arXiv subjects

L. Siess

Publications and source records attributed to L. Siess.

At least 19 recordsLinked to original sources

arXiv2026

Detection of actinides in CEMP-rs stars

The carbon-enhanced metal-poor stars with hybrid enrichments of slow- and rapid neutron-capture elements, the so-called CEMP-rs stars, still raise many questions due to their elusive abundance signatures. In our recent analysis, we found that heavy r process elements are enhanced in these objects and can be explained by the intermediate neutron-capture process (i-process) occurring in low-mass, very low-metallicity asymptotic giant branch (AGB) stars. However, the origin of actinides such as thorium and uranium is typically associated with explosive nucleosynthesis in highly neutron-rich environments, and their detection in stellar spectra remains challenging due to severe line blending from other elements and carbon-bearing molecules. In this work, we investigate the presence of thorium and uranium abundances in a sample of CEMP-rs stars using their high-resolution spectra obtained with the UVES spectrograph mounted on the UT2 (Kueyen) ESO VLT. Thorium is robustly detected in three stars, while uranium remains marginally detected, allowing only upper limits to be derived. Comparison with theoretical i-process nucleosynthesis models demonstrates that the observed abundances can be reproduced within uncertainties, supporting an i-process origin for these elements. This study reports the first detection of actinides in stars confirmed as CEMP-rs stars, providing new constraints on their nucleosynthetic history. Furthermore, these detections provide a potential way to estimate in the future the time elapsed since the proton-ingestion episode in AGB stars using cosmochronometry techniques, and more generally to place lower limits on the ages of the resulting white dwarf remnants.

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arXiv2026

Ages and ZAMS spin distribution of stars in detached eclipsing binaries

Benchmarking fundamental stellar properties is essential for calibrating evolutionary models and establishing empirical relationships between mass, radius, luminosity and related stellar properties. We determine the current ages and initial equatorial velocities at the Zero-Age Main Sequence (ZAMS) for a well-characterized sample of 108 detached main-sequence eclipsing binaries. Evolutionary tracks from the ZAMS to the present are calculated by accounting for tidal interactions, including meridional circulation, and angular momentum loss via stellar winds, under the assumption of circular orbits. System ages are derived by identifying the evolutionary stage at which the calculated radii of both stellar components best match the observed values. While initial velocities for currently synchronized systems cannot be uniquely constrained, as a wide range of initial states naturally converges to synchronisation over time, we are able to determine unique solutions for ZAMS spin velocities in systems that remain asynchronous today. Our models successfully reproduce observed present-day equatorial velocities with a precision of 1\%. Two systems, HD~71636 and V396~Cas, were found to have primaries with initially retrograde spin at ZAMS. We also find an increasing dispersion of spin velocities with age. Our results demonstrate that tidal and evolutionary effects in binary systems actively counteract rotational deceleration from stellar winds, effectively preventing the substantial spin-down that typically characterizes the evolution of isolated single stars.

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arXiv2026

Stellar nucleosynthesis in the era of large surveys: S-process polluted binaries in GALAH DR4

Binary interactions during the AGB phase can lead to the formation of chemically peculiar stars with overabundances of s-process elements. Only a few hundreds of these stars have been subject to detailed chemical or dynamical studies. This work aims at compiling a systematic sample of s-process-polluted candidates using GALAH DR4. We also want to compare their properties with those of confirmed s-polluted stars to have stronger evidence of their nature. GALAH DR4 uses neural networks and automatic spectral analysis methods as well as data of a lower spectral resolution than normally used to characterise these objects. Because of this, we built a validation sample, for which we obtained UVES@VLT and HERMES@Mercator high-resolution spectra. We compare our stellar parameters and abundances with those of the survey and use this validation to define the thresholds that a star in GALAH DR4 must pass to be flagged as a good s-process-rich candidate. Based on our comparisons, we define thresholds on [s/Fe], [Y/Fe], [Zr/Fe], [Ba/Fe], and [La/Fe]. We identified 1059 stars in GALAH DR4 that are good candidates to be s-process polluted stars, covering a broad parameter space. They share many similarities with the samples of confirmed s-rich stars, especially their ratios of heavy over light s-elements ([hs/ls]), which strengthen our confidence in the purity of the sample. We find that only 7% of the candidates have measured orbital periods and eccentricities, limiting for now a full comparison with confirmed Ba and related stars. However, their binary fraction is, as expected, higher than the one we found for the full GALAH DR4 catalogue. Our sample of candidates is almost five times larger than the number of currently confirmed polluted stars. This and the fact that it has been homogeneously treated by GALAH open very interesting avenues to confront nucleosynthesis and binary evolution models.

