arXiv subjects

Benard Nsamba

Publications and source records attributed to Benard Nsamba.

15 recordsLinked to original sources

arXiv2026

High-precision K2/TESS Photometry and HERMES Spectroscopy of Four Bright Field Stars

Space-based photometry from K2 and TESS, coupled with high-resolution spectroscopy, provides a unique opportunity to re-examine the variability and chemical nature of four bright stars (HD 73135, BD +19$^\circ$ 2045, BD +19$^\circ$ 2046, and TYC 1395-855-1), which were poorly characterised by the original ground-based campaign under the Nainital-Cape survey. We aim to establish the nature of the variability, chemical properties, and evolutionary status of these stars. We analysed ground-based Johnson $BV$ photometry combined with K2 and TESS time-series data to establish the nature of their variability. Using high-resolution HERMES spectroscopy, we determined their spectral classifications and chemical abundances. In addition, we used the inferred spectroscopic constraints with grid-based evolutionary modelling to derive their corresponding masses, radii, and ages. The frequency analysis reveals a diversity of low-amplitude variability across the sample. HD 73135 shows a persistent modulation near 1.5 d that is most consistent with rotation, although an ellipsoidal-binary interpretation cannot yet be excluded, while BD +19$^\circ$ 2045 exhibits multiple low-frequency signals and is identified as a new candidate $\gamma$ Doradus $g$-mode pulsator. BD +19$^\circ$ 2046 and TYC 1395-855-1 are non-variable or only marginally variable in K2 but display coherent low-frequency modulation in TESS of uncertain origin. Spectroscopically, HD 73135 is the only chemically peculiar star in the sample and shows a clear Am abundance pattern, whereas the other three stars are chemically normal. These results demonstrate the value of combining legacy survey data with contemporary photometric and spectroscopic analysis.

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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.

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arXiv2025

MAISTEP -- a new grid-based machine learning tool for inferring stellar parameters I. Ages of giant-planet host stars

Our understanding of exoplanet demographics partly depends on their corresponding host star parameters. With the majority of exoplanet-host stars having only atmospheric constraints available, robust inference of their parameters is susceptible to the approach used. The goal of this work is to develop a grid-based machine learning tool capable of determining the stellar radius, mass, and age using only atmospheric constraints and to analyse the age distribution of stars hosting giant planets. Our machine learning approach involves combining four tree-based machine learning algorithms (Random Forest, Extra Trees, Extreme Gradient Boosting, and CatBoost) trained on a grid of stellar models to infer stellar radius, mass, and age using Teff, [Fe/H], and luminosities. We perform a detailed statistical analysis to compare the inferences of our tool with those based on seismic data from the APOKASC and LEGACY samples. Finally, we apply our tool to determine the ages of stars hosting giant planets. Comparing the stellar parameter inferences from our machine learning tool with those from the APOKASC and LEGACY, we find a bias (and a scatter) of -0.5\% (5\%) and -0.2\% (2\%) in radius, 6\% (5\%) per cent and -2\% (3\%) in mass, and -9\% (16\%) and 7\% (23\%) in age, respectively. Therefore, our machine learning predictions are commensurate with seismic inferences. When applying our model to a sample of stars hosting Jupiter-mass planets, we find the average age estimates for the hosts of Hot Jupiters, Warm Jupiters, and Cold Jupiters to be 1.98, 2.98, and 3.51 Gyr, respectively. These statistical ages of the host stars confirm previous predictions - based on stellar model ages for a relatively small number of hosts, as well as on the average age-velocity dispersion relation - that stars hosting Hot Jupiters are statistically younger than those hosting Warm and Cold Jupiters.

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arXiv2024

Uniform characterisation of an ensemble of main-sequence benchmark stars: effect of Gaia-based data on grid search models

The inference of stellar parameters (such as radius and mass) through asteroseismic forward modelling depends on the number, accuracy, and precision of seismic and atmospheric constraints. ESA's Gaia space mission is providing precise parallaxes which yield an additional constraint to be included in the model grid search. Using a handful of main-sequence benchmark stars, we perform a uniform characterisation of these stars. We assess the accuracy and precision of stellar parameters inferred from grid-based searches when a Gaia-based luminosity is combined with different stellar constraints. We also examine the precision needed for an interferometric radius (model-independent radius) to have a significant contribution towards the determination of stellar mass in the optimisation process. Our findings show that more precise stellar masses are inferred for some stars when seismic and spectroscopic constraints are complemented with a Gaia-based luminosity, with a scatter varying from 1.9 per cent to 0.8 per cent. However, the inferred stellar radii are underestimated when compared to the interferometric radii and yield a scatter of $\sim$1.9 per cent. In addition, we demonstrate that a precisely measured interferometric radius ($\lesssim$ 1 per cent) when applied in the optimisation process yields a mass with a precision $\lesssim$ 1.5 per cent. Finally, we find that when only $l=0$ mode oscillation frequencies are available, robust masses and radii are still attainable. However, this requires precise and numerous $l=0$ mode oscillations frequencies ($>$ 8) to be coupled with atmospheric constraints.

