arXiv subjects

Luciano Piersanti

Publications and source records attributed to Luciano Piersanti.

At least 19 recordsLinked to original sources

arXiv2026

Observational Signatures and Constraints on the Intermediate Neutron-Capture Process. The Case of the CEMP star TYC 6044-714-1 (RAVE J094921.8-161722)

Observational abundances of CEMP stars with patterns in between those produced by the rapid and slow nucleosynthesis processes (CEMP-rs stars) are currently invoked as evidence of synthesis via the intermediate process in the early AGB evolutionary phase of metal-poor low mass stars. Nevertheless, discriminating between r+s- and i-process hypotheses requires high-precision abundances obtained through advanced spectral modelling techniques. Theoretical models of the i-process have become more robust, incorporating refined stellar modelling and nuclear reaction physics, providing ranges of probable elemental abundances and isotopic ratios predictions to be confronted with observational determinations. We performed a new analysis of a high resolution and high S/N UVES spectrum of TYC 6044-714-1. We derived accurate effective temperature and highly precise atmospheric parameters, element abundances, and isotopic ratios using state-of-the-art 1D non-LTE and 3D non-LTE spectral line modelling. Using the latest AGB nucleosynthesis models, we assessed the possibility of the i-process to act aside the s-process. We find that TYC~6044-714-1 was likely born as a normal in-situ halo star about 13 Gyr ago, pre-enriched by the r-process through a standard Galactic chemical-evolution pathway. The s+r model provides the best overall reproduction of the observed heavy-element abundance pattern and Ba isotopic ratios, yielding excellent agreement across all three s-process peaks. While i+s+r models with increasing overshooting efficiency improve the fit for specific elements, they do not consistently reproduce the full abundance pattern. The i+s+r models require extreme and physically implausible conditions, and predict s-process Ba fractions inconsistent with those inferred from isotopic ratios of the 4934 Å resonance line. We conclude that the pure s+r scenario is the most plausible explanation.

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arXiv2025

The Impact of Axion-Like Particles on Late Stellar Evolution From Intermediate-Mass Stars to core-collapse Supernova Progenitors

Context. Stars with masses ranging from 3 to 11 M_\odot exhibit multiple evolutionary paths. Less massive stars in this range conclude their evolution as carbon-oxygen (CO) white dwarfs. However, those that achieve carbon ignition before the pressure by degenerate electron halts the core contraction may either form massive CONe/ONe white dwarfs, or undergo an electron-capture supernova, or photo-disintegrate neon and proceed with further thermonuclear burning, ultimately leading to the formation of a gravitationally unstable iron core. Aims. An evaluation of the impact of the energy loss caused by the production of axion-like-particles (ALPs) on evolution and final destiny of these stars is the main objective of this paper. Methods. We compute various sets of stellar models, all with solar initial composition, varying the strengths of the ALP coupling with photons and electrons. Results. As a consequence of an ALP thermal production, the critical masses for off-center C and Ne ignitions are both shifted upward. When the current bounds for the ALP coupling strengths are assumed, the maximum mass for CO WD progenitors is about 1.1 M_\odot heavier than that obtained without the ALP energy loss, while the minimum mass for a core collapse supernova (CCSN) progenitor is 0.7 M_\odot higher. Conclusions. Current constraints from observed Type II-P supernova light curves and pre-explosive luminosity do not exclude an ALP production within the current bounds. However, the maximum age of CCSN progenitors, as deduced from the star formation rate of the parent stellar population, would require a smaller minimum mass. This discrepancy can be explained by assuming a moderate extra mixing (as due to core overshooting or rotational induced mixing) above the fully convective core that develops during the main sequence.

