Bump Cepheids in the Magellanic Clouds: Metallicities, the Distances to the LMC and SMC, and the Pulsation-evolution Mass Discrepancy

Abstract

We use nonlinear pulsation models to reproduce the observed light and color curves for two samples of bump Cepheid variables, 19 from the Large Magellanic Cloud and 9 from the Small Magellanic Cloud. This analysis determines the fundamental parameters mass, luminosity, effective temperature, metallicity, distance, and reddening for the sample of stars. The use of the light-curve shape alone to determine metallicity is a new modeling technique introduced here. The metallicity, distance, and reddening distributions for the two samples are in agreement with those of similar stellar populations in the literature. The distance modulus of the Large Magellanic Cloud is determined to be 18.54 ±0.018, and the distance modulus of the Small Magellanic Cloud is determined to be 18.93 ± 0.024. The mean Cepheid metallicities are Z = 0.0091 ± 0.0007 and 0.0050 ± 0.0005 for the LMC and SMC, respectively. The masses derived from pulsation analysis are significantly less than those predicted by stellar evolutionary models with no or mild convective core overshoot. We show that this discrepancy cannot be accounted for by uncertainties in our input opacities or in mass-loss physics. We interpret the observed mass discrepancy in terms of enhanced internal mixing in the vicinity of the convective core during the main-sequence lifetime and find that the overshoot parameter Λc rises from 0.688 ± 0.009Hp at the mean LMC metallicity to 0.746 ± 0.009Hp in the SMC. Show more