arXiv subjects

Marco Pignatari

Publications and source records attributed to Marco Pignatari.

At least 55 records · Page 3Linked to original sources

arXiv2021

129I and 247Cm in Meteorites Constrain the Last Astrophysical Source of Solar r-process Elements

The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron-capture process (r process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives ($\simeq$ 15.6 Myr) of the radioactive r-process nuclei 129I and 247Cm preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic 129I / 247Cm = 438 $\pm$ 184 to nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.

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arXiv2021

Project ThaiPASS: International Outreach Blending Astronomy and Python

$\textit{Thailand-UK Python+Astronomy Summer School}$ (ThaiPASS), a collaborative project comprising UK and Thai institutions and assess its impact and possible application to schools in the United Kingdom. Since its inception in 2018, the annual ThaiPASS have trained around 60 Thai high-school students in basic data handling skills using Python in the context of various astronomy topics, using current research from the teaching team. Our impact assessment of the 5 day summer schools show an overwhelmingly positive response from students in both years, with over $80\%$ of students scoring the activities above average in all activities but one. We use this data to suggest possible future improvements. We also discuss how ThaiPASS may inspire further outreach and engagement activities within the UK and beyond.

physics.ed-ph↗

arXiv2020

Heavy Elements Nucleosynthesis On Accreting White Dwarfs: building seeds for the p-process

The origin of the proton-rich trans-iron isotopes in the solar system is still uncertain. Single-degenerate thermonuclear supernovae (SNIa) with n-capture nucleosynthesis seeds assembled in the external layers of the progenitor's rapidly accreting white dwarf phase may produce these isotopes. We calculate the stellar structure of the accretion phase of five white dwarf models with initial masses >~ 0.85Msun using the stellar code MESA. The near-surface layers of the 1, 1.26, 1.32 and 1.38Msun models are most representative of the regions in which the bulk of the p nuclei are produced during SNIa explosions, and for these models we also calculate the neutron-capture nucleosynthesis in the external layers. Contrary to previous rapidly-accreting white dwarf models at lower mass, we find that the H-shell ashes are the main site of n-capture nucleosynthesis. We find high neutron densities up to several 10^15 cm^-3 in the most massive WDs. Through the recurrence of the H-shell ashes these intermediate neutron densities can be sustained effectively for a long time leading to high neutron exposures with a strong production up to Pb. Both the neutron density and the neutron exposure increase with increasing the mass of the accreting WD. Finally, the SNIa nucleosynthesis is calculated using the obtained abundances as seeds. We obtain solar to super-solar abundances for p-nuclei with A>96. Our models show that SNIa are a viable p-process production site.

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arXiv2020

Origin of large meteoritic SiC stardust grains in metal-rich AGB stars

Stardust grains that originated in ancient stars and supernovae are recovered from meteorites and carry the detailed composition of their astronomical sites of origin. We present evidence that the majority of large ($μ$m-sized) meteoritic silicon carbide (SiC) grains formed in C-rich asymptotic giant branch (AGB) stars that were more metal-rich than the Sun. In the framework of the slow neutron-captures (the s process) that occurs in AGB stars the lower-than-solar 88Sr/86Sr isotopic ratios measured in the large SiC grains can only be accompanied by Ce/Y elemental ratios that are also lower than solar, and predominately observed in metal-rich barium stars - the binary companions of AGB stars. Such an origin suggests that these large grains represent the material from high-metallicity AGB stars needed to explain the s-process nucleosynthesis variations observed in bulk meteorites (Ek et al. 2020). In the outflows of metal-rich, C-rich AGB stars SiC grains are predicted to be small ($\simeq$ 0.2 $μ$m-sized); large ($\simeq$ $μ$m-sized) SiC grains can grow if the number of dust seeds is two to three orders of magnitude lower than the standard value of $10^{-13}$ times the number of H atoms. We therefore predict that with increasing metallicity the number of dust seeds might decrease, resulting in the production of larger SiC grains.