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arXiv2025

Trans-Fe elements from Type Ia Supernovae. I. Heavy element nucleosynthesis during the formation of near-Chandrasekhar white dwarfs

Type Ia supernovae (SNIa) are thermonuclear explosions of white dwarfs in binary systems. They are central to galactic chemical evolution and serve as standardizable candles in cosmology, yet their progenitors remain uncertain. In this work, we present a grid of five models detailing the evolution and nucleosynthesis of slowly merging carbon-oxygen white dwarfs approaching the Chandrasekhar mass. These models test a variety of physics input settings, including accretion rates, nuclear reaction rates, convection parameters, and the composition of the accreted material. During the merger process, as the mass of the primary white dwarf approaches the Chandrasekhar limit, carbon burning is initiated first on the surface before eventually igniting explosively at the center. As a consequence, the 22Ne(a,n)25Mg reaction activates in the outer layers of all models. The neutrons released in this way produce a weak s-process-like abundance distribution peaking at Kr, which is overproduced by more than a factor of 1000 compared to solar. The trans-Fe elements-enriched outer layer mass varies from 0.04 Msun to 0.11 Msun, depending on the accretion rate. Our explosion simulation of these progenitor models ejects significant amount of first-peak elements (e.g., Kr, Sr) as well as of some long-lived radioactive species, such as 60Fe. In a previous theoretical study, we found that a similar nucleosynthesis process during the progenitor phase may also occur on the surface of near-Chandrasekhar white dwarfs formed through the accretion of H-rich material via the single-degenerate scenario. Therefore, these results suggest trans-Fe enrichment might be a hallmark of near-Chandrasekhar SNIa ejecta, regardless of the specific progenitor channel, and could provide a new spectral signature distinguishing them from sub-Chandrasekhar explosions.

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arXiv2024

The expected evolution of the binary system PTF J2238+743015.1

Binary systems made by a low-mass CO WD and a He-donor represent possible progenitors of explosive events via He-detonation, producing low-luminosity thermonuclear Supernovae with a peculiar nucleosynthetis. Recently, the binary system PTF J223857.11+743015.1 has been suggested as one. We investigate the evolution of the PTF J223857.11+743015.1 system, composed by a 0.75Msun CO WD and a 0.390Msun subdwarf, capped by a thin H-rich layer, considering rotation of the WD component. We compute the evolution of two stars simultaneously, accounting for the possible evolution of the orbital parameters, as determined by mass transfer between components and by mass ejection from the system during RLOF episodes. We consider that the WD gains angular momentum due to accretion and we follow the evolution of the angular velocity profile as due to angular momentum transport via convection and rotation-induced instabilities. As the donor H-rich envelope is transferred, the WD experiences recurrent very strong H-flashes triggering RLOF episodes during which the entire accreted matter is lost from the system. Due to mixing of chemicals by rotation-induced instabilities during the accretion phase, H-flashes occur inside the original WD. Hence, pulse-by pulse, the accretor mass is reduced down to 0.7453Msun. When He-rich matter is transferred, He-detonation does not occur in the rotating WD, which undergoes 6 very strong He-flashes and subsequent RLOF episodes. Also in this case, due to rotation-induced mixing of the accreted layers with the underlying core, the WD is eroded. Finally, when the mass transfer rate from the donor decreases, a massive He-buffer is piled-up onto the accretor which ends its life as a cooling WD. The binary system PTF J2238+743015.1 as all those binaries having similar components masses and orbital parameters are not good candidates as thermonuclear explosions progenitors.

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arXiv2022

Chandrasekhar-mass white dwarfs are the progenitors of a small fraction of Type Ia supernovae according to nucleosythesis constraints

The precise progenitor system of type Ia supernovae (SNe Ia), whether it is a white dwarf (WD) close to the Chandrasekhar limit or substantially less massive, has been a matter of debate for decades. Recent research by our group on the accretion and simmering phases preceding the explosion of a massive WD has shown that the central density at thermal runaway lies in the range $(3.6-6.3)\times10^9$ g cm$^{-3}$ for reasonable choices of accretion rate onto the WD and progenitor metallicity. In this work, we have computed one-dimensional simulations of the explosion of such WDs, with special emphasis on the chemical composition of the ejecta, which in all cases is extremely rich in neutronized isotopes of chromium ($^{54}$Cr) and titanium ($^{50}$Ti). We show that, in order to reconcile such a nucleosynthesis with the isotopic abundances of the Solar System, Chandrasekhar-mass white dwarfs can account for at most 26 per cent of normal-luminosity SNe Ia, or at most 20 per cent of all SNe Ia.

