The Mira-based Distance to the Galactic Center

Abstract

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 center of R 0  = 7.9 ± 0.3 kpc, which is in good agreement with other literature values. The uncertainty has two components of ∼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 , changes in the predicted K s magnitudes can be approximated by  +  , and these coefficients are nearly independent of period. The expected overestimate in the Galactic center distance from using an LMC-calibrated relation is ∼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-neighborhood Miras work well in the near-infrared; though, there are offsets from the model predictions in the optical and mid-infrared.