arXiv subjects

M. Lugaro

Publications and source records attributed to M. Lugaro.

At least 37 records · Page 2Linked to original sources

arXiv2023

The $γ$-process nucleosynthesis in core-collapse supernovae. I. A novel analysis of $γ$-process yields in massive stars

The $γ$-process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average production of $γ$-process yields from massive stars are too low to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are by a further factor of 10 lower than the yields of the other $γ$-process nuclei. We investigate the $γ$-process in 5 sets of core-collapse supernova models published in literature with initial masses 15, 20, and 25 M$_{\odot}$ at solar metallicity. We compared the $γ$-process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass, relative to their solar values. Further, we investigated the contribution of C-O shell mergers in the supernova progenitors as an additional site of the $γ$-process. Our analysis shows that a large scatter among the different models exists for both the $γ$-process integrated yields and the isotopic ratios. We found only 10 ratios that agree with their solar values, all the others differ by at least a factor of 3 from the solar values in all the considered sets of models. The $γ$-process within C-O shell mergers mostly influence the isotopic ratios that involve intermediate and heavy proton-rich isotopes with $\rm A>100$.

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arXiv2023

Aluminium-26 from massive binary stars III. Binary stars up to core-collapse and their impact on the early Solar System

Many of the short-lived radioactive nuclei that were present in the early Solar System can be produced in massive stars. In the first paper in this series (Brinkman et al. 2019), we focused on the production of $^{26}$Al in massive binaries. In our second paper (Brinkman et al. 2021), we considered rotating single stars, two more short-lived radioactive nuclei, $^{36}$Cl and $^{41}$Ca, and the comparison to the early Solar System data. In this work, we update our previous conclusions by further considering the impact of binary interactions. We used the MESA stellar evolution code with an extended nuclear network to compute massive (10-80 M$ _{\odot} $), binary stars at various initial periods and solar metallicity (Z=0.014), up to the onset of core collapse. The early Solar System abundances of $^{26}$Al and $^{41}$Ca can be matched self-consistently by models with initial masses $\geq$25 M$_{\odot}$, while models with initial primary masses $\geq$35 M$_{\odot}$ can also match $^{36}$Cl. Almost none of the models provide positive net yields for $^{19}$F, while for $^{22}$Ne the net yields are positive from 30 M$_{\odot}$ and higher. This leads to an increase by a factor of approximately 4 in the amount of $^{22}$Ne produced by a stellar population of binary stars, relative to single stars. Also, besides the impact on the stellar yields, our 10 M$_{\odot}$ primary star undergoing Case A mass-transfer ends its life as a white dwarf instead of as a core-collapse supernova. This demonstrates that binary interactions can also strongly impact the evolution of stars close to the supernova boundary.

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arXiv2022

Barium stars as tracers of s-process nucleosynthesis in AGB stars II. Using machine learning techniques on 169 stars

We aim to analyse the abundance pattern of 169 Barium (Ba) stars, using machine learning techniques and the AGB final surface abundances predicted by Fruity and Monash stellar models. We developed machine learning algorithms that use the abundance pattern of Ba stars as input to classify the initial mass and metallicity of its companion star using stellar model predictions. We use two algorithms: the first exploits neural networks to recognise patterns and the second is a nearest-neighbour algorithm, which focuses on finding the AGB model that predicts final surface abundances closest to the observed Ba star values. In the second algorithm we include the error bars and observational uncertainties to find the best fit model. The classification process is based on the abundances of Fe, Rb, Sr, Zr, Ru, Nd, Ce, Sm, and Eu. We selected these elements by systematically removing s-process elements from our AGB model abundance distributions, and identifying those whose removal has the biggest positive effect on the classification. We excluded Nb, Y, Mo, and La. Our final classification combines the output of both algorithms to identify for each Ba star companion an initial mass and metallicity range. With our analysis tools we identify the main properties for 166 of the 169 Ba stars in the stellar sample. The classifications based on both stellar sets of AGB final abundances show similar distributions, with an average initial mass of M = 2.23 MSun and 2.34 MSun and an average [Fe/H] = -0.21 and -0.11, respectively. We investigated why the removal of Nb, Y, Mo, and La improves our classification and identified 43 stars for which the exclusion had the biggest effect. We show that these stars have statistically significant different abundances for these elements compared to the other Ba stars in our sample. We discuss the possible reasons for these differences in the abundance patterns.