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arXiv2019

The Nucleosynthetic Yields of Core-Collapse Supernovae, prospects for the Next Generation of Gamma-Ray Astronomy

Though the neutrino-driven convection model for the core-collapse explosion mechanism has received strong support in recent years, there are still many uncertainties in the explosion parameters -- such as explosion energy, remnant mass, and end-of-life stellar abundances as initial conditions. Using a broad set of spherically symmetric core-collapse simulations we examine the effects of these key parameters on explosive nucleosynthesis and final explosion yields. Post-bounce temperature and density evolution of ZAMS 15, 20, and 25 solar mass progenitors are post-processed through the Nucleosynthesis Grid (NuGrid) nuclear network to obtain detailed explosive yields. In particular, this study focuses on radio-isotopes that are of particular interest to the next generation of gamma-ray astronomical observations; 43K, 47Ca, 44Sc, 47Sc, 48V, 48Cr, 51Cr, 52Mn, 59Fe, 56Co, 57Co, 57Ni. These nuclides may be key in advancing our understanding of the inner workings of core-collapse supernovae by probing the parameters of the explosion engine. We find that the isotopes that are strong indicators of explosion energy are 43K, 47Ca, 44Sc, 47Sc, and 59Fe, those that are dependent on the progenitor structure are 48V, 51Cr, and 57Co, and those that probe neither are 48Cr, 52Mn, 57Ni, and 56Co. We discuss prospects of observing these radionuclides in supernova remnants.

astro-ph.HE↗

arXiv2019

The Impact of Nuclear Physics Uncertainties on Galactic Chemical Evolution Predictions

Modeling the evolution of the elements in the Milky Way is a multidisciplinary and challenging task. In addition to simulating the 13 billion years evolution of our Galaxy, chemical evolution simulations must keep track of the elements synthesized and ejected from every astrophysical site of interest (e.g., supernova, compact binary merger). The elemental abundances of such ejecta, which are a fundamental input for chemical evolution codes, are usually taken from theoretical nucleosynthesis calculations performed by the nuclear astrophysics community. Therefore, almost all chemical evolution predictions rely on the nuclear physics behind those calculations. In this proceedings, we highlight the impact of nuclear physics uncertainties on galactic chemical evolution predictions. We demonstrate that nuclear physics and galactic evolution uncertainties both have a significant impact on interpreting the origin of neutron-capture elements in our Solar System. Those results serve as a motivation to create and maintain collaborations between the fields of nuclear astrophysics and galaxy evolution.

astro-ph.GA↗

arXiv2019

Enrichment of the Galactic disc with neutron-capture elements: Mo and Ru

We present new observational data for the heavy elements molybdenum (Mo, Z = 42) and ruthenium (Ru, Z = 44) in F-, G-, and K-stars belonging to different substructures of the Milky Way. The range of metallicity covered is --1.0 $<$ [Fe/H] $<$ +0.3. The spectra of Galactic disc stars have a high resolution of 42,000 and 75,000 and signal-to-noise ratio better than 100. Mo and Ru abundances were derived by comparing the observed and synthetic spectra in the region of Mo I lines at 5506, 5533 Å~ for 209 stars and Ru I lines at 4080, 4584, 4757 Å~ for 162 stars using the LTE approach. For all the stars, the Mo and Ru abundance determinations are obtained for the first time with an average error of 0.14 dex. This is the first extended sample of stellar observations for Mo and Ru in the Milky Way disc, and together with earlier observations in halo stars it is pivotal in providing a complete picture of the evolution of Mo and Ru across cosmic timescales. The Mo and Ru abundances were compared with those of the neutron-capture elements (Sr, Y, Zr, Ba, Sm, Eu). The complex nucleosynthesis history of Mo and Ru is compared with different Galactic Chemical Evolution (GCE) simulations. In general, present theoretical GCE simulations show underproduction of Mo and Ru at all metallicities compared to observations. This highlights a significant contribution of nucleosynthesis processes not yet considered in our simulations. A number of possible scenarios are discussed.

