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

Abstract

Recent investigations aimed at estimating the distance to t he young LMC cluster NGC 1866 have made use of Red Clump stars in the surrounding LMC field, together with em pirical and theoretical Main-Sequence fitting methods, and have found significantly different distances for the field an d the cluster, the latter being closer by in distance modulus by �(DM)�0.20 mag. In this paper we (re-)consider the Cepheid star population of NGC 1866, to try to shed some light on this discrepancy. By combining various extensive photometric datasets in B, V,I 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 determina tion 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 �(DM) ranging between 0.0 and 0.11 mag. The simultaneous comparison of the PL relationships in B, V and I for the cluster and LMC field gives an estimate of the cluster r eddening, which results to be E(B V ) = 0.12 ± 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 esti mates. The adoption of the LMC extinction law recently presented by Gordon et al. (2003) does not change these results. The cluster Main Sequence fitting distance obtained with this new redd ening is DM = 18.58 ± 0.08, fully compatible with the Red Clump value of DM = 18.53 ±0.07(random) +0.02 −0.05 (systematic) and the Cepheid constraint on �(DM). Finally, we determined the distance to the cluster by using a Cepheid Wesenheit PL relationship with slope coming from LMC observations, and absolute magnitude zero point calibrated on Hipparcos parallaxes of Galactic Cepheids, in the assumption that the relationship is independent of metallicity; the resulting DM = 18.65 ± 0.10 is not an accurate estimate of the LMC distance because of possible metallicity effects but, when compared to the revised Main Sequence fitting value, it points out to a possibly weak depen dence of the Wesenheit PL relationship on the Cepheid chemical composition, at least in the period range between 2.5 and 3.5 days.