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arXiv2022

High-resolution spectroscopic analysis of four new chemically peculiar stars

We present detailed chemical compositions of four stars on the first-ascent red giant branch that are classified as chemically peculiar, but lack comprehensive analyses at high spectral resolution. For BD+03°2688, HE 0457-1805, HE 1255-2324, and HE 2207-1746, we derived metallicities [Fe/H] $=-1.21$, $-0.19$, $-0.31$, and $-0.55$, respectively, indicating a range in Galactic population membership. In addition to atmospheric parameters, we extracted elemental abundances for 28 elements, including the evolutionary-sensitive CNO group and $^{12}$C/$^{13}$C ratios. Novel results are also presented for the heavy elements tungsten and thallium. All four stars have very large enhancements of neutron-capture elements, with high [La/Eu] ratios indicating enrichments from the slow neutron capture ($s$-process). To interpret these abundances, all indicative of [$s$/Fe] $> 1.0$, we compared our results with data from literature, as well as with predictions from the Monash and FRUITY $s$-process nucleosynthesis models. BD+03°2688, HE 1255-2324, and HE 2207-1746 show C/O $>1$, while HE 0457-1805 has C/O $<1$. Since HE 0457-1805 and HE 1255-2324 are binary stars, their peculiarities are attributable to mass transfer. We identified HE 0457-1805 as a new barium giant star, and HE 1255-2324 as a new CH star, in fact a higher metallicity analogue CEMP-$r/s$ star; the single object reported in literature so far with similar characteristics is the barium star HD 100503 ([Fe/H] $= -0.72$). A systematic monitoring is needed to confirm the binary nature of BD+03°2688 and HE 2207-1746, which are probably CH stars.

astro-ph.SR↗

arXiv2022

First direct limit on the 334 keV resonance strength in the $^{22}$Ne(α,γ)$^{26}$Mg reaction

In stars, the fusion of $^{22}$Ne and $^4$He may produce either $^{25}$Mg, with the emission of a neutron, or $^{26}$Mg and a $γ$ ray. At high temperature, the ($α,n$) channel dominates, while at low temperature, it is energetically hampered. The rate of its competitor, the $^{22}$Ne($α$,$γ$)$^{26}$Mg reaction, and, hence, the minimum temperature for the ($α,n$) dominance, are controlled by many nuclear resonances. The strengths of these resonances have hitherto been studied only indirectly. The present work aims to directly measure the total strength of the resonance at $E$_{r}$\,=\,$334$\,$keV (corresponding to $E$_{x}$\,=\,$10949$\,$keV in $^{26}$Mg). The data reported here have been obtained using high intensity $^4$He$^+$ beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched $^{22}$Ne gas target, and a 4$π$ bismuth germanate summing $γ$-ray detector. The ultra-low background rate of less than 0.5 counts/day was determined using 67 days of no-beam data and 7 days of $^4$He$^+$ beam on an inert argon target. The new high-sensitivity setup allowed to determine the first direct upper limit of 4.0$\,\times\,$10$^{-11}$ eV (at 90% confidence level) for the resonance strength. Finally, the sensitivity of this setup paves the way to study further $^{22}$Ne($α$,$γ$)$^{26}$Mg resonances at higher energy.

nucl-ex↗

arXiv2022

Horizons: Nuclear Astrophysics in the 2020s and Beyond

Nuclear Astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

nucl-ex↗

arXiv2022

Barium stars as tracers of s-process nucleosynthesis in AGB stars I. 28 stars with independently derived AGB mass

