arXiv subjects

A. Serenelli

Publications and source records attributed to A. Serenelli.

At least 19 recordsLinked to original sources

arXiv2025

Beyond the Nyquist frequency: Asteroseismic catalog of undersampled Kepler late subgiants and early red giants

Subgiants and early red giants are crucial for studying the first dredge-up, a key evolutionary phase where the convective envelope deepens, mixing previously interior-processed material and bringing it to the surface. Yet, very few have been seismically characterized with Kepler because their oscillation frequencies are close to the 30 minute sampling frequency of the mission. We developed a new method as part of the new PyA2Z code to identify super-Nyquist oscillators and infer their global seismic parameters, $\nu_\mathrm{max}$ and large separation, $\Delta\nu$. Applying PyA2Z to 2 065 Kepler targets, we seismically characterize 285 super-Nyquist and 168 close-to-Nyquist stars with masses from 0.8 to 1.6 M$_\odot$. In combination with APOGEE spectroscopy, Gaia spectro-photometry, and stellar models, we derive stellar ages for the sample. There is good agreement between the predicted and actual positions of stars on the HR diagram (luminosity vs. effective temperature) as a function of mass and composition. While the timing of dredge-up is consistent with predictions, the magnitude and mass dependence show discrepancies with models, possibly due to uncertainties in model physics or calibration issues in observed abundance scales.

astro-ph.SR↗

arXiv2024

Solar fusion III: New data and theory for hydrogen-burning stars

In stars that lie on the main sequence in the Hertzsprung-Russel diagram, like our sun, hydrogen is fused to helium in a number of nuclear reaction chains and series, such as the proton-proton chain and the carbon-nitrogen-oxygen cycles. Precisely determined thermonuclear rates of these reactions lie at the foundation of the standard solar model. This review, the third decadal evaluation of the nuclear physics of hydrogen-burning stars, is motivated by the great advances made in recent years by solar neutrino observatories, putting experimental knowledge of the proton-proton chain neutrino fluxes in the few-percent precision range. The basis of the review is a one-week community meeting held in July 2022 in Berkeley, California, and many subsequent digital meetings and exchanges. The relevant reactions of solar and stellar hydrogen burning are reviewed here, from both theoretical and experimental perspectives. Recommendations for the state of the art of the astrophysical S-factor and its uncertainty are formulated for each of them. Several other topics of paramount importance for the solar model are reviewed, as well: recent and future neutrino experiments, electron screening, radiative opacities, and current and upcoming experimental facilities. In addition to reaction-specific recommendations, also general recommendations are formed.

astro-ph.SR↗

arXiv2024

Entropy-calibrated stellar modeling: Testing and improving the use of prescriptions for entropy of adiabatic convection

The modeling of convection is a long standing problem in stellar physics. Up-to-now, all ad hoc models rely on a free parameter alpha (among others) which has no real physical justification and is therefore poorly constrained. However, a link exists between this free parameter and the entropy of the stellar adiabat. Prescriptions, derived from 3D stellar atmospheric models, are available that provide entropy as a function of stellar atmospheric parameters (effective temperature, surface gravity, chemical composition). This can provide constraints on alpha through the development of entropy-calibrated models. Several questions arise as these models are increasingly used. Which prescription should be used? How do uncertainties impact entropy-calibrated models? We aim to study the three existing prescriptions and determine which one should be used, and how. We implemented the entropy-calibration method into the stellar evolution code Cesam2k20 and performed comparisons with the Sun and the alpha Cen system. In addition, we used data from the CIFIST grid of 3D atmosphere models to evaluate the accuracy of the prescriptions. Of the three entropy prescriptions available, we determine which one best reproduces the entropies of the 3D models. We also demonstrate that the entropy obtained from this prescription should be corrected for the evolving chemical composition and for an entropy offset different between various EoS tables, following a precise procedure, otherwise classical parameters obtained from the models will be strongly biased. Finally, we also provide table with entropy of the adiabat of the CIFIST grid, as well as fits of these entropies. We performed a precise examination of entropy-calibrated modelling, and gave recommendations on which adiabatic entropy prescription to use, how to correct it and to implement the method into a stellar evolution code.

