Multiphase Relations of Magellanic Cloud Cepheids

Abstract

Cepheid variable stars comprise a very important rung in the cosmic distance ladder because of the famous relationship that exists between their luminosities and the durations of their pulsation periods. Previously, the Cepheid Period-Luminosity (PL) relation and the interstellar extinction independent Period-Wesenheit (PW) relation have only been investigated at maximum, minimum and mean light. Here we examine Cepheid luminosities expressed throughout entire cycles of pulsation in two nearby galaxies of differing metallicity, the Small and Large Magellanic Clouds. We confirm that the relationship between the luminosity and the common logarithm of the pulsational period is nonlinear at many phases and show that it varies as a function of metallicity. Properly calibrating these relations will improve cosmic distance measurements and hence give a more exact estimate of the Hubble constant. In turn, that would put strong constraints on the dark energy equation of state and improve calculations of the size and age of the visible universe.