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arXiv2026

The intermediate neutron capture process. VI. Proton ingestion and i-process in rotating magnetic asymptotic giant branch stars

The intermediate neutron-capture process (i-process) can occur during proton ingestion events (PIEs), which may take place in the early evolutionary phases of asymptotic giant branch (AGB) stars. We investigate the impact of rotational and magnetic mixing on i-process nucleosynthesis in low-metallicity, low-mass AGB stars. We computed AGB models with [Fe/H] = $-2.5$ and $-1.7$ and initial masses of 1 and 1.5 $M_{\odot}$ using the STAREVOL code, including a network of 1160 nuclei coupled to transport equations. Rotating models incorporate a calibrated Tayler-Spruit (TS) dynamo to account for core rotation rates inferred from asteroseismic observations of solar-metallicity sub-giants and giants. Initial rotation velocities of 0, 30, and 90 km s$^{-1}$ were considered, along with varying assumptions for magnetic mixing. We find that rotation without magnetic fields strongly suppresses the i-process due to the production of primary $^{14}$N, which is subsequently converted into $^{22}$Ne $-$ a potent neutron poison during the PIE. Including magnetic fields via the TS dynamo restores the models close to their non-rotating counterparts: strong core-envelope coupling suppresses shear mixing and prevents primary $^{14}$N synthesis, yielding i-process nucleosynthesis similar to non-rotating models. We also find that rotational mixing during the AGB phase is insufficient to affect the occurrence of PIEs. Proton ingestion event-driven nucleosynthesis proceeds similarly in asteroseismic-calibrated magnetic rotating AGB stars and non-rotating stars, producing identical abundance patterns.

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arXiv2025

Abundance of heavy r-process elements in CEMP-rs stars: The role of the i-process

Carbon-enhanced metal-poor (CEMP) stars are ancient stars enriched in carbon and heavy elements. Some of these stars exhibit enhanced s-process and/or r-process elements, hence are classified as CEMP-s, CEMP-rs, or CEMP-r. This classification is challenging due to the limited availability of heavy element abundances, particularly among r-process elements. Heavy r-process elements such as terbium, holmium, thulium, ytterbium, lutetium, tantalum, and iridium have rarely been measured because their sensitive lines are located in the ultraviolet. However, they provide sensitive diagnostics of the s-, r-, and i- nucleosynthetic processes. In this work, we aim to obtain a secure classification of CEMP-s and -rs stars and investigate whether the i-process can account for the measured abundance patterns in CEMP-rs stars. We derive the abundance profiles, notably for twelve heavy r-elements, including, in some cases, tantalum, using high-resolution UVES spectra of seventeen CEMP-s and -rs stars. Based on indicators such as the [s/r] abundance ratio or the model-independent 'abundance distance', nine stars are confirmed as CEMP-rs and six as CEMP-s. The classification of two objects remains uncertain. The i-process satisfactorily reproduces the abundance patterns of CEMP-rs stars. However, larger samples are needed to confirm trends with metallicity and clarify how CEMP-rs stars differ from CEMP-s stars.

