On the contradictory case of the binary system HD 81809 hosting two pulsating solar-like stars observed by TESS

TL;DR

This study provides a detailed analysis of the binary system HD 81809, revealing its stellar parameters, chemical differences, and oscillation properties, and reconstructs its evolutionary history using asteroseismology and stellar models.

Contribution

It offers the first comprehensive characterization of HD 81809's stellar components, including chemical composition, oscillation data, and evolutionary status, highlighting its significance as a benchmark for old, metal-poor stars.

Findings

HD 81809's primary is metal-poor, secondary has solar metallicity.

Both stars exhibit solar-like oscillations with distinct seismic parameters.

The system is approximately 10 billion years old, with a subgiant primary and main sequence secondary.

Abstract

We present a new comprehensive study of HD81809, a nearby binary system of two solar-like stars showing high-amplitude X-ray emission and a well-defined 8-year solar-like magnetic cycle. By analyzing high-resolution spectroscopy, alongside DR3 Gaia astrometry, and bolometric fluxes, we derive updated fundamental parameters for both components. In particular, we uncover a significant chemical difference: the primary is metal-poor ([Fe/H]$ \simeq - 0.57$), while the secondary shows solar-like metallicity ([Fe/H]$=0.00$). This suggests that the system originated in a mildly metal-poor environment, consistent with the Galactic thick disk population, and that the secondary's surface composition has been altered by a recent accretion event. Using multi-sector TESS photometry, we detected solar-like oscillations in both components, deriving global asteroseismic parameters $\Delta\nu = 43.32 \pm 3.91 \mu$Hz, $\nu_{\rm max} = 708.74^{+3.23}_{-3.74} \mu$Hz for HD81809 A, and $\Delta\nu = 97.75 \pm 4.49~\mu$Hz, $\nu_{\rm max} = 2098.07^{+3.07}_{-2.83} \mu$Hz for HD81809 B. By combining all the observational constraints with stellar evolutionary models computed using CLES and MESA codes, we reconstructed the evolutionary scenario of the system. Our results indicate that HD 81809 is an old system with an age of $\sim 10 \mathrm{Gyr}$, composed of a subgiant primary with mass $\sim 0.87M_{\odot}$ and radius $\sim1.96R_{\odot}$ - likely responsible for the reactivated dynamo cycle - and a main sequence secondary with mass $ M=0.85M_{\odot}$ and radius $R=1.10R_{\odot}$. This system represents a benchmark for studying stellar evolution, magnetic activity, and the physics of old, metal-poor stars in the Galactic thick disk.