Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance

We have applied the infrared surface brightness (ISB) technique to derive distances to thirteen Cepheid variables in the LMC which span a period range from 3 to 42 days. From the absolute magnitudes of the variables calculated from these distances, we find that the LMC Cepheids define tight period-luminosity relations in the V, I, W, J and K bands which agree exceedingly well with the corresponding Galactic PL relations derived from the same technique, and are significantly steeper than the LMC PL relations in these bands observed by the OGLE-II Project in V, I and W, and by Persson et al. in J and K. We find that the LMC Cepheid distance moduli we derive, after correcting them for the tilt of the LMC bar, depend significantly on the period of the stars, in the sense that the shortest-period Cepheids have distance moduli near 18.3, whereas the longestperiod Cepheids are found to lie near 18.6. Since such a period dependence of the tilt-corrected LMC distance moduli should not exist, there must be a systematic, period-dependent error in the ISB technique not discovered in previous work. We identify as the most likely culprit the p-factor which is used to convert the observed Cepheid radial velocities into their pulsational velocities. By demanding i) a zero slope on the distance modulus vs. period diagram, and ii) a zero mean difference between the ISB and ZAMS-fitting distance moduli of a sample of well established Galactic cluster Cepheids, we find that p=1.58 (± 0.02) 0.15 (± 0.05) log P, with the p-factor depending more strongly on Cepheid period (and thus luminosity) than indicated by past theoretical calculations. When we recalculate the distances of the LMC Cepheids with the revised p-factor suggested by our data law we not only obtain consistent distance moduli for all stars, but also decrease the slopes in the various LMC PL relations (and particularly in the reddening-independent K and W bands) to values which are consistent with the values observed by OGLE-II and Persson et al. From our 13 Cepheids, we determine the LMC distance modulus to be 18.56 ± 0.04 mag, with an additional estimated systematic uncertainty of ∼0.1 mag. Using the same corrected p-factor law to re-determine the distances of the Galactic Cepheids, the new Galactic PL relations are also found consistent with the observed optical and near-infrared PL relations in the LMC. Our main conclusion from the ISB analysis of the LMC Cepheid sample is that within current uncertainties, there seems to be no significant difference between the slopes of the PL relations in Milky Way and LMC. With literature data on more metal-poor systems, it seems now possible to conclude that the slope of the Cepheid PL relation is independent of metallicity in the broad range in [Fe/H]