astro-ph.SR↗

arXiv2024

Predicted asteroseismic detection yield for solar-like oscillating stars with PLATO

We determine the expected yield of detections of solar-like oscillations for the PLATO ESA mission. We used a formulation from the literature to calculate the probability of detection and validated it with Kepler data. We then applied this approach to the PLATO P1 and P2 samples with the lowest noise level and the much larger P5 sample, which has a higher noise level. We used the information available in in the PIC 1.1.0, including the current best estimate of the signal-to-noise ratio. We also derived relations to estimate the uncertainties of seismically inferred stellar mass, radius and age and applied those relations to the main sequence stars of the PLATO P1 and P2 samples with masses equal to or below 1.2 $\rm{M}_\odot$ for which we had obtained a positive seismic detection. We found that one can expect positive detections of solar-like oscillations for more than 15 000 FGK stars in one single field after a two-years run of observation. For main sequence stars with masses $\leq 1.2 \rm{M}_\odot$, we found that about 1131 stars satisfy the PLATO requirements for the uncertainties of the seismically inferred stellar masses, radii and ages in one single field after a two-year run of observation. The baseline observation programme of PLATO consists in observing two fields of similar size (in the Southern and Northern hemispheres) for two years each. The expected seismic yields of the mission are more 30000 FGK dwarfs and subgiants with positive detections of solar-like oscillations, enabling to achieve the mission stellar objectives. The PLATO mission should produce a sample of seismically extremely well characterized stars of quality equivalent to the Kepler Legacy sample but containing a number of stars $\sim$ 80 times larger if observing two PLATO fields for two years each. They will represent a goldmine which will make possible significant advances in stellar modelling.

astro-ph.SR↗

arXiv2023

Seismic and spectroscopic analysis of 9 bright red giants observed by Kepler

Photometric time series gathered by space telescopes such as CoRoT and Kepler allow to detect solar-like oscillations in red-giant stars and to measure their global seismic constraints, which can be used to infer global stellar properties (e.g. masses, radii, evolutionary states). Combining such precise constraints with photospheric abundances provides a means of testing mixing processes that occur inside red-giant stars. In this work, we conduct a detailed spectroscopic and seismic analysis of nine nearby (d < 200 pc) red-giant stars observed by Kepler. Both seismic constraints and grid-based modelling approaches are used to determine precise fundamental parameters for those evolved stars. We compare distances and radii derived from Gaia Data Release 3 parallaxes with those inferred by a combination of seismic, spectroscopic and photometric constraints. We find no deviations within errorsbars, however the small sample size and the associated uncertainties are a limiting factor for such comparison. We use the period spacing of mixed modes to distinguish between ascending red-giants and red-clump stars. Based on the evolutionary status, we apply corrections to the values of $Δν$ for some stars, resulting in a slight improvement to the agreement between seismic and photometric distances. Finally, we couple constraints on detailed chemical abundances with the inferred masses, radii and evolutionary states. Our results corroborate previous studies that show that observed abundances of lithium and carbon isotopic ratio are in contrast with predictions from standard models, giving robust evidence for the occurrence of additional mixing during the red-giant phase.

astro-ph.SR↗

arXiv2023

Observational constraints on the origin of the elements. VIII. Constraining the Barium, Strontium and Yttrium chemical evolution in metal-poor stars