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arXiv2022

On the stellar core physics of the 16 Cyg binary system: constraining the central hydrogen abundance using asteroseismology

The unprecedented quality of the asteroseismic data of solar-type stars made available by space missions such as NASA's Kepler telescope are making it possible to explore stellar interior structures. This offers possibilities of constraining stellar core properties (such as core sizes, abundances, and physics) paving the way for improving the precision of the inferred stellar ages. We employ 16 Cyg A and B as our benchmark stars for an asteroseismic study in which we present a novel approach aimed at selecting from a sample of acceptable stellar models returned from Forward Modelling techniques, down to the ones that better represent the core of each star. This is accomplished by comparing specific properties of the observed frequency ratios for each star to the ones derived from the acceptable stellar models. We demonstrate that in this way we are able to constrain further the hydrogen mass fraction in the core, establishing the stars' precise evolutionary states and ages. The ranges of the derived core hydrogen mass fractions are [0.01 - 0.06] and [0.12 - 0.19] for 16 Cyg A and B, respectively, and, considering that the stars are coeval, the age and metal mass fraction parameters span the region [6.4 - 7.4] Gyr and [0.023 - 0.026], respectively. In addition, our findings show that using a single helium-to-heavy element enrichment ratio, ($ΔY/ΔZ$), when forward modelling the 16 Cyg binary system, may result in a sample of acceptable models that do not simultaneously fit the observed frequency ratios, further highlighting that such an approach to the definition of the helium content of the star may not be adequate in studies of individual stars.

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arXiv2021

A 20-Second Cadence View of Solar-Type Stars and Their Planets with TESS: Asteroseismology of Solar Analogs and a Re-characterization of pi Men c

We present an analysis of the first 20-second cadence light curves obtained by the TESS space telescope during its extended mission. We find a precision improvement of 20-second data compared to 2-minute data for bright stars when binned to the same cadence (~10-25% better for T<~8 mag, reaching equal precision at T~13 mag), consistent with pre-flight expectations based on differences in cosmic ray mitigation algorithms. We present two results enabled by this improvement. First, we use 20-second data to detect oscillations in three solar analogs (gamma Pav, zeta Tuc and pi Men) and use asteroseismology to measure their radii, masses, densities and ages to ~1%, ~3%, ~1% and ~20% respectively, including systematic errors. Combining our asteroseismic ages with chromospheric activity measurements we find evidence that the spread in the activity-age relation is linked to stellar mass and thus convection-zone depth. Second, we combine 20-second data and published radial velocities to re-characterize pi Men c, which is now the closest transiting exoplanet for which detailed asteroseismology of the host star is possible. We show that pi Men c is located at the upper edge of the planet radius valley for its orbital period, confirming that it has likely retained a volatile atmosphere and that the "asteroseismic radius valley" remains devoid of planets. Our analysis favors a low eccentricity for pi Men c (<0.1 at 68% confidence), suggesting efficient tidal dissipation (Q/k <~ 2400) if it formed via high-eccentricity migration. Combined, these early results demonstrate the strong potential of TESS 20-second cadence data for stellar astrophysics and exoplanet science.

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arXiv2021

Weighing stars from birth to death: mass determination methods across the HRD

The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exists a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between $[0.3,2]\%$ for the covered mass range of $M\in [0.1,16]\,\msun$, $75\%$ of which are stars burning hydrogen in their core and the other $25\%$ covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars.

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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

Robust asteroseismic properties of the bright planet host HD 38529

The Transiting Exoplanet Survey Satellite (TESS) is recording short-cadence, high duty-cycle timeseries across most of the sky, which presents the opportunity to detect and study oscillations in interesting stars, in particular planet hosts. We have detected and analysed solar-like oscillations in the bright G4 subgiant HD 38529, which hosts an inner, roughly Jupiter-mass planet on a 14.3 d orbit and an outer, low-mass brown dwarf on a 2136 d orbit. We combine results from multiple stellar modelling teams to produce robust asteroseismic estimates of the star's properties, including its mass $M = 1.48 \pm 0.04 \mathrm{M}_\odot$, radius $R = 2.68 \pm 0.03 \mathrm{R}_\odot$ and age $t = 3.07 \pm 0.39 \,\mathrm{Gyr}$. Our results confirm that HD 38529 has a mass near the higher end of the range that can be found in the literature and also demonstrate that precise stellar properties can be measured given shorter timeseries than produced by CoRoT, Kepler or K2.

