arXiv subjects

Mikkel N. Lund

Publications and source records attributed to Mikkel N. Lund.

At least 19 recordsLinked to original sources

arXiv2026

Asteroseismology and interferometry of the F7V spectroscopic binary $χ$ Draconis A in the TESS CVZ

We present a detailed analysis of the asteroseismic main-sequence benchmark star $χ$ Dra A in the TESS northern CVZ. We aim to derive stellar mass and radius from asteroseismic modelling of individual mode frequencies and test the asteroseismic mass and radius against an independent dynamical mass and interferometric radius measurement. We determined the dynamical mass of $χ$ Dra using 618 radial velocity measurements obtained with the SONG telescope at Tenerife, and 53 relative astrometric measurements. With the PAVO beam combiner at CHARA, we obtained the interferometric radius of $χ$ Dra A. We determined asteroseismic parameters from 16 sectors of 20-sec cadence TESS photometry. We determined $T_{\rm eff}=6277\pm30$ K, $\rm [Fe/H] =-0.51\pm0.03$ dex, and $[α/\rm Fe]=0.08\pm0.03$ dex from the spectroscopic analysis. For the dynamical fit we obtained a mass of $M_{\rm A}=1.028 \pm 0.004$ $\rm M_{\odot}$ for $χ$ Dra A, and $M_{\rm B}=0.735 \pm 0.003$ $\rm M_{\odot}$ for $χ$ Dra B. Combining the derived interferometric angular diameter with dynamical parallax yields an interferometric radius of $R_{\rm A} = 1.159^{+0.029}_{-0.028}$ $\rm R_{\odot}$. In the TESS power spectrum, we identified 38 individual oscillating modes. Using these modes, we modelled the star with nine independent pipelines to test the resulting model mass and radius against our independently calculated mass and radius. All models yielded masses slightly lower than the dynamical mass. Using asteroseismic scaling relations, we found that scaling masses from corrected scaling relations best reproduce the dynamical mass. The combination of spectroscopy, interferometry, and asteroseismology has yielded precise results for the main component of the $χ$ Dra system, making it one of the best-characterised main-sequence solar-like oscillators.

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arXiv2026

The Kepler and TESS Asteroseismic Consortia - A Historical Overview

This overview is, in short, about people. About an idea that grew into a consortium, then a community, and finally a way of doing asteroseismology in the space era. The Kepler Asteroseismic Science Consortium (KASC) and later the TESS Asteroseismic Science Consortium (TASC) are not just organisational structures to access data. They became collaborative ecosystems in which students, postdocs, and senior scientists learned to work together across continents, time zones, and occasionally differing opinions about filters. The idea for this booklet did not emerge from a formal meeting or a funding call. It emerged-appropriately enough for an international collaboration-over a beer. More precisely, over a beer (truth be told, more than one) at Beer Lab University in Honolulu, during the TASC7/KASC14 Workshop in 2023. Google Maps now informs us that this particular Beer Lab location is "permanently closed", which seemed at first like an ominous metaphor for fading inspiration. Fortunately, we have since learned that Beer Lab simply moved across the street, proving that good ideas, like good consortia, tend to reinvent themselves rather than disappear entirely. Somewhere between discussions that night and many others over the years, we found ourselves revisiting stories we had long heard from our former supervisors and more senior colleagues; stories of how KASC was negotiated, how the first working groups were formed, how data were filtered (a saga in its own right!), and how trust was built. This book is therefore an attempt to capture some of that collective memory, while we still have access to the primary sources. It is timed to celebrate the return of the KASC/TASC Workshop Series to Aarhus in 2026, where the consortia's original framework was set in motion. There is something fitting about reflecting on the journey of KASC and TASC as the community gathers again where it all began.

