THE DISTANCE TO THE LARGE MAGELLANIC CLOUD CLUSTER NGC 1866 AND THE SURROUNDING FIELD

摘要

We use the main-sequence stars in the LMC cluster NGC 1866 and red clump stars in the local field to obtain two independent estimates of the LMC distance. We apply an empirical main-sequence-fitting technique based on a large sample of subdwarfs with accurate Hipparcos parallaxes in order to estimate the cluster distance modulus, and the multicolor red clump method to derive distance and reddening of the LMC field. We find that the main-sequence fitting and the red clump distance moduli are in significant disagreement; the NGC 1866 distance modulus is equal to (m ― M)~(0,NGC 1866) = 18.33 +- 0.08 (consistent with a previous estimate using the same data and theoretical main-sequence isochrones), while the field stars provide (m ― M)_(0,field) = 18.53 +- 0.07. This difference reflects the more general dichotomy in the LMC distance estimates found in the literature. Various possible causes for this disagreement are explored, with particular attention paid to the still uncertain metallicity of the cluster and the star formation history of the field stars.