Recently Lian et al. (2023), thanks to Gaia-ESO data, studied the chemical evolution of neutron-capture elements in the regime [Fe/H]>-1. We aim here to complement this study down to [Fe/H]=-3, and focus on Ba, Y, Sr, and abundance ratios of [Ba/Y] and [Sr/Y], which give comprehensive views on s-process nucleosynthesis channels. We measured LTE and NLTE abundances of Ba, Y, and Sr in 323 Galactic metal-poor stars using high-resolution optical spectra with high S/N. We used the spectral fitting code TSFitPy, together with 1D model atmospheres using previously determined LTE and NLTE atmospheric parameters. The NLTE effects are on the order of -0.1 to ~0.2dex depending on the element. T he ratio between heavy and light s-process elements [Ba/Y] varies weakly with [Fe/H] even in the metal-poor regime, consistently with the behavior in the metal-rich regime. The [Ba/Y] scatter at a given metallicity is larger than the abundance measurement uncertainties. Homogeneous chemical evolution models with different yields prescriptions are unable to accurately reproduce the [Ba/Y] scatter at low-[Fe/H]. Adopting the stochastic chemical evolution model by Cescutti & Chaippini (2014) allows to reproduce the observed scatter in the abundance pattern of [Ba/Y] and [Ba/Sr]. With our observations, we rule out the need for an arbitrary scaling of the r-process contribution as previously suggested by the model authors. We have showed how important it is to properly include NLTE effects when measuring chemical abundances, especially in the metal-poor regime. This work shows that the choice of the Galactic chemical evolution model (stochastic vs. 1-zone) is key when comparing models to observations. The upcoming surveys such as 4MOST and WEAVE will deliver high quality spectra of many thousands of metal-poor stars, and this work gives a typical case study of what could be achieved with such surveys.

astro-ph.SR↗

arXiv2021

The Future of Solar Neutrinos

In this article we review the current state of the field of solar neutrinos, including flavour oscillations, non-standard effects, solar models, cross section measurements, and the broad experimental program thus motivated and enabled. We discuss the historical discoveries that contributed to current knowledge, and define critical open questions to be addressed in the next decade. We discuss the state of the art of standard solar models, including uncertainties and problems related to the solar composition, and review experimental and model solar neutrino fluxes, including future prospects. We review the state of the art of the nuclear reaction data relevant for solar fusion in the proton-proton chain and carbon-nitrogen-oxygen cycle. Finally, we review the current and future experimental program that can address outstanding questions in this field.

hep-ph↗

arXiv2020

The influence of planetary engulfment on stellar rotation in metal-poor main-sequence stars

The method of gyrochronology relates the age of its star to its rotation period. However, recent evidence of deviations from gyrochronology relations was reported in the literature. Here, we study the influence of tidal interaction between a star and its companion on the rotation velocity of the star, in order to explain peculiar stellar rotation velocities. The interaction of a star and its planet is followed using a comprehensive numerical framework that combines tidal friction, magnetic braking, planet migration, and detailed stellar evolution models from the GARSTEC grid. We focus on close-in companions from 1 to 20 M$_{Jup}$ orbiting low-mass, 0.8 and 1 M$_{\odot}$, main-sequence stars with a broad metallicity range from [Fe/H] = -1 to solar. Our simulations suggest that the dynamical interaction between a star and its companion can have different outcomes, which depend on the initial semi-major axis and the mass of the planet, as well as the mass and metallicity of its host star. In most cases, especially in the case of planet engulfment, we find a catastrophic increase in stellar rotation velocity from 1 kms$^{-1}$ to over 40 kms$^{-1}$, while the star is still on the main-sequence. The main prediction of our model is that low-mass main-sequence stars with abnormal rotation velocities should be more common at low-metallicity, as lower [Fe/H] favours faster planet engulfment, provided occurrence rate of close in massive planets is similar at all metallicities. Our scenario explains peculiar rotation velocities of low-mass main-sequence stars by the tidal interaction between the star and its companion. Current observational samples are too small and incomplete, and thus do not allow us to test our model.

