arXiv subjects

Nuno Moedas

Publications and source records attributed to Nuno Moedas.

9 recordsLinked to original sources

arXiv2026

Chemical paradox in a binary system: Exploring metal enrichment in HD 81809B

The HD 81809 system presents peculiar chemical composition with a large metallicity difference between its two components: the primary has low metallicity ([Fe/H]=-0.57 dex), while the secondary has approximately solar metallicity ([Fe/H]=0.0 dex). This study investigates whether the chemical enrichment of HD 81809B can be reconciled by a planetary engulfment event, consistent with the star's evolutionary constraints. Using Modules for Experiments in Stellar Astrophysics (MESA) code, we model HD 81809B introducing accretion events with different ranges of masses and chemical mixtures to understand their impact in the surface chemical composition. Our simulation shows that reproducing the observed surface [Fe/H] in HD 81809B requires a substantial accretion event. To reach the observed metallicity, the star must engulf 25 to 75 $M_\oplus$ of metals near its current age. Accretion brings the effective temperature of stellar models closer to the observed one; however, the lithium results over-enriched. In order to reproduce the abundance of this element, the star must accrete less than 6 $M_\oplus$. These results suggest that a planetary accretion event is plausible in the evolution of HD 81809B; however, the chemical composition of the accreted material may differ from the assumptions adopted here. Nevertheless, this study emphasizes the importance of considering such external events when modeling chemically anomalous binary systems.

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arXiv2026

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 $\Delta\nu = 43.32 \pm 3.91 \mu$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.

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arXiv2026

Revisiting the exoplanet radius valley with host stars from SWEET-Cat

The radius valley,a deficit of planets near 2 $\mathrm{R_{\oplus}}$, was observed among exoplanets of radius $\lesssim$ 5 $\mathrm{R_{\oplus}}$ with periods $<$ 100 days by NASA's $Kepler$ mission. It separates super-Earths (rocky, $\lesssim 1.9$ $\mathrm{R_{\oplus}}$) from sub-Neptunes (volatile-rich, $\gtrsim 2$ $\mathrm{R_{\oplus}}$) and may arise from formation conditions or atmospheric loss. Disentangling these mechanisms has led to numerous studies of population-level trends, although the resulting interpretations remain sensitive to sample selection and the robustness of host-star parameters. We re-examine its existence, depth, and dependence on period, flux, stellar mass, and age. Using SWEET-Cat and MAISTEP tool, we derived stellar parameters for 1,221 main-sequence stars (1,405 planets), with effective temperatures 4400--7500 K and radii 0.62--2.75 $\mathrm{R_{\odot}}$, achieving 2\% precision in radius and mass. Planetary radii were recomputed from radius ratios, yielding 5\% median uncertainty. The valley is partially filled near 2 $\mathrm{R_{\oplus}}$ and depends on period, flux, and stellar mass, with slopes $-0.12^{+0.02}_{-0.01}$, $0.10^{+0.02}_{-0.03}$, and $0.19^{+0.09}_{-0.07}$. Sub-Neptunes show a stronger stellar mass-dependent trend than super-Earths ($0.17^{+0.04}_{-0.04}$ vs $0.11^{+0.05}_{-0.05}$). With stellar age, the super-Earth/sub-Neptune ratio rises from $0.51^{+0.11}_{-0.08}$ ($<3$ Gyr) to $0.64^{+0.11}_{-0.11}$ ($\gtrsim3$ Gyr), and the valley becomes shallower and shifts to larger radii. A 4D fit shows consistent slopes with 2D analyses and a weaker age trend ($0.07^{+0.03}_{-0.04}$). These results suggest prolonged atmospheric loss, which is consistent with a core-powered mass loss scenario and emphasize the need for improved determinations, a goal expected to be achieved by future missions like PLATO.

