On the Contradictory Case of the Binary System HD 81809 Hosting Two Pulsating Solar-like Stars Observed by TESS

Abstract We present a new comprehensive study of HD 81809, a nearby binary system of two solar-like stars showing high-amplitude X-ray emission and a well-defined 8 yr solar-like magnetic cycle. By analyzing high-resolution spectroscopy, alongside Gaia Data Release 3 (DR3) 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] ≃ −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 multisector TESS photometry, we detected solar-like oscillations in both components, deriving global asteroseismic parameters Δ ν = 43.32 ± 3.91 μ Hz and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>708.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.74</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3.23</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> </mml:math> Hz for HD 81809 A, and Δ ν = 97.75 ± 4.49 μ Hz and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2098.0</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.83</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3.07</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> </mml:math> Hz for HD 81809 B. By combining all the observational constraints with stellar evolutionary models computed using the 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 ∼10 Gyr, composed of a subgiant primary with mass M ∼ 0.87 ⊙ and radius R ∼ 1.96 R ⊙ —likely responsible for the reactivated dynamo cycle—and a main-sequence secondary with mass M ∼ 0.85 M ⊙ and radius R ∼ R 1.10 R ⊙ . This system represents a benchmark for studying stellar evolution, magnetic activity, and the physics of old, metal-poor stars in the Galactic thick disk.