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arXiv2022

Pre-explosive accretion and simmering phases of Type Ia Supernovae

In accreting WDs approaching the Chandrasekhar limit, hydrostatic carbon burning precedes the dynamical breakout. During this \textit{simmering} phase, $e-$captures are energetically favored in the central region of the star, while $β-$decays are favored more outside, and the two zones are connected by a growing convective instability. We analyze the interplay between weak interactions and convection, the so-called convective URCA process, during the simmering phase of SNe Ia progenitors and its effects on the physical and chemical properties at the explosion epoch. At variance with previous studies, we find that the convective core powered by the carbon burning remains confined within the ${^{21}(Ne,F)}$ URCA shell. As a result, a much larger amount of carbon has to be consumed before the explosion which eventually occurs at larger density than previously estimated. In addition, we find that the extension of the convective core and its average neutronization depend on the the WD progenitor initial metallicity. For the average neutronization in the convective core at the explosion epoch we obtain ${\overlineη_{exp}} = (1.094\pm 0.143)\times 10^{-3} + (9.168\pm 0.677)\times 10^{-2}\times Z$. Outside the convective core, the neutronization is instead determined by the initial amount of C+N+O in the progenitor star. Since S, Ca, Cr and Mn, the elements usually exploited to evaluate the pre-explosive neutronization, are mainly produced outside the heavily neutronized core, the problem of too high metallicity estimated for the progenitors of the historical Tycho and Kepler SNe Ia remains unsolved.

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arXiv2020

On the mass and metallicity distribution of parent AGB stars of presolar SiC

The vast majority (>=90%) of presolar SiC grains identified in primitive meteorites are relics of ancient asymptotic giant branch (AGB) stars, whose ejecta were incorporated into the Solar System during its formation. Detailed characterization of these ancient stardust grains has revealed precious information on mixing processes in AGB interiors in great detail. However, the mass and metallicity distribution of their parent stars still remains ambiguous, although such information is crucial to investigating the slow neutron capture process, whose efficiency is mass- and metallicity-dependent. Using a well-known Milky Way chemo-dynamical model, we follow the evolution of the AGB stars that polluted the Solar System at 4.57 Gyr ago and weighted the stars based on their SiC dust productions. We find that presolar SiC in the Solar System predominantly originated from AGB stars with M~2 Msun and Z~Zsun. Our finding well explains the grain-size distribution of presolar SiC identified in situ in primitive meteorites. Moreover, it provides complementary results to very recent papers dealing with the characterization of parent stars of presolar SiC.

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arXiv2020

The luminosity constraint in the era of precision solar physics

The luminosity constraint is a very precise relationship linking the power released by the Sun as photons and the solar neutrino fluxes. Such a relation, which is a direct consequence of the physical processes controlling the production and the transport of energy in the solar interior, is of great importance for the studies of solar neutrinos and has a special role for the search of neutrinos from the CNO cycle, whose first detection with a 5$σ$ significance has been recently announced by the Borexino collaboration. Here we revise the luminosity constraint, discussing and validating its underlying hypotheses, in the light of latest solar neutrino and luminosity measurements. We generalize the current formulation of the luminosity constraint relation so that it can be easily used in future analysis of solar neutrino data, and we provide a specific application showing the link between CNO and pp neutrino fluxes.

