The age of the Methuselah star in the light of stellar evolution models with tailored abundances

Abstract

Context. HD140283, or the Methuselah star, is a well-known reference object in stellar evolution. Its peculiar chemical composition, proximity and absence of reddening makes it an interesting case-study of Pop II stars. Thanks to recent observational efforts, we now have precise interferometric and spectroscopic constraints, as well as revised astrometric parallaxes from the Gaia mission.Aims. We determine the age of HD140283 with these latest constraints, and we quantify the impact of systematics from physical inaccuracies on the stellar evolution models.Methods. Using recent spectroscopic abundances from the literature, including 3D non-local thermal equilibrium values for C, O, and Fe, we computed opacity tables specific to HD140283. We then used them in grids of stellar evolution models coupled to a Markov chain Monte Carlo tool to determine the age of HD140283.Results. With our tailored models we found an age of 12.3 Gy. When we instead used a solar-scaled mixture, we found an age value of 14 Gy, which contradicts theories about the age of the Universe (13.77 ± 0.06 Gy). We also found that a reduction of the mixing-length parameter from its solar calibrated value leads to an even younger age, and this agrees with other recent studies. However, we found no direct evidence that would favour a lower value of the mixing-length parameter based on our modelling.Conclusions. It is crucial to take the specific elemental abundances into account to model HD140283 because this leads to significant differences in the inferred age. However, this effect is degenerate with a decreasing mixing-length parameter. In this respect, asteroseismic constraints might play a key role in accurately deriving the mass of HD140283, and therefore, in strongly constraining its age.