The distance to the LMC cluster NGC 1866; clues from the cluster Cepheid population

Abstract

(abridged) Recent investigations aimed at estimating the distance to the young LMC cluster NGC1866 have made use of Red Clump stars in the surrounding LMC field, together with empirical and theoretical Main-Sequence fitting methods, and have found significantly different distances for the field and the cluster, the latter being closer by in distance modulus by Δ(DM) \sim 0.20 mag. In this paper we (re-)consider the Cepheid star population of NGC1866, to try to shed some light on this discrepancy. By combining various extensive photometric datasets in BVI and single-epoch 2MASS JHK photometry, PL relationships for the cluster Cepheids are obtained. A comparison between the field LMC and cluster PL relationships for the reddening free Wesenheit index gives a firm determination of the distance between the cluster and the LMC main body (0.04 mag in distance modulus, the cluster being more distant) which, coupled to a model for the geometry of the LMC disk, provides Δ(DM) ranging between 0.0 and -0.11 mag. The simultaneous comparison of the PL relationships in BVI for the cluster and LMC field gives an estimate of the cluster reddening, which results to be E(B-V) = 0.12 \pm 0.02. This determination is higher than the canonical value of 0.06 mag used in all previous studies, but we show that it is not in contradiction with a re-analysis of independent estimates. The cluster Main Sequence fitting distance obtained with this new reddening is DM = 18.58 \pm 0.08, fully compatible with the Red Clump value of DM = 18.53 \pm 0.07(random)^{+0.02}_{-0.05} (systematic) and the Cepheid constraint on Δ(DM).