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

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 $Δν= 43.32 \pm 3.91 μ$Hz, $ν_{\rm max} = 708.74^{+3.23}{-3.74} μ$Hz for HD81809 A, and $Δν= 97.75 \pm 4.49~μ$Hz, $ν{\rm max} = 2098.07^{+3.07}{-2.83} μ$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.

💡 Research Summary

HD 81809 (HIP 46404, TIC 46802551, HR 3750) is a nearby visual binary composed of two solar‑type stars separated by 0.487″ and orbiting with a period of ~34.5 yr at a distance of 30.9 pc (Gaia DR3 parallax π = 32.3 mas). The authors combine high‑resolution spectroscopy (HERMES, HIRES, FEROS, ELODIE), Gaia astrometry, and multi‑sector TESS photometry to obtain a self‑consistent set of fundamental parameters for both components, resolve previous contradictions in the literature, and place the system in a coherent evolutionary context.

Orbital solution and dynamical masses
Radial velocities (RVs) of both stars were extracted by fitting double‑Gaussian profiles to the cross‑correlation functions of 12 HERMES spectra (R≈85 000) and supplemented with archival RVs spanning more than two decades. Simultaneous fitting of the spectroscopic RVs and the visual orbit (semi‑major axis α = 0.4089″, eccentricity e = 0.366, inclination i ≈ 85°) using an MCMC approach yields dynamical masses M_A = 0.92 ± 0.09 M☉ and M_B = 0.79 ± 0.06 M☉. The orbital parallax (π = 32.3 ± 0.8 mas) agrees with the Gaia value, confirming the distance.

Atmospheric parameters and chemical composition
A high‑S/N (≈500) averaged HERMES spectrum was used to determine effective temperatures via line‑depth ratios (T_eff ≈ 5580 K for A, 5520 K for B), surface gravities from Fe I/Fe II ionisation balance (log g ≈ 3.8 and 4.2, respectively), microturbulence, and projected rotation (v sin i ≈ 4 km s⁻¹ for both). Detailed abundance analysis of 27 elements reveals a striking metallicity dichotomy: the primary is metal‑poor with