astro-ph.EP↗

arXiv2026

Studying the surface effect in Procyon A as an F-type star

Procyon A is an F-type main-sequence star in a binary system. It has been the subject of numerous ground-based and space-based observing campaigns, providing precise classical constraints, including a well-determined mass. It was also among the first stars in which individual frequencies were detected, making it a crucial benchmark for F-type stars. Our goal is to investigate the surface effect, namely the discrepancy between observed and model oscillation frequencies due to inadequate modeling of the surface stellar layers, especially important in F-type stars. Using Procyon A as a case study, we aim to understand how different surface correction prescriptions impact the inference of the fundamental properties of this star, and compare the results with those obtained when the surface corrections are neglected. We inferred the fundamental stellar properties employing a grid of models computed with MESA, including gravitational settling, radiative accelerations, and turbulent mixing. We selected the best-fit models using the AIMS code taking into account different methods to fit the individual frequencies. We find that the use of surface corrections can introduce uncertainties up to 7\% in the inferred stellar mass. We identify that the most reliable stellar mass estimates are obtained when using frequency ratios, the Sonoi et al. (2015) surface correction or directly fitting the individual frequencies. Our results indicate that the surface effects in F-type stars differ from those found in the Sun and in solar-like stars, highlighting the need to be careful when considering the surface corrections for these stars.

astro-ph.SR↗

arXiv2025

Impact of radiative accelerations on the stellar characterization of FGK-type stars using spectroscopic and seismic constraints

Chemical transport mechanisms are fundamental processes in stellar evolution models. They are responsible for the chemical distribution, and their impact determines how accurately we can characterize stars. Radiative accelerations are one of these processes. They allow the accumulation of elements at different depths in the star. We aim to assess the impact of radiative accelerations on the modeling of FGK-type stars and their impact on the prediction of surface abundances. To reduce the cost of the computation of radiative accelerations, we implemented the single-valued parameters (SVP) method in the stellar evolution code MESA. The SVP method is more efficient in calculating radiative accelerations, which enables computations of large enough grids of models for stellar characterization. Compared to models that include atomic diffusion (with only gravitational settling), the inclusion of radiative accelerations has a small effect on the inference of fundamental properties, with an impact of 2\%, 0.7\%, and 5\% for mass, radius, and age. However, the treatment of radiative accelerations is necessary to predict the chemical composition of and accurately characterize stars.

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arXiv2024

Characterisation of FG-type stars with an improved transport of chemical elements

Context. The modelling of chemical transport mechanisms is crucial for accurate stellar characterizations. Atomic diffusion is one of these processes and it is commonly included in stellar models. However, it is usually neglected for F-type or more massive stars because it produces surface abundance variations that are unrealistic. Additional mechanisms to counteract atomic diffusion must therefore be considered. It has been demonstrated that turbulent mixing can prevent the surface abundance over-variations, and can also be calibrated to mimic the effects of radiative accelerations on iron. Aims. We aim to evaluate the effect of a calibrated turbulent mixing on the characterisation of a sample of F-type stars, and how the estimates compare with those obtained when the chemical transport mechanisms are neglected. Methods. We selected stars from two samples - one from the Kepler LEGACY sample and the other from a sample of Kepler planet-hosting stars. We inferred their stellar properties using two grids. The first grid considers atomic diffusion only in models that do not show chemical over-variations at the stellar surface. The second grid includes atomic diffusion in all the stellar models and the calibrated turbulent mixing to avoid unrealistic surface abundances. Results. Comparing the derived results from the two grids, we found that the results for the more massive stars in our sample will have higher dispersion in the inferred values of mass, radius and age, due to the absence of atomic diffusion in one of the grids. This can lead to relative uncertainties for individual stars of up to 5% for masses, 2% for radii and 20% for ages. Conclusions. This work shows that a proper modelling of the microscopic transport processes is key for an accurate estimation of their fundamental properties not only for G-type stars, but also for F-type stars.

