The occurrence of classical Cepheids in binary systems

TL;DR

This study uses population synthesis models to analyze the binary nature of classical Cepheids, revealing that most binary Cepheids have orbital periods longer than one year and that binary interactions influence their evolution.

Contribution

The paper introduces population synthesis models that match observed properties of binary Cepheids and explains the impact of binary interactions on Cepheid evolution.

Findings

Most binary Cepheids have orbital periods longer than one year.

Binary interactions on the red giant branch prevent some stars from becoming Cepheids.

The binary fraction of Cepheids is lower than that of their main-sequence progenitors.

Abstract

Classical Cepheids, like binary stars, are laboratories for stellar evolution and Cepheids in binary systems are especially powerful ones. About one-third of Galactic Cepheids are known to have companions and Cepheids in eclipsing binary systems have recently been discovered in the Large Magellanic Cloud. However, there are no known Galactic binary Cepheids with orbital periods less than one year. We compute population synthesis models of binary Cepheids to compare to the observed period and eccentricity distributions of Galactic Cepheids as well as to the number of observed eclipsing binary Cepheids in the LMC. We find that our population synthesis models are consistent with observed binary properties of Cepheids. Furthermore, we show that binary interaction on the red giant branch prevents some red giant stars from becoming classical Cepheids. Such interactions suggest that the binary fraction of Cepheids should be significantly less than that of their main-sequence progenitors, and that almost all binary Cepheids have orbital periods longer than one year. If the Galactic Cepheid spectroscopic binary fraction is about 35%, then the spectroscopic binary fraction of their intermediate mass main sequence progenitors is about 40-45%.