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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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arXiv2020

TESS Asteroseismic Analysis of the Known Exoplanet Host Star HD 222076

The Transiting Exoplanet Survey Satellite (TESS) is an all-sky survey mission aiming to search for exoplanets that transit bright stars. The high-quality photometric data of TESS are excellent for the asteroseismic study of solar-like stars. In this work, we present an asteroseismic analysis of the red-giant star HD~222076 hosting a long-period (2.4 yr) giant planet discovered through radial velocities. Solar-like oscillations of HD~222076 are detected around $203 \, μ$Hz by TESS for the first time. Asteroseismic modeling, using global asteroseismic parameters as input, yields a determination of the stellar mass ($M_\star = 1.12 \pm 0.12\, M_\odot$), radius ($R_\star = 4.34 \pm 0.21\,R_\odot$), and age ($7.4 \pm 2.7\,$Gyr), with precisions greatly improved from previous studies. The period spacing of the dipolar mixed modes extracted from the observed power spectrum reveals that the star is on the red-giant branch burning hydrogen in a shell surrounding the core. We find that the planet will not escape the tidal pull of the star and be engulfed into it within about $800\,$Myr, before the tip of the red-giant branch is reached.

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arXiv2019

TESS Asteroseismology of the known red-giant host stars HD 212771 and HD 203949

The Transiting Exoplanet Survey Satellite (TESS) is performing a near all-sky survey for planets that transit bright stars. In addition, its excellent photometric precision enables asteroseismology of solar-type and red-giant stars, which exhibit convection-driven, solar-like oscillations. Simulations predict that TESS will detect solar-like oscillations in nearly 100 stars already known to host planets. In this paper, we present an asteroseismic analysis of the known red-giant host stars HD 212771 and HD 203949, both systems having a long-period planet detected through radial velocities. These are the first detections of oscillations in previously known exoplanet-host stars by TESS, further showcasing the mission's potential to conduct asteroseismology of red-giant stars. We estimate the fundamental properties of both stars through a grid-based modeling approach that uses global asteroseismic parameters as input. We discuss the evolutionary state of HD 203949 in depth and note the large discrepancy between its asteroseismic mass ($M_\ast = 1.23 \pm 0.15\,{\rm M}_\odot$ if on the red-giant branch or $M_\ast = 1.00 \pm 0.16\,{\rm M}_\odot$ if in the clump) and the mass quoted in the discovery paper ($M_\ast = 2.1 \pm 0.1\,{\rm M}_\odot$), implying a change $>30\,\%$ in the planet's mass. Assuming HD 203949 to be in the clump, we investigate the planet's past orbital evolution and discuss how it could have avoided engulfment at the tip of the red-giant branch. Finally, HD 212771 was observed by K2 during its Campaign 3, thus allowing for a preliminary comparison of the asteroseismic performances of TESS and K2. We estimate the ratio of the observed oscillation amplitudes for this star to be $A_{\rm max}^{\rm TESS}/A_{\rm max}^{\rm K2} = 0.75 \pm 0.14$, consistent with the expected ratio of $\sim0.85$ due to the redder bandpass of TESS.

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arXiv2019

A Hot Saturn Orbiting An Oscillating Late Subgiant Discovered by TESS

We present the discovery of TOI-197.01, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. TOI-197 (HIP116158) is a bright (V=8.2 mag), spectroscopically classified subgiant which oscillates with an average frequency of about 430 muHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2-minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (2.943+/-0.064 Rsun), mass (1.212 +/- 0.074 Msun) and age (4.9+/-1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (9.17+/-0.33 Rearth) with an orbital period of ~14.3 days, irradiance of 343+/-24 Fearth, moderate mass (60.5 +/- 5.7 Mearth) and density (0.431+/-0.062 gcc). The properties of TOI-197.01 show that the host-star metallicity - planet mass correlation found in sub-Saturns (4-8 Rearth) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ~15%, TOI-197.01 is one of the best characterized Saturn-sized planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.

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arXiv2019

On the nature of the core of $α$ Centauri A: the impact of the metallicity mixture

Forward asteroseismic modelling plays an important role towards a complete understanding of the physics taking place in deep stellar interiors. With a dynamical mass in the range over which models develop convective cores while in the main sequence, the solar-like oscillator $α$ Centauri A presents itself as an interesting case study. We address the impact of varying the metallicity mixture on the determination of the energy transport process at work in the core of $α$ Centauri A. We find that $\gtrsim$ 70$\%$ of models reproducing the revised dynamical mass of $α$ Centauri A have convective cores, regardless of the metallicity mixture adopted. This is consistent with the findings of Nsamba et al., where nuclear reaction rates were varied instead. Given these results, we propose that $α$ Centauri A be adopted in the calibration of stellar model parameters when modelling solar-like stars with convective cores.

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arXiv2018

Model physics in low-mass solar-type stars: atomic diffusion and metallicity mixture

Using asteroseismic data from the Kepler satellite, we explore the systematic uncertainties arising from changes in the input physics used when constructing evolution models of solar-type stars. We assess the impact of including atomic diffusion and of varying the metallicity mixture on the determination of global stellar parameters (i.e., radius, mass, and age). We find significant systematic uncertainties on global stellar parameters when diffusion is included in stellar grids. Furthermore, we find the systematic uncertainties on the global stellar parameters to be comparable to the statistical uncertainties when a different metallicity mixture is employed in stellar grids.

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