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arXiv2020

A high-precision abundance analysis of the nuclear benchmark star HD 20

We present our chemical abundance investigation of the metal-poor ([Fe/H]=-1.60 dex), r-process-enriched ([Eu/Fe]=0.73 dex) halo star HD 20 using novel and archival high-resolution spectra at outstanding signal-to-noise ratios. By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with non-LTE analyses of iron lines, we derive a set of highly accurate and precise stellar parameters. These allow us to delineate a chemical pattern comprised of solid detections of 48 elements, including 28 neutron-capture elements, which establishes HD 20 among the few benchmark stars that have almost complete patterns with low systematic dependencies on the stellar parameters. Our light-element (Z<30) abundances are representative of other, similarly metal-poor stars in the Galactic halo with contributions from core-collapse supernovae of type II. A comparison to the scaled solar r-pattern shows that the lighter neutron-capture elements (37<Z<60) are poorly matched. In particular, we find imprints of the weak r-process acting at low metallicities. Nonetheless, by comparing our detailed abundances to the observed metal-poor star BD +17 3248, we find a persistent residual pattern that is indicative of enrichment contributions from the s-process. We show that mixing with material from predicted yields of massive, rotating AGB stars at low metallicity considerably improves the fit. Based on a solar ratio of heavy- to light-s elements -- at odds with model predictions for the i-process -- and a missing clear residual pattern with respect to other stars with claimed contributions from this process, we refute (strong) contributions from such astrophysical sites providing intermediate neutron densities. Finally, nuclear cosmochronology is used to tie our detection of the radioactive element Th to an age estimate for HD 20 of $11.0\pm3.8$ Gyr. [abridged]

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arXiv2019

Comprehensive models of novae at metallicity $Z = 0.02$ and $Z = 10^{-4}$

Novae are the observational manifestations of thermonuclear runaways on the surface of accreting white dwarfs (WDs). Although novae are an ubiquitous phenomenon, their properties at low metallicity are not well understood. Using the publicly-available stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we model the evolution of accreting carbon-oxygen WDs and consider models which accrete matter with metallicity Z=0.02 or $10^{-4}$. We consider both models without mixing and with matter enriched by CO-elements assuming that mixing occurs in the process of accretion (with mixing fraction 0.25). We present and contrast ignition mass, ejected mass, recurrence period and maximum luminosity of novae for different WD masses and accretion rates for these metallicities and mixing cases. We find that models with Z = 0.02 have ignition masses and recurrence periods smaller than models with low Z, while the ejected mass and maximum luminosity are larger. Retention efficiency during novae outbursts decreases with increasing metallicity. In our implementation, inclusion of mixing at the H/He interface reduces accreted mass, ejected mass and recurrence period as compared to the no-mixing case, while the maximum luminosity becomes larger. Retention efficiency is significantly reduced, becoming negative in most of our models. For ease of use, we provide a tabular summary of our results.

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arXiv2019

The initial mass-final luminosity relation of type II supernova progenitors. Hints of new physics?

We revise the theoretical initial mass-final luminosity relation for progenitors of type IIP and IIL supernovae. The effects of the major uncertainties, as those due to the treatment of convection, semiconvection, rotation, mass loss, nuclear reaction rates and neutrinos production rates are discussed in some details. The effects of mass transfer between components of close-binary systems are also considered. By comparing the theoretical predictions to a sample of type II supernovae for which the initial mass of the progenitors and the pre-explosive luminosity are available, we conclude that stellar rotation may explain a few progenitors which appear brighter than expected in case of non-rotating models. In the most extreme case, SN2012ec, an initial rotational velocity up to 300 km s$^{-1}$ is required. Alternatively, these objects could be mass-loosing components of close binaries. However, most of the observed progenitors appear fainter than expected. This occurrence seems to indicate that the Compton and pair neutrino energy-loss rates, as predicted by the standard electro-weak theory, are not efficient enough and that an additional negative contribution to the stellar energy balance is required. We show that axions coupled with parameters accessible to currently planned experiments, such as IAXO and, possibly, Baby-IAXO and ALPS II, may account for the missing contribution to the stellar energy-loss.

