Pulsation Models for Ultra-Low (Z=0.0004) Metallicity Classical Cepheids

TL;DR

This study models ultra-low metallicity Cepheids to understand their pulsation properties and refine the Period-Luminosity relation at very low chemical compositions, crucial for accurate cosmic distance measurements.

Contribution

It provides the first detailed nonlinear convective models and updated PL relations for Z=0.0004 Cepheids, exploring their pulsation characteristics at extremely low metallicity.

Findings

PL relations are steeper at Z=0.0004 compared to higher Z

PL relations at Z=0.0004 are similar to those at Z=0.004

Metallicity effect on PL relation may level off at very low Z

Abstract

Classical Cepheids are primary distance indicators playing a fundamental role in the calibration of the extragalactic distance scale. The possible dependence of their characteristic Period-Luminosity (PL) relation on chemical composition is still debated in the literature, and the behaviour of these pulsators at very low metallicity regimes is almost unexplored. In order to derive constraints on the application of the Period-Luminosity relation at low metal abundances, we investigate the properties of the few ultra-low metallicity (Z ~ 0.0004) Cepheids recently discovered in the Blue Compact Dwarf galaxy IZw18. To this purpose we have computed an updated and extended set of nonlinear convective models for Z=0.0004 and Y=0.24, spanning a wide range of stellar masses, and taking into account the evolutionary constraints for selected luminosity levels. As a result we are able to predict the topology of the instability strip, the variations of all the relevant quantities along the pulsation cycle, including the morphology of the light curves, the theoretical Period-Luminosity-Color, Period-Wesenheit and Period-Luminosity relations at such a low metallicity. For each of these relations we provide the appropriate coefficients for fundamental mode pulsators with Z=0.0004. By comparing these results with the properties of more metal rich Cepheids we find that the synthetic PL relations for Z=0.0004 are steeper than at higher Z, but similar to the Z=0.004 ones, thus suggesting a leveling off of the metallicity effect towards the lowest Zs.