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arXiv2026

Ensemble asteroseismology: An ensemble approach to detecting signatures of solar-like oscillations in K-dwarfs

Solar-like oscillations have to date been observed in hundreds of main-sequence and sub-giant stars. However, only a handful of detections have been made in K-type dwarfs, using ground-based extreme precision radial velocity observations and space-based photometric observations made by the NASA \emph{Kepler} and TESS missions. Whilst the upcoming ESA PLATO Mission promises to add to these individual detections, it will do so only in a similar, modest number of stars. Here, we propose a new ensemble strategy to exploit the PLATO data, in which frequency power spectra on hundreds of K-dwarfs lying in constrained ranges of effective temperature are combined in a weighted manner to significantly improve the detectability of the oscillations. Whilst this approach means it is not possible to extract usable constraints on individual oscillation frequencies, it provides a way to detect and measure the characteristics of the composite envelope of oscillation power given by the ensemble, which in turn provides diagnostics of granulation and magneto-convection and the impact of magnetic activity on the modes. We use data in the PLATO Input Catalogue (PIC) to make discrete numerical predictions of the detectability of the ensemble spectra. We also derive a simple analytical approximation of our method that obviates the need to perform numerical calculations over a discrete sample of targets, and which serves as a useful tool to make quick predictions for other future or planned missions. Our predictions indicate that PLATO has the potential to provide solid ensemble detections well into the K-dwarf regime. In summary, PLATO offers an ideal opportunity to exploit this new approach.

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arXiv2026

The $μ$ Herculis system solved after nearly three centuries

$μ$ Herculis is a bright, nearby quadruple system. Its brightest member, $μ$ Her Aa, displays solar-like oscillations, establishing the system as a crucial benchmark for asteroseismology, provided that its mass can be determined independently of stellar models. We aim to resolve the full hierarchical architecture of the system and determine precise, model-independent dynamical masses for all four components (Aa, Ab, B, and C), along with a consistent astrometric solution for the system's centre of mass. We performed a joint fit of radial velocities, relative astrometry and absolute astrometry from \textit{Hipparcos}, \textit{Gaia} DR3, and ground-based catalogues, spanning nearly three centuries. Our forward-modelling framework simultaneously constrains the Keplerian orbits of the inner Aa--Ab and B--C subsystems, the wide A--BC orbit, and the sky motion and parallax of the total centre of mass. Leveraging several complementary datasets and the decisive 2023 periastron passage of the Aa--Ab pair, we precisely determine all orbital parameters and obtain sub-percent precision on the component masses: $M_{\rm Aa} = 1.134 \pm 0.007\,M_{\odot}$, $M_{\rm Ab} = 0.2286 \pm 0.0006\,M_{\odot}$, $M_{\rm C} = 0.445 \pm 0.005\,M_{\odot}$, and $M_{\rm B} = 0.417 \pm 0.005\,M_{\odot}$. We derive a system parallax of $\varpi_{\rm CM} = 120.069 \pm 0.089\,\mathrm{mas}$ that reconciles and improves upon the individual \textit{Hipparcos} and \textit{Gaia} DR3 values.

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arXiv2026

The PLATO Science Calibration and Validation Plan: Targets for the First Long-pointing Field

In order to meet the science goals of the PLATO space mission, an extensive science calibration and validation plan has been designed. This paper describes this plan, as well as the methodology adopted to select the science calibration and validation stars that have entered its input catalogue. This is the so-called {\tt scvPIC}, which is part of the general PLATO Input Catalogue (PIC) for the first selected long pointing field in the Southern Hemisphere known as LOPS2. While many of PLATO's science requirements needed dedicated stars as calibrators as discussed here, its most stringent requirement is the delivery of the age of the host stars of exoplanetary systems with an accuracy better than 10\% for a G0V star of {\it V} = 10 mag, i.e. a nearby Sun-like star. This is presently not within reach for large populations of dwarfs and subgiants in the Milky Way as it requires the models of their stellar interiors to be improved. We discuss how this ambitious age requirement led to the selection of tens of thousands of red giants, and of thousands of main-sequence early F-type gravity-mode pulsators in order to deduce their internal rotation profile across stellar evolution. This asteroseismic observable will then be imported as key information into improved models of dwarfs and subgiants in the Milky Way as optimal modelling tools for ever better age-dating of the exoplanet hosts as the PLATO mission moves along. Additional calibrators and validators included in the {\tt scvPIC} are a few thousands of binaries, a few hundreds of legacy and benchmark stars, a few hundred photometrically stable stars, and six transiting brown dwarfs.

