THE DISTANCE TO THE LARGE MAGELLANIC CLOUD - CONSTRAINTS FROM CEPHEIDS IN LARGE MAGELLANIC CLOUD STAR-CLUSTERS

We have used recent observational data for the Cepheids in the rich, young Large Magellanic Cloud clusters NGC 1866 and NGC 2031 to constrain the cluster distances with the mass-equivalency (ME) method. The basis of this approach is to fix the cluster distances by requiring the Cepheid evolutionary and pulsational masses to be equal. Using evolutionary models incorporating a mild amount of core and envelope over-shooting along with recent pulsational models, we derive distance moduli of 18.51+/-0.21 and 18.32+/-0.20 for NGC 1866 and NGC 2031, respectively. The quoted errors are dominated by the uncertainties in the heavy element abundances of the clusters (assumed to be 0.3 dex for both clusters), with a smaller contribution due to the apparently intrinsic spread in the masses of the Cepheids in each cluster. For the ME method, we find that DELTA(m - M)0/DELTAZ1 = 0.69, where Z1 = log (Z/0.016). This result implies that the cluster distances can be determined to better than +/-5% if the cluster abundances can be measured to better than about +/-0.15 dex. The distance moduli we derive for NGC 1866 and NGC 2031 are consistent with other recent values derived for the LMC, meaning that the models used in this analysis avoid the classical evolutionary/pulsational Cepheid mass discrepancy. The results of our analysis are based on models using the Los Alamos opacities; had we used models incorporating the new Livermore opacities instead, we estimate that the cluster distance moduli would be larger by at most 0.1 mag.