Revisiting Kepler-444

Abstract

Context. The CoRoT and Kepler missions have paved the way for synergies between exoplanetology and asteroseismology. The use of seismic data helps providing stringent constraints on the stellar properties which directly impact the results of planetary studies. Amongst the most interesting planetary systems discovered by Kepler, Kepler-444 is unique by the quality of its seismic and classical stellar constraints. Its magnitude, age and the presence of 5 small-sized planets orbiting this target makes it an exceptional testbed for exoplanetology. Aims. We aim at providing a detailed characterization of Kepler-444, focusing on the dependency of the results on variations of key ingredients of the theoretical stellar models. This thorough study will serve as a basis for future investigations of the planetary evolution of the system orbiting Kepler-444. Methods. We use local and global minimization techniques to study the internal structure of the exoplanet-host star Kepler-444. We combine seismic observations from the Kepler mission, Gaia DR2 data, and revised spectroscopic parameters to precisely constrain its internal structure and evolution. Results. We provide updated robust and precise determinations of the fundamental parameters of Kepler-444 and demonstrate that this low-mass star bore a convective core during a significant portion of its life on the main sequence. Using seismic data, we are able to estimate the lifetime of the convective core to approximately 8 Gyr out of the 11 Gyr of the evolution of Kepler-444. The revised stellar parameters found by our thorough study are M = 0.754 ± 0.03 M⊙, R = 0.753 ± 0.01 R⊙, and Age = 11 ± 1 Gyr.

Probing the interior physics of stars through asteroseismology

“…Inversion methods have led to an evaluation of the interior structure for 16 Cyg A and B, revealing discrepancies between the sound speed in the cores of these two stars with respect to those in the 1D models at the level of ∼5% (Bellinger et al, 2017), although this binary-exoplanet system is the best calibrated solar analog (Davies et al, 2015). Inversions applied to the exoplanet host star Kepler-444 (also known as KOI-3158) resulted in high-precision mass and radius estimates of M ⋆ ¼ ð0.75 AE 0.03ÞM ⊙ , R ⋆ ¼ ð0.75 AE 0.01ÞR ⊙ and revealed that this star must have had a convective core during the first 8 Gyr of its 11-Gyr lifetime (Buldgen et al, 2019).…”

Asteroseismology of $β$ Cephei stars: The stellar inferences tested in hare and hound exercises

“…As our method enables us to refine the region of the parameter space enclosing the solution, we could also use it as an exploratory method for providing initial guesses of a local method, such as the Levenberg-Marquardt algorithm (a two-step approach which was already followed for red-giants, e.g. Buldgen et al 2019 or solar-like pulsators, e.g. Farnir et al 2020).…”

Coralie radial-velocity search for companions around evolved stars (CASCADES) III. A new Jupiter host-star: in-depth analysis of HD 29399 using TESS data

“…The final modelling step is carried out using a Levenberg-Marquardt algorithm (used e.g. in Buldgen et al 2016;Farnir et al 2019;Buldgen et al 2019a) with the following set of constraints: ρInv , the inverted mean density, L, T eff , [Fe/H], and the individual small frequency separations, denoted d 0,2 = ν n,0 − ν n−1,2 , using the mass, M, the age, t, the mixing-length parameter, α MLT , and the initial hydrogen, X 0 , and metal content, Z 0 , as free parameters.…”