 Miras from SAAO and MACHO observations to examine their use as distance estimators in the Milky Way .
We measure a distance to the Galactic centre of R _ { 0 } = 7.9 \pm 0.3 kpc , which is in good agreement with other literature values .
The uncertainty has two components of \sim 0.2 kpc each : the first is from our analysis and predominantly due to interstellar extinction , the second is due to zero-point uncertainties extrinsic to our investigation , such as the distance to the Large Magellanic Cloud ( LMC ) .
In an attempt to improve existing period-luminosity calibrations , we use theoretical models of Miras to determine the dependence of the period-luminosity relation on age , metallicity , and helium abundance , under the assumption that Miras trace the bulk stellar population .
We find that at a fixed period of \log P = 2.4 , changes in the predicted K _ { s } magnitudes can be approximated by \Delta M _ { Ks } \approx - 0.109 ( \Delta [ Fe / H ] ) +0.033 ( { \Delta } t / Gyr ) +0.021 ( % { \Delta } Y / 0.01 ) , and these coefficients are nearly independent of period .
The expected overestimate in the Galactic centre distance from using an LMC-calibrated relation is \sim 0.3 kpc .
This prediction is not validated by our analysis ; a few possible reasons are discussed .
We separately show that while the predicted color-color diagrams of solar-neighbourhood Miras work well in the near-infrared , though there are offsets from the model predictions in the optical and mid-infrared .