The strange evolution of the Large Magellanic Cloud Cepheid OGLE-LMC-CEP1812

TL;DR

This paper investigates the evolution of a Cepheid in the OGLE-LMC-CEP1812 system, revealing it is a merger product that explains its apparent age discrepancy with its companion, advancing understanding of Cepheid evolution.

Contribution

It demonstrates that the Cepheid in OGLE-LMC-CEP1812 is a merger product, providing new insights into Cepheid evolution and resolving previous age discrepancy issues.

Findings

The Cepheid is a merger of two main sequence stars.

The merger product has evolved across the Hertzsprung gap.

The age discrepancy with the companion is resolved by the merger scenario.

Abstract

Classical Cepheids are key probes of both stellar astrophysics and cosmology as standard candles and pulsating variable stars. It is important to understand Cepheids in unprecedented detail in preparation for upcoming GAIA, JWST and extremely-large telescope observations. Cepheid eclipsing binary stars are ideal tools for achieving this goal, however there are currently only three known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions about the evolution of classical Cepheids because of an apparent age discrepancy between the Cepheid and its red giant companion. We show that the Cepheid component is actually the product of a stellar merger of two main sequence stars that has since evolved across the Hertzsprung gap of the HR diagram. This post-merger product appears younger than the companion, hence the apparent age discrepancy is resolved. We discuss this idea and consequences for understanding Cepheid evolution.