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arXiv2025

Statistical framework for nuclear parameter uncertainties in nucleosynthesis modeling of r- and i-process

Propagating nuclear uncertainties to nucleosynthesis simulations is key to understand the impact of theoretical uncertainties on the predictions, especially for processes far from the stability region, where nuclear properties are scarcely known. While systematic (model) uncertainties have been thoroughly studied, the statistical (parameter) ones have been more rarely explored, as constraining them is more challenging. We present here a methodology to determine coherently parameter uncertainties by anchoring the theoretical uncertainties to the experimentally known nuclear properties through the use of the Backward Forward Monte Carlo method. We use this methodology for two nucleosynthesis processes: the intermediate neutron capture process (i-process) and the rapid neutron capture process (r-process). We determine coherently for the i-process the uncertainties from the (n,$\gamma$) rates while we explore the impact of nuclear mass uncertainties for the r-process. The effect of parameter uncertainties on the final nucleosynthesis is in the same order as model uncertainties, suggesting the crucial need for more experimental constraints on key nuclei of interest. We show how key nuclear properties, such as relevant (n,$\gamma$) rates impacting the i-process tracers, could enhance tremendously the prediction of stellar evolution models by experimentally constraining them.

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arXiv2025

An accreting dwarf star orbiting the S-type giant star pi1 Gru

Aims. We aim to characterize the properties of the inner companion of the S-type AGB star pi1 Gru and to identify plausible future evolutionary scenarios for this triple system. Methods. We observed pi1 Gru with ALMA and VLT/SPHERE. In addition, we collected archival photometry data and used the Hipparcos-Gaia proper motion anomaly. We derive the best orbital parameters from Bayesian inference. Results. In June-July 2019, the inner companion, pi1 Gru C, was located at 37.4 +/- 2.0 mas from the primary (a projected separation of 6.05 +/- 0.55 au at 161.7 +/- 11.7 pc). The best orbital solution yields a companion mass of 0.86 (+0.22/-0.20) Msun (using the derived mass of the primary) and a semi-major axis of 7.05(+0.54/-0.57) au, corresponding to an orbital period of 11.0 (+1.7/-1.5) yr. The preferred solution is an elliptical orbit with eccentricity e = 0.35(+0.18/-0.17), although a circular orbit cannot be fully excluded. The close companion could be either a K1V (F9.5V to K7V) star or a white dwarf (WD). Ultraviolet and millimeter continuum photometry are consistent with the presence of an accretion disk around the close companion. The ultraviolet emission may originate from hot spots in an overall cooler disk, or from a hot disk if the companion is a WD. Conclusions. Although the close companion and the AGB star are interacting and an accretion disk is observed around the companion, the mass-accretion rate is too low to trigger a Type Ia supernova, but it could produce novae every ~900 yr. Short-wavelength, spatially resolved observations are required to further constrain the nature of the C companion. Searches for close-in companions similar to this system will improve our understanding of the physics of mass and angular momentum transfer, as well as orbital evolution during late evolutionary stages.

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arXiv2025

Formation of Ba stars : impact of wind Roche lobe overflow and circumbinary disk in shaping the orbital parameters

After more than three decades of investigation, the distribution of Ba stars in the e-log P diagram still defies our understanding. Recent smooth particle hydrodynamic simulations involving an asymptotic giant branch (AGB) primary have shown that a circumbinary disk (CB) can form around the binary and that the presence of dust in the wind of evolved low- and intermediate-mass stars can significantly affect the systemic angular momentum loss and mass accretion onto the companion through the wind Roche lobe overflow (WRLOF) phase. We used the binary evolution code BINSTAR, where we updated the modeling of the progenitors of Ba stars including a CB disk, the WRLOF, tidally enhanced wind mass loss, and non-conservative RLOF with their effects on the orbital evolution. In our approach, we considered that a CB disk forms when WRLOF is activated. The coupling between the CB disk and the binary follows the standard resonant interaction theory. We constructed grids of 2.0 + 1.0 $M_\odot{}$ and 1.2 + 0.8 $M_\odot{}$ binaries for initial orbital parameters that result in WRLOF, and evolved these systems until the end of the primary's AGB phase. WRLOF resulted in a significant shrinkage of the orbital separation during the AGB phase, leading to binaries with initial periods on the order of $\lesssim 12000$ d undergoing Roche lobe overflow (RLOF). The combination of WRLOF, eccentricity pumping from the CB disk, and/or tidally enhanced wind mass loss can lead to RLOF on eccentric orbits down to periods of $P_\mathrm{orb} \sim 3000$d. Non-conservative RLOF enabled a reduction of the period before circularization down to $\sim 2000$d, provided at least 50 percent of the transferred mass left the system. Our models still cannot account for the eccentricity distribution of Ba stars with periods shorter than $P_\mathrm{orb} \lesssim 2000$d, where a common envelope evolution appears unavoidable.

