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

TL;DR

This work presents a comprehensive, multi-technique study of the wide binary HD 81809, revealing a chemically dichotomous pair with a metal-poor, $\alpha$-enhanced primary and a near-solar metallicity secondary likely polluted by material from a debris disk. By combining new RVs, a joint orbital solution, detailed atmospheric analyses, bolometric luminosities, and TESS-based asteroseismology, the authors constrain the system's dynamical masses and fundamental parameters, and reconstruct its old, $\sim$10 Gyr, evolution within the Galactic thick disk. Stellar modeling with CLES and MESA indicates a subgiant primary of $M_A\approx0.87$–$0.89\,M_\odot$ and a main-sequence secondary of $M_B\approx0.83$–$0.94\,M_\odot$, though matching all observational constraints is challenged by the surface-pollution scenario and a bolometric luminosity discrepancy for the primary. The detected $P_{cyc}\approx8.2$ yr magnetic cycle in the primary, coupled with a dynamo-rejuvenation explanation in the subgiant through weakened magnetic braking, positions HD 81809 as a key benchmark for stellar evolution, magnetic activity, and the chemistry of old, metal-poor stars in the thick disk.

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 $Δν= 43.32 \pm 3.91 μ$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.

Figures (10)

• Figure 1: The cross-correlation function for one HERMES spectrum of HD 81809 with the double Gaussian fit used to measure the radial velocities of the two stars.
• Figure 2: Parameter correlation plot for parameters of interest given in Table \ref{['tab:orbit']} in the fit of the visual and spectroscopic orbits to radial velocity, position angle and angular separation measurements for HD 81909.
• Figure 3: Left panel: Radial velocity measurements for HD 81809 A (blue points) and HD 81809 B (red points) with the best-fit spectroscopic orbit (solid lines). Symbols denote the source of the radial velocity measurements as follows: cross -- HERMES; diamond -- HIRES; triangle -- FEROS; star -- ELODIE; dots -- SB9. Right panel: Measurements of the relative positions of the two stars with the best-fit astrometric orbit. Points measured by speckle interferometry are plotted with open circles, micrometer measurements are plotted with dots, and the Hipparcos measurement is shown with a cross. Lines connect measurements to the point on the visual orbit corresponding to their time of observation. The point of periastron on the orbit is marked with a filled circle.
• Figure 4: Atmospheric abundances obtained by present analysis for HD 81809 A (black dots) and HD 81809 B (red dots) compared with those of the Sun grevesse2011chemical (dashed line). In the right panel, we show the histogram of the distribution of the abundances around the average with overplotted Gaussian fit, colors as in left panel.
• Figure 5: In the upper panel the SED of HD 81809 is shown. The best-fit SED is plotted as dark blue line. The observed fluxes are plotted as blue points with error bars while predicted fluxes for the best-fit SED integrated over the response functions shown in grey are plotted with open circles. The SEDs of the two stars are also plotted (green -- primary, orange -- secondary). Lower panel: Flux ratio as a function of wavelength for the best-fit SEDs. The observed flux ratios are plotted as points and the predicted flux ratios in the Tycho $B_T$ and $V_T$ bands are plotted as open circles.