Barium (Ba) stars are polluted by material enriched in the slow neutron capture (s-process) elements synthesised in the interior of their former asymptotic giant branch (AGB) companion star, which is now a white dwarf. We compare individual Ba star abundance patterns to AGB nucleosynthesis models to verify if the AGB model mass is compatible with independently derived AGB mass. We selected a sample of 28 Ba stars for which both self-consistent spectroscopic observation and analysis are available and stellar mass determinations, via positioning the star on the HR diagram and comparing with evolutionary tracks. For this sample stars we considered both previously and recently derived elemental abundances. Then, we performed a detailed comparison of these s-process elemental abundances to different AGB nucleosynthesis models from the Monash and the FRUITY theoretical data sets. We simplified the binary mass transfer by calculating dilution factors to match the [Ce/Fe] value of each star when using different AGB models, and we then compared the diluted model abundances to the complete Ba-star abundance pattern. Our comparison confirms that low mass, non-rotating AGB stellar models with 13C as the main neutron source are the polluters of the vast majority of the considered Ba stars. Out of the 28 stars, in 21 cases the models are in good agreement with both the determined abundances and the independently derived AGB mass, although in 16 cases higher observed abundances of Nb, Ru, Mo and/or Nd, Sm than predicted. For 3 stars we obtain a match to the abundances only by considering models with masses lower than those independently determined. Finally, 4 stars show much higher first s-process peak abundance values than the model predictions, which may represent the signature of a physical and/or nucleosynthetic process that is not represented in the set of models considered here.

astro-ph.SR↗

arXiv2021

Galactic Chemical Evolution of Radioactive Isotopes with an s-process Contribution

Analysis of inclusions in primitive meteorites reveals that several short-lived radionuclides (SLRs) with half-lives $0.1-100$ Myr existed in the early Solar System (ESS). We investigate the ESS origin of $^{107}$Pd, $^{135}$Cs, and $^{182}$Hf, which are produced by $slow$ neutron captures (the $s$-process) in asymptotic giant branch (AGB) stars. We modelled the galactic abundances of these SLRs using the OMEGA+ galactic chemical evolution (GCE) code and two sets of mass- and metallicity-dependent AGB nucleosynthesis yields (Monash and FRUITY). Depending on the ratio of the mean life $τ$ of the SLR to the average length of time between the formation of AGB progenitor $γ$, we calculate timescales relevant for the birth of the Sun. If $τ/γ\gtrsim2$, we predict self-consistent isolation times between 9 and 26 Myr by decaying the GCE predicted $^{107}$Pd/$^{108}$Pd, $^{135}$Cs/$^{133}$Cs, and $^{182}$Hf/$^{180}$Hf ratios to their respective ESS ratios. The predicted $^{107}$Pd/$^{182}$Hf ratio indicates that our GCE models are missing $9-73\%$ of $^{107}$Pd and $^{108}$Pd in the ESS. This missing component may have come from AGB stars of higher metallicity than those that contributed to the ESS in our GCE code. If $τ/γ\lesssim0.3$, we calculate instead the time ($T_{\rm LE}$) from the last nucleosynthesis event that added the SLRs into the presolar matter to the formation of the oldest solids in the ESS. For the 2 M$_\odot$, $Z=0.01$ Monash model we find a self-consistent solution of $T_{\rm LE}=25.5$ Myr.

astro-ph.GA↗

arXiv2021

First models of s process in AGB stars of solar metallicity for the stellar evolutionary code ATON with a novel stable explicit numerical solver

Aims. We describe the first s-process post-processing models for asymptotic giant branch (AGB) stars of masses 3, 4 and 5 M at solar metallicity (Z=0.018) computed using the input from the stellar evolutionary code aton. Methods. The models are computed with the new code snuppat(S-process NUcleosynthesis Post-Processing code for aton), including an advective scheme for the convective overshoot that leads to the formation of the main neutron source, 13C. Each model is post-processed with 3 different values of the free overshoot parameter. Included in the code snuppat is the novel Patankar-Euler-Deflhard explicit numerical solver, that we use to solve the nuclear network system of differential equations. Results. The results are compared to those from other s-process nucleosynthesis codes (Monash,fruity, and NuGrid), as well as observations of s-process enhancement in AGB stars, planetary nebulae, and barium stars. This comparison shows that the relatively high abundance of12C in the He-rich intershell in aton results in as-process abundance pattern that favours the second over the first s-process peak for all the masses explored. Also, our choice of an advective as opposed to diffusive numerical scheme for the convective overshoot results in significants-process nucleosynthesis also for the 5 M models, which may be in contradiction with observations.