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arXiv2026

Bolometric corrections of stellar oscillation mode amplitudes as observed by the PLATO mission. I. Planck-spectrum estimates

We derive bolometric correction functions for oscillation mode amplitudes observed by the different cameras of the ESA PLATO mission. Such corrections between bolometric (full light) and mission instrument-specific amplitudes enable comparisons to theoretical expectations and amplitude conversion between different photometric missions, which is essential for proper detectability yields and target selection. Bolometric correction functions were calculated assuming a Planck function approximation for the stellar spectral flux distribution. The calculations follow the procedures applied in earlier analyses for the NASA Kepler and TESS missions. We derived power-law and polynomial parametrisations of the bolometric corrections with $T_{\rm eff}$. We find that on average, oscillation mode amplitudes from PLATO's normal cameras (N-CAMs) are expected to be ~6.7% lower compared to Kepler, and ~12.5% higher compared to TESS. A significant average amplitude ratio of ~25% is expected for amplitudes measured using the blue PLATO fast camera (F-CAM) compared to TESS. We find that observations of bright solar-like oscillators, especially with PLATO's F-CAMs, would provide an important test of the predicted corrections.

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arXiv2026

Optimising the global detection of solar-like oscillations. Tuning the frequency range for asteroseismic detection predictions and searches

A well-established method exists for predicting the detectability of solar-like oscillations and has been widely used to support target selection for space-based photometric missions. The method evaluates the probability of an asteroseismic detection from the expected global signal-to-noise ratio (SNR) of the oscillation signal relative to the broadband background from shot noise and granulation. Stellar parameters are used to estimate the oscillation and granulation signals, while instrumental properties and apparent stellar brightness determine the expected shot noise. We investigate whether there is an optimal choice for the frequency range, $W$, over which the global SNR is calculated. The oscillation power is assumed to follow a Gaussian-like envelope with full width at half maximum $Γ_{\rm env}$ centred on the frequency of maximum oscillation power. It has commonly been assumed that $W \simeq 2Γ_{\rm env}$ when predicting detections. We compute numerical predictions of the global SNR and corresponding detection probabilities for a range of stellar and observational parameters, adopting widths $W=αΓ_{\rm env}$ where $α$ is a multiplicative factor. We also examine the impact of this choice on detection yields across a population of targets using bright solar-like oscillators observed by TESS as a representative sample. We find that the commonly adopted value $α\simeq 2$ is suboptimal and that $α\simeq 1.2$ maximises the detection probability. This choice can also significantly affect predicted detection yields for stellar samples. We therefore recommend adopting $W \simeq 1.2Γ_{\rm env}$ both when computing detection probabilities and when searching for oscillations in real data via tests of excess mode power, as it optimises the probability of robust detections.

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arXiv2026

Halo Photometry and Asteroseismology for 98 of the Brightest Stars Observed by TESS

The Transiting Exoplanet Survey Satellite (TESS) mission has facilitated studies of asteroseismology, eclipsing binaries, and transits in many stars. However, the brightest stars saturate TESS, yet they are the most amenable to photon-hungry high-resolution studies and have long observational histories. In this work, we adapted the halo photometry used in $K$2 to extract light curves from the unsaturated halo pixels of the star's point spread function. We used this method to extract light curves for 98 of the brightest stars observed by TESS in Sectors 1-93. These bright stars include 15 red giants, five $δ$ Scuti variables, eight stochastic low-frequency variables, eight eclipsing binaries, and 46 other variables. We measured $ν_{\rm max}$ for 13 red giants using pyMON and $Δν$ for one of them, $β$ Gem (Pollux). For five of them, this represents the first time that oscillations were detected. We derived their stellar masses using the measured $ν_{\rm max}$ and previous interferometric and radiometric angular diameters. We also discovered $δ$ Scuti and $γ$ Doradus variability in $α$ Cep, possible asteroseismic binary signatures in $ε$ Car, and a new eclipsing binary, $γ$ And. Furthermore, we identified 18 stars in our sample that will be observed by the future PLAnetary Transits and Oscillations of stars (PLATO) mission, and 69 stars that have Stellar Observations Network Group (SONG) observations, including some simultaneous with TESS. The light curves are publicly available on the Mikulski Archive for Space Telescopes.

