Did Kepler-444 have a long-lived convective core?

Abstract

ABSTRACT With the greater power to infer the state of stellar interiors provided by asteroseismology, it has become possible to study the survival of initially convective cores within stars during their main-sequence evolution. Standard theories of stellar evolution predict that convective cores in subsolar mass stars have lifetimes below $1\, {\rm Gyr}$. However, a recent asteroseismic study of the star Kepler-444 concluded that the initial convective core had survived for nearly $8\, {\rm Gyr}$. The goal of this paper is to study the convective-core evolution of Kepler-444 and to investigate its proposed longevity. We modify the input physics of stellar models to induce longer convective-core lifetimes and vary the associated parameter across a dense grid of evolutionary tracks. The observations of metallicity, effective temperature, mean density, and asteroseismic frequency ratios are fitted to the models using the basta pipeline. We explore different choices of constraints, from which a long convective-core lifetime is only recovered for a few specific combinations: mainly from the inclusion of potentially unreliable frequencies and/or excluding the covariances between the frequency ratios, whereas for the classical parameters, the derived luminosity has the largest influence. For all choices of observables, our analysis reliably constrains the convective-core lifetime of Kepler-444 to be short, with a median around $0.6\, {\rm Gyr}$ and a 1σ upper bound around $3.5\, {\rm Gyr}$.