astro-ph.SR↗

arXiv2020

The solar abundance problem and eMSTOs in clusters

We study the impact of accretion from protoplanetary discs on stellar evolution of AFG-type stars. We use a simplified disc model computed using the Two-Pop-Py code that contains the growth and drift of dust particles in the protoplanetary disc. It is used to model the accretion scenarios for a range of physical conditions of protoplanetary discs. Two limiting cases are combined with the evolution of stellar convective envelopes computed using the Garstec stellar evolution code. We find that the accretion of metal-poor (gas) or metal-rich (dust) material has a significant impact on the chemical composition of the stellar convective envelope. As a consequence, the evolutionary track of the star diverts from the standard scenario predicted by canonical stellar evolution models, which assume a constant and homogeneous chemical composition after the assembly of the star has finished. In the case of the Sun, we find a modest impact on the solar chemical composition. Accretion of metal-poor material indeed reduces the overall metallicity of the solar atmosphere, and it is consistent, within the uncertainty, with the solar Z reported by Caffau et al. (2011), but our model is not consistent with the measurement by Asplund et al. (2009). Another effect is the change of the position of the star in the colour-magnitude diagram. We compare our predictions to a set of open clusters from the Gaia DR2 and show that it is possible to produce a scatter close to the turn-off of young clusters that could contribute to explain the observed scatter in CMDs. Detailed measurements of metallicities and abundances in the nearby open clusters will provide a stringent observational test of our proposed scenario.

astro-ph.SR↗

arXiv2020

Tess asteroseismology of the known planet host star $λ^2$ Fornacis

The Transiting Exoplanet Survey Satellite (TESS) is observing bright known planet-host stars across almost the entire sky. These stars have been subject to extensive ground-based observations, providing a large number of radial velocity (RV) measurements. In this work we use the new TESS photometric observations to characterize the star $λ^2$ Fornacis, and following this to update the parameters of the orbiting planet $λ^2$ For b. We measure the p-mode oscillation frequencies in $λ^2$ For, and in combination with non-seismic parameters estimate the stellar fundamental properties using stellar models. Using the revised stellar properties and a time series of archival RV data from the UCLES, HIRES and HARPS instruments spanning almost 20 years, we refit the orbit of $λ^2$ For b and search the RV residuals for remaining variability. We find that $λ^2$ For has a mass of $1.16\pm0.03$M$_\odot$ and a radius of $1.63\pm0.04$R$_\odot$, with an age of $6.3\pm0.9$Gyr. This and the updated RV measurements suggest a mass of $λ^2$ For b of $16.8^{+1.2}_{-1.3}$M$_\oplus$, which is $\sim5$M$_\oplus$ less than literature estimates. We also detect a periodicity at 33 days in the RV measurements, which is likely due to the rotation of the host star. While previous literature estimates of the properties of $λ^2$ are ambiguous, the asteroseismic measurements place the star firmly at the early stage of its subgiant evolutionary phase. Typically only short time series of photometric data are available from TESS, but by using asteroseismology it is still possible to provide tight constraints on the properties of bright stars that until now have only been observed from the ground. This prompts a reexamination of archival RV data from the past few decades to update the characteristics of the planet hosting systems observed by TESS for which asteroseismology is possible.

astro-ph.SR↗

arXiv2020

An updated discussion of the solar abundance problem

We discuss the level of agreement of a new generation of standard solar models (SSMs), Barcelona 2016 or B16 for short, with helioseismic and solar neutrino data, confirming that models implementing the AGSS09met surface abundances, based on refined three-dimensional hydrodynamical simulations of the solar atmosphere, do not not reproduce helioseismic constraints. We clarify that this solar abundance problem can be equally solved by a change of the composition and/or of the opacity of the solar plasma, since effects produced by variations of metal abundances are equivalent to those produced by suitable modifications of the solar opacity profile. We discuss the importance of neutrinos produced in the CNO cycle for removing the composition-opacity degeneracy and the perspectives for their future detection.

astro-ph.SR↗

arXiv2020

The Aarhus red giants challenge II. Stellar oscillations in the red giant branch phase

Context. The large quantity of high-quality asteroseismic data that obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties. Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies. Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences. Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties, in particular for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I; here we illustrate particular examples of this. Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential of the mixed modes of red-giant stars for the diagnostics of stellar interiors.