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arXiv2026

Granulation signatures as seen by Kepler short-cadence data. I. A decoupling between granulation and oscillation timescales for dwarfs

Granulation is the observable signature of convection in envelopes of low-mass stars, forming the background in stellar power spectra. While well-studied in evolved giants, granulation on the MS has received less attention. We here study and characterise granulation signatures of MS and SGB stars, extending previous studies of giants to provide a continuous physical picture across evolutionary stages. We analyse 753 Kepler short-cadence stars using a Bayesian nested-sampling framework to evaluate three background descriptions and compare model preferences. This yields full posterior distributions for all parameters, enabling robust comparisons across a diverse stellar sample. No universal preference between background models is found. Assuming a Gaussian oscillation envelope, $ν_\mathrm{max}$ estimates are sensitive to model misspecification, with the resulting systematics exceeding the formal uncertainties. The envelope width scales with $ν_\mathrm{max}$ across models and shows a dependence on effective temperature. Total granulation amplitudes in dwarfs broadly follow giant-based scalings, however a decoupling appears between the timescale of the primary granulation and the oscillations for MS stars cooler than the Sun. The prolonged granulation timescale is reproduced by 3D simulations of a K-dwarf, driven by reduced convective velocities due to more efficient convective energy transport in denser envelopes. The prolonged granulation timescale increases the frequency separation to the oscillation excess, potentially aiding seismic detectability, while the reduced convective velocities may influence the excitation of stellar oscillations and relate to the low amplitudes observed in cool dwarfs. Finally, we contribute a dataset linking granulation, oscillations, and stellar parameters, providing a foundation for future investigations into their interdependence across the HR diagram.

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arXiv2026

Asteroseismology of solar-like oscillators: emulating individual mode frequencies with a branching neural network

Accurately measuring stellar ages and internal structures is challenging, but the inclusion of asteroseismic observables can substantially improve precision. However, the curse of dimensionality means this comes at a high computational cost when using standard interpolation methods across grids of stellar models. Furthermore, without a rigorous treatment of random uncertainties in grid-based modelling, it is not possible to address systematic errors in stellar models. We present PITCHFORK -- a multilayer perceptron neural network with a branching architecture capable of rapid emulation of both classical stellar observables and individual asteroseismic oscillation modes of solar-like oscillators. PITCHFORK can predict the classical observables $T_{\text{eff}}$, $L$, and $\left[\mathrm{Fe}/\mathrm{H}\right]$ with precisions of $5.88\,\text{K}$, $0.014\,\text{L}_{\odot}$, and $0.001\,\text{dex}$, respectively, and can predict 35 individual radial mode frequencies with a uniform precision of $0.02$ per cent. PITCHFORK is coupled to a vectorised Bayesian inference pipeline to return well-sampled and fully marginalised posterior distributions. We validate our rigorous treatment of the random uncertainties -- including the asteroseismic surface effect -- in an extensive hare-and-hounds exercise. We also demonstrate our ability to infer the stellar properties of benchmark stars -- namely, the Sun and the binary stars 16 Cygni A and B. This work demonstrates a computationally scalable and statistically robust framework for stellar parameter inference of solar-like oscillators using individual asteroseismic mode frequencies. This provides a foundation for the treatment of systematics in preparation for the imminent abundance of asteroseismic data from future missions.

