On the Mass Discrepancy of the Cepheid Stars

In this paper we examine the problem of the mass discrepancy of Cepheid stars in the young, rich LMC cluster NGC 2157 observed by Mateo and coworkers, which is known to possess three Cepheids. The simultaneous fit of the color-magnitude diagram with theoretical simulations based on models for intermediate-mass stars incorporating either semiconvection or overshoot in the central cores, and with suited chemical composition (the evolutionary masses of the Cepheids are therefore automatically known), and the derivation of the pulsational masses from the Cepheid models of Chiosi & Wood allow us to show that the problem of mass discrepancy likely originates from the adoption of semiconvective models and insufficient accuracy in the determination of the mass by one of the two methods. When this is feasible, as in the ideal laboratory given by the young LMC clusters with Cepheids, the discrepancy no longer exists. Finally, an independent estimate of the distance modulus based on the equality between the evolutionary and pulsational mass of the Cepheid stars is presented. The new method gives a distance modulus in agreement with previous determinations.