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arXiv2025

ATOMIUM: Continuum emission and evidence of dust enhancement from binary motion

Low- and intermediate-mass stars on the asymptotic giant branch (AGB) account for a significant portion of the dust and chemical enrichment in their host galaxy. Here we present ALMA observations of the continuum emission at 1.24 mm around a sample of 17 stars from the ATOMIUM survey. From our analysis of the stellar contributions to the continuum flux, we find that the semi-regular variables all have smaller physical radii and fainter monochromatic luminosities than the Mira variables. Comparing these properties with pulsation periods, we find a positive trend between stellar radius and period only for the Mira variables with periods above 300 days and a positive trend between the period and the monochromatic luminosity only for the red supergiants and the most extreme AGB stars with periods above 500 days. We find that the continuum emission at 1.2 mm can be classified into four groups. "Featureless" continuum emission is confined to the (unresolved) regions close to the star for five stars in our sample, relatively uniform extended flux is seen for four stars, tentative elongated features are seen for three stars, and the remaining five stars have unique or unusual morphological features in their continuum maps. These features can be explained by binary companions to 10 out of the 14 AGB stars in our sample. Based on our results we conclude that there are two modes of dust formation: well established pulsation-enhanced dust formation and our newly proposed companion-enhanced dust formation. If the companion is located close to the AGB star, in the wind acceleration region, then additional dust formed in the wake of the companion can increase the mass lost through the dust driven wind. This explains the different dust morphologies seen around our stars and partly accounts for a large scatter in literature mass-loss rates, especially among semiregular stars with small pulsation periods.

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arXiv2024

Production of Lithium and Heavy Elements in AGB Stars Experiencing PIEs

Asymptotic giant branch (AGB) stars can experience proton ingestion events (PIEs), leading to a rich nucleosynthesis. During a PIE, the intermediate neutron capture process (i-process) develops, leading to the production of trans-iron elements. It is also suggested that lithium is produced during these events. We investigate the production of lithium and trans-iron elements in AGB stars experiencing a PIE with $1<M_{\rm ini}/M_{\odot}< 3$ and $-3< \mathrm{[Fe/H]} <0$. We find that lithium is produced in all PIE models with surface abundances $3<$ A(Li) $<5$. The surface enrichment and overall AGB lithium yield increases with decreasing stellar mass. The lithium enrichment is accompanied by a production of $^{13}$C with $3<^{12}$C/$^{13}$C$<9$ at the surface just after the PIE. AGB stars experiencing PIE may be related to J-type carbon stars whose main features are excesses of lithium and $^{13}$C. In addition to Li and $^{13}$C, heavy elements (e.g., Sr, Ba, Eu, Pb) are significantly produced in low-metallicity stars up to [Fe/H]$\simeq-1$. The yields of our models are publicly available. Additionally, of interest to the Li nucleosynthesis, we provide an updated fitting formula for the $^{7}$Be($e^-,\nu_e$)$^{7}$Li electron capture rate.

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arXiv2024

Proton ingestion in asymptotic giant branch stars as a possible explanation for J-type stars and AB2 grains