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arXiv2025

Investigating magnetic activity cycles in solar-like oscillators using asteroseismic data from the K2 mission

We present the results of an investigation into the possible presence of magnetic activity cycles in stars observed in two observational campaigns by the K2 mission. This study was based on the KEYSTONE asteroseismic sample of solar-like oscillators, which contained 20 stars for which we were able to determine whether the asteroseismic p-mode frequencies varied in time. These frequency shifts ($δν$) were determined using a cross-correlation method and using the individual mode frequencies, obtained by fitting power spectra. Three stars were found to exhibit $δν$ larger than their associated errors ($σ_{δν}$) using both methods, while two more stars exhibited $δν>σ_{δν}$ when the cross correlation was used and a further two stars exhibited $δν>σ_{δν}$ when the fitted frequencies were used. When considering the whole sample of 20 stars, the amplitude of $δν$ showed no dependence on the large frequency separation and metallicity. However, $δν$ was observed to increase with rotation rate and effective temperature. Our sample contained a number of evolved subgiant stars, allowing us to expand the parameter space usually considered when comparing $δν$ with stellar parameters. While $δν$ was small for all of the evolved stars, one was found to have $δν>σ_{δν}$, raising the possibility that these evolved stars may still exhibit variable magnetic activity.

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arXiv2025

Peakbagging the K2 KEYSTONE sample with PBjam: characterising the individual mode frequencies in solar-like oscillators

The pattern of individual mode frequencies in solar-like oscillators provides valuable insight into their properties and interior structures. The identification and characterisation of these modes requires high signal-to-noise and frequency resolution. The KEYSTONE project unlocks the asteroseismic potential of the K2 mission by providing individually reduced, high-quality time series data, global asteroseismic parameters, and spectroscopic analysis for 173 solar-like oscillators. In this work, we build on the KEYSTONE project and present the first analysis of the pattern of individual modes in the oscillation spectra for the K2 KEYSTONE stars. We perform a robust identification and characterisation of the modes through peakbagging methods in the open-source analysis tool PBjam. We present over 6000 mode frequencies, widths, and heights for 168 stars in the sample, covering the HR diagram from FGK dwarfs to sub-giants and the lower red giant branch, providing a significant increase in the number of individual mode frequency detections for main sequence and sub-giant oscillators. This study also presents sample-wide trends of oscillation patterns as a function of the fundamental stellar properties, and improves the precision of the global asteroseismic parameters. These measurements are part of the legacy of the K2 mission, and can be used to perform detailed modelling to improve the precision of fundamental properties of these stars. The results of this analysis provides evidence for the validity of using PBjam to identify and characterise the modes resulting from the observations of the future PLATO mission.

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arXiv2025

Luminaries in the Sky: The TESS Legacy Sample of Bright Stars. I. Asteroseismic detections in naked-eye main-sequence and sub-giant solar-like oscillators

We aim to detect and characterise solar-like oscillations in bright naked-eye (V<6) main-sequence (MS) and subgiant stars observed by TESS. We seek to expand the current benchmark sample of oscillators, provide accurate global asteroseismic parameters for these bright targets, and assess their potential for future detailed investigations -- including missions such as the HWO and PLATO. Our sample of bright stars was selected from the Hipparcos/Tycho catalogues. We analysed TESS 120-s and 20-s cadence photometry using SPOC light curves and custom apertures from target pixel files. After applying a filtering of the light curves, we extracted global asteroseismic parameters ($ν_{\rm max}$ and $Δν$) using the pySYD pipeline. Results were cross-validated with independent pipelines and compared to predictions from the ATL, while noise properties were evaluated to quantify improvements from a 20-s observing cadence. We detect solar-like oscillations in a total of 196 stars -- including 128 new detections -- with extracted $ν_{\rm max}$ and $Δν$ values showing strong conformity to expected scaling relations. This corresponds to an increase by more than an order of magnitude in the number of MS stars with detection of solar-like oscillations from TESS. Nearly 40% of our new detections are prime HWO targets, enabling systematic asteroseismic age determinations relevant for interpreting atmospheric biosignatures. Our analysis confirms that 20-s cadence data yields lower high-frequency noise levels compared to 120-s data. Moreover, the precise stellar parameters obtained through asteroseismology establish these bright stars as benchmarks for seismic investigations and provide useful constraints for refining stellar evolution models and for complementary analyses in interferometry, spectroscopy, and exoplanet characterisation.

