The age of an Ancient: systematic study of the Methuselah star/HD140283

Abstract

This master thesis focuses on the accurate determination of fundamental parameters such as mass and age of a very metal-poor subgiant of PopII: HD140283, known as the Methuselah star.

It is one if the oldest star of the Galaxy. Thanks to its peculiar chemical composition, evolutionary stage, proximity and lack of reddening, it is a benchmark for the study of PopII stars.

The age of this ancient star has been a subject of debate among researchers. Bond et al. (2013) and Vandenberg et al. (2014) find ages of 14.46 ±0.31 Gyr and 14.27 ±0.38 Gyr, respectively. More recent work by Creevey et al. (2014) estimate an age of 13.7 ±0.7 Gyr, while Joyce et al. (2018) report a range between 12.5 and 14.9 Gyr. A subsequent study by Tang & Joyce (2022) further revise the age of HD140283 to 12.5 ±0.5 Gyr. Most of these ages are in tension with the age of the Universe precisely measured at 13.77 ±0.06 Gyr.

Thanks to recent efforts, we now have precise observational constraints from spectroscopy and interferometry and a revised astrometric parallax measurement from Gaia mission DR3. Therefore, we may use detailed spectroscopic abundances with a dedicated opacity table (AESOPUS+OP).

We first investigate the impact of physical ingredients on evolutionary sequences computed using the \textit{Code liégeois d'évolution stellaire} (Clés) based on scaled solar composition for HD140283. We test individually different parametrisations of atomic diffusion, macroscopic turbulence, electronic screening, and a change of opacity table.

We then compute six grids of evolutionary models covering a range of mass and metallicity (M from 0.66 to 1.02 solar mass, with a step of 0.01, and Z between 5E-05 and 0.0018 with a step of 2E-04), with a primordial helium mass fraction, Yp=0.251, considering the effect of diffusion and turbulence.

These grids differ by their composition and convective mixing length parameter. We use two compositions, a solar-scaled one and one based on the spectroscopic abundances for HD140283. For each composition, we use three values of the mixing length parameter : a solar-calibrated value and two reduced values (-6.5% and -9%) based on the works of Magic et al. (2015).

We carry out MCMC analyses using emcee python package with the SPInS software, to find optimal parameters reproducing the observational constraints. Starting from these new initial parameters, we test the influence of systematics (diffusion and turbulence, screening and opacity) on our best solution inferred from SPInS.

Obtained results suggest an age for HD140283 of 14.10 ±0.98 Gyr with a solar composition, and 13.08 ±0.85 with a dedicated composition, showing the importance of considering detailed individual abundances.

Our work also suggest that an age not in tension with the age of the Universe is achievable with a slightly lower mass than the ones estimated in previous research.

Mixing length parameter variations have revealed a notable influence on the evolution of HD140283.

Our work also supports the importance of diffusion and turbulent processes on the evolution and internal structure of HD140283, especially close to the base of the convective envelope. Nevertheless, the influence of systematics on the optimal solution is negligible and do not significantly alter the inferred age.

More observational constraints, such as asteroseismic data, would be crucial to refine models and figure out whether an adjustment of the mixing length parameter is needed.

This master thesis opens the way to future studies, particularly in the field of galactic archeology or stellar structure and evolution.