J-type stars are a subclass of carbon stars that are generally Li-rich, not enriched in s-elements, and have low $^{12}$C/$^{13}$C ratios. They were suggested to be the manufacturers of the pre-solar grains of type AB2 (having low $^{12}$C/$^{13}$C and supersolar $^{14}$N/$^{15}$N). In this Letter, we investigate the possibility that J-type stars are early asymptotic giant branch (AGB) stars that experienced a proton ingestion event (PIE). We used the stellar evolution code STAREVOL to compute AGB stellar models with initial masses of 1, 2, and 3 $M_{\odot}$ and metallicities [Fe/H] $= -0.5$ and 0.0. We included overshooting above the thermal pulse and used a network of 1160 nuclei coupled to the transport equations. In solar-metallicity AGB stars, PIEs can be triggered if a sufficiently high overshoot is considered. These events lead to low $^{12}$C/$^{13}$C ratios, high Li abundances, and no enrichment in s-elements. We find that the $2-3$ $M_{\odot}$ AGB models experiencing a PIE can account for most of the observational features of J-type stars and AB2 grains. The remaining tensions between models and observations are (1) the low $^{14}$N/$^{15}$N ratio of some AB2 grains and of 2 out of 13 J-type stars, (2) the high $^{26}$Al/$^{27}$Al of some AB2 grains, and (3) the J-type stars with A(Li) $<2$. Extra mixing mechanisms can alleviate some of these tensions, such as thermohaline or rotation. This work highlights a possible match between AGB stellar models that undergo a PIE and J-type stars and AB2 grains. To account for other types of carbon stars, such as N-type stars, PIEs should only develop in a fraction of solar-metallicity AGB stars. Additional work is needed to assess how the occurrence of PIEs depends on mixing parameters and initial conditions, and therefore to further confirm or exclude the proposed scenario.

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arXiv2024

An impressionist view of V Hydrae. When MATISSE paints Asymmetric Giant Blobs

Our purpose is to study the effect of binary companions located within the first 10 stellar radii from the primary AGB star. In this work, we target the mass-losing carbon star V Hydrae (V Hya), looking for signatures of its companion in the dust forming region of the atmosphere. The star was observed in the L- and N-bands with the VLTI/MATISSE instrument at low spectral resolution. We reconstructed images of V Hya's photosphere and surroundings using the two bands and compared our interferometric observables with VLTI/MIDI and VISIR archival data. To constrain the dust properties, we used DUSTY to model the spectral energy distribution. The star is dominated by dust emission in the L- and N- bands. The VISIR image confirms the presence of a large-scale dusty circumstellar envelope surrounding V Hya. The MATISSE reconstructed images show asymmetric and elongated structures in both infrared bands. In the L-band, we detected an elongated shape of approximately 15 mas, likely to be of photospheric origin. In the N-band, we found a 20 mas extension North-East from the star, and perpendicular to the L-band elongated axis. The position angle and the size of the N-band extension match the prediction of the companion position at MATISSE epoch. By comparing MATISSE N-band with MIDI data, we deduce that the elongation axis in the N-band has rotated since the previous interferometric measurements 13 years ago, supporting the idea that the particle enhancement is related to the dusty clump moving along with the companion. The MATISSE images unveil the presence of a dust enhancement at the companion position, opening new doors for further analysis on the binary interaction with an AGB component.

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arXiv2024

The intermediate neutron capture process. V. The i-process in AGB stars with overshoot

The intermediate neutron capture process (i-process) can develop during proton ingestion events (PIE), potentially during the early stages of low-mass low-metallicity asymptotic giant branch (AGB) stars. We examine the impact of overshoot mixing on the triggering and development of i-process nucleosynthesis in AGB stars of various initial masses and metallicities. We computed AGB stellar models, with initial masses of 1, 2, 3, and 4 M$_{\odot}$ and metallicities in the $-2.5 \le $ [Fe/H] $\le 0$ range, using the stellar evolution code STAREVOL with a network of 1160 nuclei coupled to the transport equations. We considered different overshooting profiles below and above the thermal pulses, and below the convective envelope. The occurrence of PIEs is found to be primarily governed by the amount of overshooting at the top of pulse ($f_{\rm top}$) and to increase with rising $f_{\rm top}$. For $f_{\rm top} =$ 0, 0.02, 0.04, and 0.1, we find that 0 %, 6 %, 24 %, and 86 % of our 21 AGB models with $-2<$ [Fe/H] $<0$ experience a PIE, respectively. We also find that PIEs leave a $^{13}$C-pocket at the bottom of the pulse that can give rise to an additional radiative s-process nucleosynthesis, and ultimately produce a noticeable mixed i+s chemical signature at the surface. Finally, the chemical abundance patterns of 22 observed r/s-stars candidates with $-2<$ [Fe/H] $<-1$ are found to be in reasonable agreement with our AGB model predictions. The binary status of the dwarfs/giants being unclear, we suggest that these stars have acquired their chemical pattern either from the mass transfer of a now-extinct AGB companion or from an early generation AGB star that polluted the natal cloud. Stricter constraints from multi-dimensional hydrodynamical models on overshoot coefficients could deliver new insights into the contribution of AGB stars to heavy elements in the Universe.