astro-ph.SR↗

arXiv2021

Direct measurement of the 13C(α,n)16O cross section into the s-process Gamow peak

One of the main neutron sources for the astrophysical s-process is the reaction 13C(α,n)16O, taking place in thermally pulsing Asymptotic Giant Branch stars at temperatures around 90 MK. To model the nucleosynthesis during this process the reaction cross section needs to be known in the 150-230keV energy window (Gamow peak). At these sub-Coulomb energies cross section direct measurements are severely affected by the low event rate, making us rely on input from indirect methods and extrapolations from higher-energy direct data. This leads to an uncertainty in the cross section at the relevant energies too high to reliably constrain the nuclear physics input to s-process calculations. We present the results from a new deep-underground measurement of 13C(α,n)16O, covering the energy range 230-300keV, with drastically reduced uncertainties over previous measurements and for the first time providing data directly inside the s-process Gamow peak. Selected stellar models have been computed to estimate the impact of our revised reaction rate. For stars of nearly solar composition, we find sizeable variations of some isotopes, whose production is influenced by the activation of close-by branching points that are sensitive to the neutron density, in particular the two radioactive nuclei 60Fe and 205Pb, as well as 152Gd

nucl-ex↗

arXiv2021

The Status and Future of Direct Nuclear Reaction Measurements for Stellar Burning

The study of stellar burning began just over 100 years ago. Nonetheless, we do not yet have a detailed picture of the nucleosynthesis within stars and how nucleosynthesis impacts stellar structure and the remnants of stellar evolution. Achieving this understanding will require precise direct measurements of the nuclear reactions involved. This report summarizes the status of direct measurements for stellar burning, focusing on developments of the last couple of decades, and offering a prospectus of near-future developments.

nucl-ex↗

arXiv2021

Aluminium-26 from massive binary stars II. Rotating single stars up to core-collapse and their impact on the early Solar System

Radioactive nuclei were present in the early Solar System, as inferred from analysis of meteorites. Many are produced in massive stars, either during their lives or their final explosions. In the first paper in this series (Brinkman et al. 2019), we focused on the production of $^{26}$Al in massive binaries. Here, we focus on the production of another two short-lived radioactive nuclei, $^{36}$Cl and $^{41}$Ca, and the comparison to the early Solar System data. We used the MESA stellar evolution code with an extended nuclear network and computed massive (10-80 M$ _{\odot} $), rotating (with initial velocities of 150 and 300 km/s) and non-rotating single stars at solar metallicity (Z=0.014) up to the onset of core collapse. We present the wind yields for the radioactive isotopes $^{26}$Al, $^{36}$Cl, and $^{41}$Ca, and the stable isotopes $^{19}$F and $^{20}$Ne. In relation to the stable isotopes, we find that only the most massive models, $\geq$ 60M$_{\odot}$ and $\geq$ 40M$_{\odot}$ give positive $^{19}$F and $^{20}$Ne yields, respectively, depending on the initial rotation rate. In relation to the radioactive isotopes, we find that the early Solar System abundances of $^{26}$Al and $^{41}$Ca can be matched with by models with initial masses $\geq$40M$_{\odot}$, while $^{36}$Cl is matched only by our most massive models, $\geq$60M$_{\odot}$. $^{60}$Fe is not significantly produced by any wind model, as required by the observations. Therefore, massive star winds are a favoured candidate for the origin of the very short-lived $^{26}$Al, $^{36}$Cl, and $^{41}$Ca in the early Solar System.