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arXiv2022

Atomic diffusion and turbulent mixing in solar-like stars: Impact on the fundamental properties of FG-type stars

Chemical composition is an important factor that affects stellar evolution. The element abundance on the stellar surface evolves along the lifetime of the star because of transport processes, including atomic diffusion. However, models of stars with masses higher than about 1.2Msun predict unrealistic variations at the stellar surface. This indicates the need for competing transport processes that are mostly computationally expensive for large grids of stellar models. The purpose of this study is to implement turbulent mixing in stellar models and assess the possibility of reproducing the effect of radiative accelerations with turbulent mixing for elements like iron in order to make the computation of large grids possible. We computed stellar models with MESA and assessed the effects of atomic diffusion (with radiative acceleration) in the presence of turbulent mixing. We parametrised the effect of radiative accelerations on iron with a turbulent diffusion coefficient. Finally, we tested this parametrisation by modelling two F-type stars of the Kepler Legacy sample. We found that, for iron, a parametrisation of turbulent mixing that simulates the effect of radiative acceleration is possible. This leads to an increase in the efficiency of the turbulent mixing to counteract the effect of gravitational settling. This approximation does not affect significantly the surface abundances of the other elements we studied, except for oxygen and calcium. We demonstrate that this parametrisation has a negligible impact on the accuracy of the seismic properties inferred with these models. Moreover, turbulent mixing makes the computation of realistic F-type star models including the effect atomic diffusion possible. This leads to differences of about 10% in the inferred ages compared to results obtained with models that neglect these processes.

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arXiv2020

Asteroseismic modelling of solar-type stars: A deeper look at the treatment of initial helium abundance

Detailed understanding of stellar physics is essential towards a robust determination of stellar properties (e.g. radius, mass, and age). Among the vital input physics used in the modelling of solar-type stars which remain poorly constrained, is the initial helium abundance. To this end, when constructing stellar model grids, the initial helium abundance is estimated either (i) by using the semi-empirical helium-to-heavy element enrichment ratio, (${ΔY}/{ΔZ}$), anchored to the standard Big Bang Nucleosynthesis value or (ii) by setting the initial helium abundance as a free variable. Adopting 35 low-mass, solar-type stars with multi-year Kepler photometry from the asteroseismic "LEGACY" sample, we explore the systematic uncertainties on the inferred stellar parameters (i.e., radius, mass, and age) arising from the treatment of the initial helium abundance in stellar model grids . The stellar masses and radii derived from grids with free initial helium abundance are lower compared to those from grids based on a fixed ${ΔY}/{ΔZ}$ ratio. We find the systematic uncertainties on mean density, radius, mass, and age arising from grids which employ a fixed value of ${ΔY}/{ΔZ}$ and those with free initial helium abundance to be $\sim$ 0.9%, $\sim$ 2%, $\sim$ 5% and $\sim$ 29%, respectively. We report that the systematic uncertainties on the inferred masses and radii arising from the treatment of initial helium abundance in stellar grids lie within the expected accuracy limits of ESA's PLATO, although this is not the case for the age.

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arXiv2020

Asteroseismic stellar modelling: systematics from the treatment of the initial helium abundance

Despite the fact that the initial helium abundance is an essential ingredient in modelling solar-type stars, its abundance in these stars remains a poorly constrained observational property. This is because the effective temperature in these stars is not high enough to allow helium ionization, not allowing any conclusions on its abundance when spectroscopic techniques are employed. To this end, stellar modellers resort to estimating the initial helium abundance via a semi-empirical helium-to-heavy element ratio, anchored to the the standard Big Bang nucleosynthesis value. Depending on the choice of solar composition used in stellar model computations, the helium-to-heavy element ratio, ($ΔY/ΔZ$) is found to vary between 1 and 3. In this study, we use the Kepler "LEGACY" stellar sample, for which precise seismic data is available, and explore the systematic uncertainties on the inferred stellar parameters (radius, mass, and age) arising from adopting different values of $ΔY/ΔZ$, specifically, 1.4 and 2.0. The stellar grid constructed with a higher $ΔY / ΔZ$ value yields lower radius and mass estimates. We found systematic uncertainties of 1.1 per cent, 2.6 per cent, and 13.1 per cent on radius, mass, and ages, respectively.

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