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arXiv2025

Asteroseismology of the G8 subgiant beta Aquilae with SONG-Tenerife, SONG-Australia and TESS

We present time-series radial velocities of the G8 subgiant star beta Aql obtained in 2022 and 2023 using SONG-Tenerife and, for the first time, SONG-Australia. We also analyse a sector of TESS photometry that overlapped with the 2022 SONG data. The resulting power spectrum clearly shows solar-like oscillations centred at 430 muHz. The TESS light curve shows the oscillations at lower signal-to-noise, reflecting the fact that photometric measurements are much more affected by the granulation background than are radial velocities. The simultaneous observations in velocity and photometry represent the best such measurements for any star apart from the Sun. They allowed us to measure the ratio between the bolometric photometric amplitude and the velocity amplitude to be 26.6 +/- 3.1 ppm/(m/s). We measured this ratio for the Sun from published SOHO data to be 19.5 +/- 0.7 ppm/(m/s) and, after accounting for the difference in effective temperatures of and the Sun, these values align with expectations. In both the Sun and beta Aql, the photometry-to-velocity ratio appears to be a function of frequency. We also measured the phase shift of the oscillations in beta Aql between SONG and TESS to be -113 +/- 7 deg, which agrees with the value for the Sun and also with a 3-D simulation of a star with similar properties to beta Aql. Importantly for exoplanet searches, we argue that simultaneous photometry can be used to predict the contribution of oscillations to radial velocities. We measured frequencies for 22 oscillation modes in beta Aql and carried out asteroseismic modelling, yielding an excellent fit to the frequencies. We derived accurate values for the mass and age, and were able to place quite strong constraints on the mixing-length parameter. Finally, we show that the oscillation properties of beta Aql are very similar to stars in the open cluster M67.

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arXiv2024

The K2 Asteroseismic KEYSTONE sample of Dwarf and Subgiant Solar-Like Oscillators. I: Data and Asteroseismic parameters

The KEYSTONE project aims to enhance our understanding of solar-like oscillators by delivering a catalogue of global asteroseismic parameters (${Δν}$ and ${ν_{\rm max}}$) for 173 stars, comprising mainly dwarfs and subgiants, observed by the K2 mission in its short-cadence mode during campaigns 6-19. We derive atmospheric parameters and luminosities using spectroscopic data from TRES, astrometric data from $\textit{Gaia}$, and the infrared flux method (IRFM) for a comprehensive stellar characterisation. Asteroseismic parameters are robustly extracted using three independent methods, complemented by an iterative refinement of the spectroscopic analyses using seismic ${\log g}$ values to enhance parameter accuracy. Our analysis identifies new detections of solar-like oscillations in 159 stars, providing an important complement to already published results from previous campaigns. The catalogue provides homogeneously derived atmospheric parameters and luminosities for the majority of the sample. Comparison between spectroscopic ${T_{\rm eff}}$ and those obtained from the IRFM demonstrates excellent agreement. The iterative approach to spectroscopic analysis significantly enhances the accuracy of the stellar properties derived.