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arXiv2019

On the mass of supernova progenitors: the role of the $^{12}$C$+^{12}$C reaction

A precise knowledge of the masses of supernova progenitors is essential to answer various questions of modern astrophysics, such as those related to the dynamical and chemical evolution of Galaxies. In this paper we revise the upper bound for the mass of the progenitors of CO white dwarfs (\mup) and the lower bound for the mass of the progenitors of normal type II supernovae (\mups). In particular, we present new stellar models with mass between 7 and 10 \msun, discussing their final destiny and the impact of recent improvements in our understanding of the low energy rate of the \c12c12 reaction.

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arXiv2017

Observational Properties of SNe Ia Progenitors Close to the Explosion

We determine the expected signal in various observational bands of Supernovae Ia progenitors just before the explosion by assuming the rotating Double Degenerate scenario. Our results are valid also for all the evolutionary scenarios invoking rotation as the driving mechanism of the accretion process as well as the evolution up to the explosion. We find that the observational properties depend mainly on the mass of the exploding object, even if the angular momentum evolution after the end of the mass accretion phase and before the onset of C-burning plays a non-negligible role. Just before the explosion the magnitude M_V ranges between 9 and 11 mag, while the colour (F225W-F555W) is about -1.64 mag. The photometric properties remain constant for a few decades before the explosion. During the last few months the luminosity decreases very rapidly. The corresponding decline in the optical bands varies from few hundredths up to one magnitude, the exact value depending on both the WD total mass and the braking efficiency at the end of the mass transfer. This feature is related to the exponentially increasing energy production which drives the formation of a convective core rapidly extending over a large part of the exploding object. Also a drop in the angular velocity occurs. We find that observations in the soft X band (0.5 -2 keV) may be used to check if the SNe Ia progenitors evolution up to explosion is driven by rotation and, hence, to discriminate among different progenitor scenarios.

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arXiv2017

Nucleation of small silicon carbide dust clusters in AGB stars

Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB stars. The formation pathways of these grains are, however, not fully understood.\ We calculate ground states and energetically low-lying structures of (SiC)$_n$, $n=1,16$ clusters by means of simulated annealing (SA) and Monte Carlo simulations of seed structures and subsequent quantum-mechanical calculations on the density functional level of theory. We derive the infrared (IR) spectra of these clusters and compare the IR signatures to observational and laboratory data.\ According to energetic considerations, we evaluate the viability of SiC cluster growth at several densities and temperatures, characterising various locations and evolutionary states in circumstellar envelopes.\ We discover new, energetically low-lying structures for Si$_{4}$C$_{4}$, Si$_{5}$C$_{5}$, Si$_{15}$C$_{15}$ and Si$_{16}$C$_{16}$, and new ground states for Si$_{10}$C$_{10}$ and Si$_{15}$C$_{15}$. The clusters with carbon-segregated substructures tend to be more stable by 4-9 eV than their bulk-like isomers with alternating Si-C bonds. However, we find ground states with cage ("bucky"-like) geometries for Si$_{12}$C$_{12}$ and Si$_{16}$C$_{16}$ and low-lying, stable cage structures for n $\ge$ 12. The latter findings indicate thus a regime of clusters sizes that differs from small clusters as well as from large-scale crystals. Thus, and owing to their stability and geometry, the latter clusters may mark a transition from a quantum-confined cluster regime to crystalline, solid bulk-material. The calculated vibrational IR spectra of the ground-state SiC clusters shows significant emission. They include the 10-13 $μ$m wavelength range and the 11.3 $μ$m feature inferred from laboratory measurements and observations, respectively, though the overall intensities are rather low.