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arXiv2023

Systematic study of the low-lying electric dipole strength in Sn isotopes and its astrophysical implications

The $\gamma$-ray strength functions (GSF) and nuclear level densities (NLD) below the neutron threshold have been extracted for $^{111-113,116-122,124}$Sn from particle-$\gamma$ coincidence data with the Oslo method. The evolution of bulk properties of the low-lying electric dipole response has been investigated on the basis of the Oslo GSF data and results of a recent systematic study of electric and magnetic dipole strengths in even-even Sn isotopes with relativistic Coulomb excitation. The obtained GSFs reveal a resonance-like peak on top of the tail of the isovector giant dipole resonance, centered at $\approx$8 MeV and exhausting $\approx$2\% of the classical Thomas-Reiche-Kuhn (TRK) sum. In contrast to predictions of the relativistic quasiparticle random-phase and time-blocking approximation calculations (RQRPA and RQTBA), no monotonous increase in the total low-lying $E1$ strength was observed in the experimental data from $^{111}$Sn to $^{124}$Sn, demonstrating rather similar strength distributions in these nuclei. The Oslo GSFs and NLDs were further used as inputs to constrain the cross sections and Maxwellian-averaged cross sections of $(n,\gamma)$ reactions in the Sn isotopic chain using TALYS. The obtained results agree well with other available experimental data and the recommended values from the JINA REACLIB, BRUSLIB, and KADoNiS libraries. Despite relatively small exhausted fractions of the TRK sum rule, the low-lying electric dipole strength makes a noticeable impact on the radiative neutron-capture cross sections in stable Sn isotopes. Moreover, the experimental Oslo inputs for the $^{121,123}$Sn$(n,\gamma)$$^{122,124}$Sn reactions were found to affect the production of Sb in the astrophysical $i$-process, providing new constraints on the uncertainties of the resulting chemical abundances from multi-zone low-metallicity Asymptotic Giant Branch stellar models.

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arXiv2023

MELCHIORS: The Mercator Library of High Resolution Stellar Spectroscopy

Over the past decades, libraries of stellar spectra have been used in a large variety of science cases, including as sources of reference spectra for a given object or a given spectral type. Despite the existence of large libraries and the increasing number of projects of large-scale spectral surveys, there is to date only one very high-resolution spectral library offering spectra from a few hundred objects from the southern hemisphere (UVES-POP) . We aim to extend the sample, offering a finer coverage of effective temperatures and surface gravity with a uniform collection of spectra obtained in the northern hemisphere. Between 2010 and 2020, we acquired several thousand echelle spectra of bright stars with the Mercator-HERMES spectrograph located in the Roque de Los Muchachos Observatory in La Palma, whose pipeline offers high-quality data reduction products. We have also developed methods to correct for the instrumental response in order to approach the true shape of the spectral continuum. Additionally, we have devised a normalisation process to provide a homogeneous normalisation of the full spectral range for most of the objects. We present a new spectral library consisting of 3256 spectra covering 2043 stars. It combines high signal-to-noise and high spectral resolution over the entire range of effective temperatures and luminosity classes. The spectra are presented in four versions: raw, corrected from the instrumental response, with and without correction from the atmospheric molecular absorption, and normalised (including the telluric correction).