astro-ph.SR↗

arXiv2021

The $^{59}$Fe(n, γ)$^{60}$Fe Cross Section from the Surrogate Ratio Method and Its Effect on the $^{60}$Fe Nucleosynthesis

The long-lived $^{60}$Fe (with a half-life of 2.62 Myr) is a crucial diagnostic of active nucleosynthesis in the Milky Way galaxy and in supernovae near the solar system. The neutron-capture reaction $^{59}$Fe(n,$γ$)$^{60}$Fe on $^{59}$Fe (half-life = 44.5 days) is the key reaction for the production of $^{60}$Fe in massive stars. This reaction cross section has been previously constrained by the Coulomb dissociation experiment, which offered partial constraint on the $E$1 $γ$-ray strength function but a negligible constraint on the $M$1 and $E$2 components. In this work, for the first time, we use the surrogate ratio method to experimentally determine the $^{59}$Fe(n,$γ$)$^{60}$Fe cross sections in which all the components are included. We derived a Maxwellian-averaged cross section of 27.5 $\pm$ 3.5 mb at $kT$= 30 keV and 13.4 $\pm$ 1.7 mb at $kT$= 90 keV, roughly 10 - 20% higher than previous estimates. We analyzed the impact of our new reaction rates in nucleosynthesis models of massive stars and found that uncertainties in the production of $^{60}$Fe from the $^{59}$Fe(n,$γ$)$^{60}$Fe rate are at most of 25%. We conclude that stellar physics uncertainties now play a major role in the accurate evaluation of the stellar production of $^{60}$Fe.

astro-ph.GA↗

arXiv2021

Heavy elements in barium stars

New elemental abundances for the neutron-capture elements Sr, Nb, Mo, Ru, La, Sm, and Eu are presented for a large sample of 180 barium (Ba) giant stars, a class of chemically peculiar objects that exhibit in their spectra enhancements of the elements created by the $s$-process, as a consequence of mass transfer between the components of a binary system. The content of heavy elements in these stars, in fact, points to nucleosynthesis mechanisms that took place within a former asymptotic giant branch (AGB) companion, now an invisible white dwarf. From high-resolution ($R=48000$) spectra in the optical, we derived the abundances either by equivalent width measurements or synthetic spectra computations, and compared them with available data for field giant and dwarf stars in the same range of metallicity. A re-determination of La abundances resulted in [La/Fe] ratios up to 1.2 dex lower than values previously reported in literature. The program Ba stars show overabundance of neutron-capture elements, except for Eu, for which the observational data set behave similarly to field stars. Comparison to model predictions are satisfactory for second-to-first $s$-process peak ratios (e.g., [La/Sr]) and the ratios of the predominantly $r$-process element Eu to La. However, the observed [Nb,Mo,Ru/Sr] and [Ce,Nd,Sm/La] ratios show median values higher or at the upper limits of the ranges of the model predictions. This unexplained feature calls for new neutron capture models to be investigated.

astro-ph.SR↗

arXiv2021

Gas and dust from extremely metal-poor AGB stars

We study the evolution of extremely metal-poor AGB stars, with metallicities down to [Fe/H]=-5, to understand the main evolutionary properties, the efficiency of the processes able to alter their surface chemical composition and to determine the gas and dust yields. We calculate two sets of evolutionary sequences of stars in the 1-7.5Msun mass range, evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries we adopted the metallicities Z=3x10^{-5} and Z=3x10^{-7} which correspond, respectively to [Fe/H]=-3 and [Fe/H]=-5. The results from stellar evolution modelling are used to calculate the yields of the individual chemical species. We also modelled dust formation in the wind, to determine the dust produced by these objects. The evolution of AGB stars in the extremely metal-poor domain explored here proves tremendously sensitive to the initial mass of the star. M<2Msun stars experience several third dredge-up events, which favour the gradual surface enrichment of C12 and the formation of significant quantities of carbonaceous dust, of the order of 0.01Msun. The C13 and nitrogen yiel are found to be significantly smaller than in previous explorations of low-mass, metal-poor AGB stars, owing to the weaker proton ingestion episodes experienced during the initial AGB phases. M>5Msun stars experience hot bottom burning and their surface chemistry reflects the equilibria of a very advanced proton-capture nucleosynthesis; little dust production takes place in their wind. Intermediate mass stars experience both third dredge-up and hot bottom burning: they prove efficient producers of nitrogen, which is formed by proton captures on C12 nuclei of primary origin dredged-up from the internal regions.