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arXiv2017

Type Ia Supernovae keep memory of their progenitor metallicity

The ultimate understanding of Type Ia Supernovae diversity is one of the most urgent issues to exploit thermonuclear explosions of accreted White Dwarfs (WDs) as cosmological yardsticks. In particular, we investigate the impact of the progenitor system metallicity on the physical and chemical properties of the WD at the explosion epoch. We analyze the evolution of CO WDs through the accretion and simmering phases by using evolutionary models based on time-dependent convective mixing and an extended nuclear network including the most important electron captures, beta decays and URCA processes. We find that, due to URCA processes and electron-captures, the neutron excess and density at which the thermal runaway occurs are substantially larger than previously claimed. Moreover, we find that the higher the progenitor metallicity, the larger the neutron excess variation during the accretion and simmering phases and the higher the central density and the convective velocity at the explosion. Hence, the simmering phase acts as an amplifier of the differences existing in SNe Ia progenitors. When applying our results to the neutron excess estimated for the Tycho and Kepler young Supernova remnants, we derive that the metallicity of the progenitors should be in the range Z=0.030-0.032, close to the average metallicity value of the thin disk of the Milky Way. As the amount of ${^{56}}$Ni produced in the explosion depends on the neutron excess and central density at the thermal runaway, our results suggest that the light curve properties depend on the progenitor metallicity.

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arXiv2015

He-Accreting WDs: AM CVn stars with WD Donors

We study the physical and evolutionary properties of the "WD family" of AM CVn stars by computing realistic models of IDD systems. We evaluate self-consistently both the mass transfer rate from the donor, as determined by GW emission and interaction with the binary companion, and the thermal response of the accretor to mass deposition. We find that, after the onset of mass transfer, all the considered systems undergo a strong non-dynamical He-flash. However, due to the compactness of these systems, the expanding accretors fill their Roche lobe very soon, thus preventing the efficient heating of the external layers of the accreted CO WDs. Moreover, due to the loss of matter from the systems, the orbital separations enlarge and mass transfer comes to a halt. The further evolution depends on the value of $\dot{M}$, after the donors fill again their lobe. On one hand, if the accretion rate, as determined by the actual value of (M$_{don}$,M$_{acc}$), is high enough, the accretors experience several He-flashes of decreasing strength and then quiescent He-burning sets in. Later on, since the mass transfer rate in IDD is a permanently decreasing function of time, accretors experience several recurrent strong flashes. On the other hand, for intermediate and low values of $\dot{M}$, the accretors enter the strong flashes accretion regime. As expected, in all the considered systems the last He-flash is the strongest one, even if a dynamical event never occurs. When the mass accretion rate decreases below (2-3)$\times 10^{-8} M_\odot yr^{-1}$, the compressional heating of the He-shell becomes less efficient than the neutrino cooling, so that all the accretors in the considered systems evolve into massive degenerate objects. Our results suggest that SNe .Ia or type Ia Supernovae due to Edge-Lit Detonation in the WD family of AM CVn stars should be much more rare than previously expected.

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arXiv2015

On the need of the Light Elements Primary Process (LEPP)

Extant chemical evolution models underestimate the Galactic production of Sr, Y and Zr as well as the Solar System abundances of s-only isotopes with 90 4.0 M_SUN) are negligible; 4) the inclusion of rotation implies a downward shift of the whole distribution with an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only distribution; 5) different prescriptions on convection or mass-loss produce nearly rigid shifts of the whole distribution. In summary, a variation of the standard paradigm of AGB nucleosynthesis would allow to reconcile models predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot be definitely ruled out, because of the uncertainties still affecting stellar and Galactic chemical evolution models.

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arXiv2014

Heavy elements in Globular Clusters: the role of AGB stars

Recent observations of heavy elements in Globular Clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common feature of halo stars, s-process enhancements are found in a few Globular Clusters only. We show that the combined pollution of AGB stars with mass ranging between 3 to 6 M$_\odot$ may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two different neutron-capture nucleosynthesis episodes. The first is due to the 13C(a,n)16O reaction and takes place during the interpulse periods. The second is due to the 22Ne(a,n)25Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Sr, Y, Zr). The first mainly operates in the less-massive AGB stars, while the second dominates in the more-massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for $150\pm 50$ Myr, a period short enough compared to the formation timescale of the Globular Cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.

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