astro-ph.SR↗

arXiv2019

The Asteroseismic Target List (ATL) for solar-like oscillators observed in 2-minute cadence with the Transiting Exoplanet Survey Satellite (TESS)

We present the target list of solar-type stars to be observed in short-cadence (2-min) for asteroseismology by the NASA Transiting Exoplanet Survey Satellite (TESS) during its 2-year nominal survey mission. The solar-like Asteroseismic Target List (ATL) is comprised of bright, cool main-sequence and subgiant stars and forms part of the larger target list of the TESS Asteroseismic Science Consortium (TASC). The ATL uses Gaia DR2 and the Extended Hipparcos Compilation (XHIP) to derive fundamental stellar properties, calculate detection probabilities and produce a rank-ordered target list. We provide a detailed description of how the ATL was produced and calculate expected yields for solar-like oscillators based on the nominal photometric performance by TESS. We also provide publicly available source code which can be used to reproduce the ATL, thereby enabling comparisons of asteroseismic results from TESS with predictions from synthetic stellar populations.

astro-ph.SR↗

arXiv2018

Discovery of extended main sequence turn offs in Galactic open clusters

By far, the color-magnitude diagrams (CMDs) of Galactic open clusters have been considered proto-types of single stellar populations. By using photometry in ultraviolet and optical bands we discovered that the nearby young cluster NGC6705 (M11) exhibits an extended main-sequence turn off (eMSTO) and a broadened main-sequence (MS). This is the first evidence of multiple stellar populations in a Galactic open cluster. By using high-resolution VLT spectra we provide direct evidence that the multiple sequences along the CMD correspond to stellar populations with different rotation rates. Specifically, the blue MS is formed of slow-rotating stars, while red-MS stars are fast rotators. Moreover, we exploit photometry from Gaia DR2 to show that three Galactic open clusters, namely NGC2099, NGC2360, and NGC2818, exhibit the eMSTO, thus suggesting that it is a common feature among these objects. Our previous work on the Large Magellanic Cloud star cluster NGC1818 shows that slowly and fastly-rotating stars populate the blue and red MS observed in its CMD. The similarities between M11 and the young clusters of the Magellanic Clouds suggest that rotation is responsible for the appearance of multiple populations in the CMDs of both Milky Way open clusters and Magellanic Clouds young clusters.

astro-ph.SR↗

arXiv2018

The first continuous optical monitoring of the transitional millisecond pulsar PSR J1023+0038 with Kepler

We report on the first continuous, 80 day optical monitoring of the transitional millisecond pulsar PSR J1023+0038 carried out in mid-2017 with Kepler in the K2 configuration, when an X-ray subluminous accretion disk was present in the binary. Flares lasting from minutes to 14 hr were observed for 15.6% of the time, which is a larger fraction than previously reported on the basis of X-ray and past optical observations, and more frequently when the companion was at the superior conjunction of the orbit. A sinusoidal modulation at the binary orbital period was also present with an amplitude of ~16%, which varied by a few percent over timescales of days, and with a maximum that took place 890 +/- 85 s earlier than the superior conjunction of the donor. We interpret these phenomena in terms of reprocessing of the X-ray emission by an asymmetrically heated companion star surface and/or a non-axisymmetric outflow possibly launched close to the inner Lagrangian point. Furthermore, the non-flaring average emission varied by up to ~ 40% over a time scale of days in the absence of correspondingly large variations of the irradiating X-ray flux. The latter suggests that the observed changes in the average optical luminosity might be due to variations of the geometry, size, and/or mass accretion rate in the outer regions of the accretion disk.

astro-ph.HE↗

arXiv2017

The Gaia-ESO Survey: Matching Chemo-Dynamical Simulations to Observations of the Milky Way