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arXiv2023

Three low-mass companions around aged stars discovered by TESS

We report the discovery of three transiting low-mass companions to aged stars: a brown dwarf (TOI-2336b) and two objects near the hydrogen burning mass limit (TOI-1608b and TOI-2521b). These three systems were first identified using data from the Transiting Exoplanet Survey Satellite (TESS). TOI-2336b has a radius of $1.05\pm 0.04\ R_J$, a mass of $69.9\pm 2.3\ M_J$ and an orbital period of 7.71 days. TOI-1608b has a radius of $1.21\pm 0.06\ R_J$, a mass of $90.7\pm 3.7\ M_J$ and an orbital period of 2.47 days. TOI-2521b has a radius of $1.01\pm 0.04\ R_J$, a mass of $77.5\pm 3.3\ M_J$ and an orbital period of 5.56 days. We found all these low-mass companions are inflated. We fitted a relation between radius, mass and incident flux using the sample of known transiting brown dwarfs and low-mass M dwarfs. We found a positive correlation between the flux and the radius for brown dwarfs and for low-mass stars that is weaker than the correlation observed for giant planets. We also found that TOI-1608 and TOI-2521 are very likely to be spin-orbit synchronized, leading to the unusually rapid rotation of the primary stars considering their evolutionary stages. Our estimates indicate that both systems have much shorter spin-orbit synchronization timescales compared to their ages. These systems provide valuable insights into the evolution of stellar systems with brown dwarf and low-mass stellar companions influenced by tidal effects.

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arXiv2023

The odd bunch: chrono-chemo-dynamics of sixteen unusual stars from Kepler

In this study we combine asteroseismic, spectroscopic and kinematic information to perform a detailed analysis of a sample of 16 stars from the Kepler field. Our selection focuses on stars that appear to contradict Galactic chemical evolution models: young and $α$-rich, old and metal-rich, as well as other targets with unclear classification in past surveys. Kinematics are derived from Gaia DR3 parallaxes and proper motions, and high-resolution spectra from HIRES/Keck are used to calculate chemical abundances for over 20 elements. This information is used to perform careful checks on asteroseismic masses and ages derived via grid-based modelling. Among the seven stars previously classified as young and $α$-rich, only one seems to be an unambiguously older object masking its true age. We confirm the existence of two very old ($\geq$11 Gyr), super metal rich ($\geq$0.1 dex) giants. These two stars have regular thin disc chemistry and in-plane solar circle orbits which fit well in the picture of radial migration via the churning mechanism. The alternative explanation that these stars have younger ages would require mass-loss rates which strongly increases with increasing metallicity. Finally, we suggest further investigations to explore the suitability of Zn as a chemical clock in red giants.

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arXiv2022

Scaling relations of convective granulation noise across the HR diagram from 3D stellar atmosphere models

High-precision photometric data from space missions have improved our understanding of stellar granulation. These observations have shown with precision the stochastic brightness fluctuations of stars across the HR diagram, allowing us to better understand how stellar surface convection reacts to a change in stellar parameters. These fluctuations need to be understood and quantified in order to improve the detection and characterization of exoplanets. In this work, we provide new scaling relations of two characteristic properties of the brightness fluctuations time series, the standard deviation ($σ$) and the auto-correlation time ($τ\rm_{eff}$). This was done by using long time series of 3D stellar atmosphere models at different metallicities and across the HR diagram, generated with a 3D radiative hydrodynamical code: the STAGGER code. We compared our synthetic granulation properties with the values of a large sample of Kepler stars, and analyzed selected stars with accurate stellar parameters from the Kepler LEGACY sample. Our 3D models showed that $σ\proptoν\rm_{max}^{-0.567\pm0.012}$ and $τ\rm_{eff}\proptoν\rm_{max}^{-0.997\pm0.018}$ for stars at solar metallicity. We showed that both $σ$ and $τ\rm_{eff}$ decrease with metallicity, although the metallicity dependence is more significant on $σ$. Unlike previous studies, we found very good agreement between $σ$ from Kepler targets and the 3D models at $\log{g}\leq3.5$, and a good correlation between the stars and models with $\log{g}\geq3.5$. For $τ\rm_{eff}$, we found that the 3D models reproduced well the Kepler LEGACY star values. Overall, this study shows that 3D stellar atmosphere models reproduce the granulation properties of stars across the HR diagram.

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