Anomalously low-mass core-He-burning star in NGC 6819 as a post-common-envelope phase product

¹ Department of Physics & Astronomy “Augusto Righi”, University of Bologna, via Gobetti 93/2, 40129 Bologna, Italy
² INAF-Astrophysics and Space Science Observatory of Bologna, via Gobetti 93/3, 40129 Bologna, Italy
³ Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, UK

^⋆ Corresponding author; massimilia.matteuzz2@unibo.it

Received: 12 June 2024
Accepted: 14 August 2024

Abstract

Precise masses of red giant stars enable a robust inference of their ages, but there are cases where these age estimates are very precise but also very inaccurate. Examples are core-helium-burning (CHeB) stars that have lost more mass than predicted by standard single-star evolutionary models. Members of star clusters in the Kepler database represent a unique opportunity to identify such stars because they combine exquisite asteroseismic constraints with independent age information (members of a star cluster share a similar age and chemical composition). We focus on the single metal-rich (Z ≈ Z_⊙) Li-rich low-mass CHeB star KIC4937011, which is a member of the open cluster NGC 6819 (turn-off mass of ≈1.6 M_⊙, i.e. an age of ≈2.4 Gyr). This star has a lower mass by ≈1 M_⊙ than expected for its age and metallicity, which might be explained by binary interactions or mass loss along the red giant branch (RGB). To infer formation scenarios for this object, we performed a Bayesian analysis by combining the binary stellar evolutionary framework BINARY_C V2.2.3 with the dynamic nested-sampling approach contained in the DYNESTY V2.1.1 package. We find that this star probably is the result of a common-envelope evolution (CEE) phase during the RGB stage of the primary star in which the low-mass (< 0.71 M_⊙) main-sequence companion does not survive. The mass of the primary star at the zero-age main sequence is in the range [1.46, 1.71] M_⊙, with a log-orbital period in the range [0.06, 2.4] log₁₀(days). During the CEE phase, ≈1 M_⊙ of material is ejected from the system, and the final star reaches the CHeB stage after helium flashes as if it were a single star with a mass of ≈0.7 M_⊙, which is what we observe today. Although the proposed scenario is consistent with photometric and spectroscopic observations, a quantitative comparison with detailed stellar evolution calculations is needed to quantify the systematic skewness of the radius, luminosity, and effective temperature distributions towards higher values than observations.