The typical methodology for comparing simulated galaxies with observational surveys is usually to apply a spatial selection to the simulation to mimic the region of interest covered by a comparable observational survey sample. In this work we compare this approach with a more sophisticated post-processing in which the observational uncertainties and selection effects (photometric, surface gravity and effective temperature) are taken into account. We compare a `solar neighbourhood analogue' region in a model Milky Way-like galaxy simulated with RAMSES-CH with fourth release Gaia-ESO survey data. We find that a simple spatial cut alone is insufficient and that observational uncertainties must be accounted for in the comparison. This is particularly true when the scale of uncertainty is large compared to the dynamic range of the data, e.g. in our comparison, the [Mg/Fe] distribution is affected much more than the more accurately determined [Fe/H] distribution. Despite clear differences in the underlying distributions of elemental abundances between simulation and observation, incorporating scatter to our simulation results to mimic observational uncertainty produces reasonable agreement. The quite complete nature of the Gaia-ESO survey means that the selection function has minimal impact on the distribution of observed age and metal abundances but this would become increasingly more important for surveys with narrower selection functions.

astro-ph.GA↗

arXiv2017

PLATO as it is: a legacy mission for Galactic archaeology

Deciphering the assembly history of the Milky Way is a formidable task, which becomes possible only if one can produce high-resolution chrono-chemo-kinematical maps of the Galaxy. Data from large-scale astrometric and spectroscopic surveys will soon provide us with a well-defined view of the current chemo-kinematical structure of the Milky Way, but will only enable a blurred view on the temporal sequence that led to the present-day Galaxy. As demonstrated by the (ongoing) exploitation of data from the pioneering photometric missions CoRoT, Kepler, and K2, asteroseismology provides the way forward: solar-like oscillating giants are excellent evolutionary clocks thanks to the availability of seismic constraints on their mass and to the tight age-initial-mass relation they adhere to. In this paper we identify five key outstanding questions relating to the formation and evolution of the Milky Way that will need precise and accurate ages for large samples of stars to be addressed, and we identify the requirements in terms of number of targets and the precision on the stellar properties that are needed to tackle such questions. By quantifying the asteroseismic yields expected from PLATO for red-giant stars, we demonstrate that these requirements are within the capabilities of the current instrument design, provided that observations are sufficiently long to identify the evolutionary state and allow robust and precise determination of acoustic-mode frequencies. This will allow us to harvest data of sufficient quality to reach a 10% precision in age. This is a fundamental pre-requisite to then reach the more ambitious goal of a similar level of accuracy, which will only be possible if we have to hand a careful appraisal of systematic uncertainties on age deriving from our limited understanding of stellar physics, a goal which conveniently falls within the main aims of PLATO's core science.

astro-ph.SR↗

arXiv2015

Oscillating red giants observed during Campaign 1 of the Kepler K2 mission: New prospects for galactic archaeology

NASA's re-purposed Kepler mission -- dubbed K2 -- has brought new scientific opportunities that were not anticipated for the original Kepler mission. One science goal that makes optimal use of K2's capabilities, in particular its 360-degree ecliptic field of view, is galactic archaeology -- the study of the evolution of the Galaxy from the fossil stellar record. The thrust of this research is to exploit high-precision, time-resolved photometry from K2 in order to detect oscillations in red giant stars. This asteroseismic information can provide estimates of stellar radius (hence distance), mass and age of vast numbers of stars across the Galaxy. Here we present the initial analysis of a subset of red giants, observed towards the North Galactic Gap, during the mission's first full science campaign. We investigate the feasibility of using K2 data for detecting oscillations in red giants that span a range in apparent magnitude and evolutionary state (hence intrinsic luminosity). We demonstrate that oscillations are detectable for essentially all cool giants within the $\log g$ range $\sim 1.9-3.2$. Our detection is complete down to $\mathit{Kp}\sim 14.5$, which results in a seismic sample with little or no detection bias. This sample is ideally suited to stellar population studies that seek to investigate potential shortcomings of contemporary Galaxy models.

astro-ph.GA↗