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arXiv2023

The intermediate neutron capture process: IV. Impact of nuclear model and parameter uncertainties

We investigate both the systematic and statistical uncertainties associated with theoretical nuclear reaction rates of relevance during the i-process and explore their impact on the i-process elemental production, and subsequently on the surface enrichment, for a low-mass low-metallicity star during the early AGB phase. We use the TALYS reaction code (Koning et al. 2023) to estimate both the model and parameter uncertainties affecting the photon strength function and the nuclear level densities, hence the radiative neutron capture rates. The STAREVOL code (Siess et al. 2006) is used to determine the impact of nuclear uncertainties on the i-process nucleosynthesis in a 1 $M_{\odot}$ [Fe/H] = - 2.5 model star during the proton ingestion event in the early AGB phase. A large nuclear network of 1160 species coherently coupled to the transport processes is solved to follow the i-process nucleosynthesis. We find that the non-correlated parameter uncertainties lead the surface abundances uncertainties of element with $Z\geq 40$ to range between 0.5 and 1.0 dex, with odd-$Z$ elements displaying higher uncertainties. The correlated model uncertainties are of the same order of magnitude, and both model and parameter uncertainties have an important impact on potential observable tracers such as Eu and La. Both the correlated model and uncorrelated parameter uncertainties need to be estimated coherently before being propagated to astrophysical observables through multi-zone stellar evolution models. Many reactions are found to affect the i-process predictions and will require improved nuclear models guided by experimental constraints. Priority should be given to the reactions influencing the observable tracers.

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arXiv2023

Nuclear Level Density and $\gamma$-ray Strength Function of $^{67}\mathrm{Ni}$ and the impact on the i-process

Proton-$\gamma$ coincidences from $(\mathrm{d},\mathrm{p})$ reactions between a $^{66}\mathrm{Ni}$ beam and a deuterated polyethylene target have been analyzed with the inverse-Oslo method to find the nuclear level density (NLD) and $\gamma$-ray strength function ($\gamma$SF) of $^{67}\mathrm{Ni}$. The $^{66}\mathrm{Ni}(n,\gamma)$ capture cross section has been calculated using the Hauser-Feshbach model in TALYS using the measured NLD and $\gamma$SF as constraints. The results confirm that the $^{66}\mathrm{Ni}(n,\gamma)$ reaction acts as a bottleneck when relying on one-zone nucleosynthesis calculations. However, the impact of this reaction is strongly dampened in multi-zone models of low-metallicity AGB stars experiencing i-process nucleosynthesis.

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arXiv2023

Does the i-process operate at nearly solar metallicity?

A sample of 895 s-process-rich candidates has been found among the 454180 giant stars surveyed by LAMOST at low spectral resolution (R~1800). In a previous study, taking advantage of the higher resolution (R~86 000) offered by the the HERMES-Mercator spectrograph, we performed the re-analysis of 15 among the brightest stars of this sample. Among these 15 program stars, having close-to-solar metallicities, 11 showed mild to strong heavy element overabundances. The nucleosynthesis process(es) at the origin of these overabundances were however not questioned in our former study. We derive the abundances in s- and r-process elements of the 15 targets in order to investigate whether some stars also show an i-process signature, as sometimes found in their lower metallicity counterparts (namely, the Carbon-Enhanced Metal-Poor (CEMP)-rs stars). Abundances are derived from the high-resolution HERMES spectra for Pr, Nd, Sm, and Eu, using the TURBOSPECTRUM radiative transfer LTE code with MARCS model atmospheres. Using the new classification scheme proposed in our recent study we find that two stars show overabundances in both s- and r-process elements well above the level expected from the Galactic chemical evolution, an analogous situation to the one of CEMP-rs stars at lower metallicities. We compare the abundances of the most enriched stars with the nucleosynthetic predictions from the STAREVOL stellar evolutionary code and find abundances compatible with an i-process occurring in AGB stars. Despite a larger number of heavy elements to characterize the enrichment pattern, the limit between CEMP-s and CEMP-rs stars remains fuzzy. It is however interesting to note that an increasing number of extrinsic stars are found to have abundances better reproduced by an i-process pattern even at close-to-solar metallicities.

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