astro-ph.SR↗

arXiv2021

The Gaia-ESO Survey: A new approach to chemically characterising young open clusters II. Abundances of the neutron-capture elements Cu, Sr, Y, Zr, Ba, La, and Ce

Young open clusters (t<200 Myr) have been observed to exhibit several peculiarities in their chemical compositions, from a slightly sub-solar iron content, super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of [Ba/Fe], with values up to +0.7 dex. Regarding the behaviour of the other $s$-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature. In this work we expand upon our previous analysis of a sample of five young open clusters (IC2391, IC2602, IC4665, NGC2516, and NGC2547) and one star-forming region (NGC2264), with the aim of determining abundances of different neutron-capture elements, mainly CuI, SrI, SrII, YII, ZrII, BaII, LaII, and CeII. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the \textit{Gaia}-ESO survey. We find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm the super-solar [Ba/Fe] values (from +0.22 to +0.64 dex). Our analysis also points to mildly enhanced [Y/Fe] values (from 0 and +0.3 dex). For the other $s$-process elements we find that [X/Fe] ratios are solar at all ages. It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. Thus, we explore different possible scenarios related to the behaviour of spectral lines, from the sensitivity to the presence of magnetic fields to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to higher levels of activity in such young stars. We are still unable to explain these enhancements, but we suggest that other elements (i.e. La) might be more reliable tracer of the $s$-process at young ages and encourage further observations.

astro-ph.SR↗

arXiv2021

Monte Carlo Investigation of the Ratios of Short-Lived Radioactive Isotopes in the Interstellar Medium

Short-lived radioactive nuclei (SLR) with mean lives below 100 Myr provide us with unique insights into current galactic nucleosynthetic events, as well as events that contributed to the material of our Solar System more that 4.6 Gyr ago. Here we present a statistical analysis of the ratios of these radioactive nuclei at the time of early Solar System (ESS) using both analytical derivations and Monte Carlo methods. We aim to understand the interplay between the production frequency and the mean lives of these isotopes, and its impact on their theoretically predicted ratios in the interstellar medium (ISM). We find that when the ratio of two SRLs, instead of the ratios of each single SLR relative to its stable or long-lived isotope, is considered, not only the uncertainties related to the galactic chemical evolution of the stable isotope are completely eliminated, but also the statistical uncertainties are much lower. We identify four ratios, 247Cm/129I, 107Pd/182Hf, 97Tc/98Tc, and 53Mn/97Tc, that have the potential to provide us with new insights into the r-, s-, and p-process nucleosynthesis at the time of the formation of the Sun, and need to be studied using variable stellar yields. Additionally, the latter two ratios need to be better determined in the ESS to allow us to fully exploit them to investigate the galactic sites of the p process.

astro-ph.GA↗

arXiv2020

Setup commissioning for an improved measurement of the D(p,gamma)3He cross section at Big Bang Nucleosynthesis energies

Among the reactions involved in the production and destruction of deuterium during Big Bang Nucleosynthesis, the deuterium-burning D(p,gamma)3He reaction has the largest uncertainty and limits the precision of theoretical estimates of primordial deuterium abundance. Here we report the results of a careful commissioning of the experimental setup used to measure the cross-section of the D(p,gamma)3He reaction at the Laboratory for Underground Nuclear Astrophysics of the Gran Sasso Laboratory (Italy). The commissioning was aimed at minimising all sources of systematic uncertainty in the measured cross sections. The overall systematic error achieved (< 3 %) will enable improved predictions of BBN deuterium abundance.

physics.ins-det↗