astro-ph.GA↗

arXiv2019

The i-process yields of rapidly-accreting white dwarfs from multicycle He-shell flash stellar evolution models with mixing parameterizations from 3D hydrodynamics simulations

We have modelled the multicycle evolution of rapidly-accreting CO white dwarfs (RAWDs) with stable H burning intermittent with strong He-shell flashes on their surfaces for $0.7\leq M_\mathrm{RAWD}/M_\odot\leq 0.75$ and [Fe/H] ranging from $0$ to $-2.6$. We have also computed the i-process nucleosynthesis yields for these models. The i process occurs when convection driven by the He-shell flash ingests protons from the accreted H-rich surface layer, which results in maximum neutron densities $N_\mathrm{n,max}\approx 10^{13}$-$10^{15}\ \mathrm{cm}^{-3}$. The H-ingestion rate and the convective boundary mixing (CBM) parameter $f_\mathrm{top}$ adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through 3D hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter $f_\mathrm{top}$ have been determined from 3D hydrodynamic simulations. We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency ($η< 10\%$). A new result is that RAWDs with [Fe/H]$< -2$ have $η> 20\%$, therefore their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa. This result and the good fits of the i-process yields from the metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars suggest that some of the present-day CEMP-r/s stars could be former distant members of triple systems, orbiting close binary systems with RAWDs that may have later exploded as SNeIa.

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arXiv2019

Chromium Nucleosynthesis and Silicon-Carbon Shell Mergers in Massive Stars

We analyze the production of the element Cr in galactic chemical evolution (GCE) models using the NuGrid nucleosynthesis yields set. We show that the unusually large [Cr/Fe] abundance at [Fe/H] $\approx 0$ reported by previous studies using those yields and predicted by our Milky Way model originates from the merging of convective Si-burning and C-burning shells in a 20 $M_\odot$ model at metallicity $Z=0.01$, about an hour before the star explodes. This merger mixes the incomplete burning material in the Si shell, including $^{51}$V and $^{52}$Cr, out to the edge of the carbon/oxygen (CO) core. The adopted supernova model ejects the outer 2 $M_\odot$ of the CO core, which includes a significant fraction of the Cr-rich material. When including this 20 $M_\odot$ model at $Z=0.01$ in the yields interpolation scheme of our GCE model for stars in between 15 and 25 $M_\odot$, we overestimate [Cr/Fe] by an order of magnitude at [Fe/H] $\approx$ 0 relative to observations in the Galactic disk. This raises a number of questions regarding the occurrence of Si-C shell mergers in nature, the accuracy of different simulation approaches, and the impact of such mergers on the pre-supernova structure and explosion dynamics. According to the conditions in this 1D stellar model, the substantial penetration of C-shell material into the Si-shell could launch a convective-reactive global oscillation, if a merger does take place. In any case, GCE provides stringent constraints on the outcome of this stellar evolution phase.

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arXiv2019

NuGrid stellar data set - III. Updated low-mass AGB models and s-process nucleosynthesis with metallicities Z=0.01, Z=0.02 and Z=0.03

The production of the neutron-capture isotopes beyond iron that we observe today in the solar system is the result of the combined contribution of the r-process, the s- process and possibly the i-process. Low-mass AGB (2 < M/Msun < 3) and massive (M >10 Msun ) stars have been identified as the sites of the s-process. In this work we consider the evolution and nucleosynthesis of low-mass AGB stars. We provide an update of the NuGrid Set models, adopting the same general physics assumptions but using an updated convective-boundary mixing model accounting for the contribution from internal gravity waves. The combined data set includes the initial masses Mzams/Msun = 2, 3 for Z = 0.03, 0.02, 0.01. These models are computed with the MESA stellar code and the evolution is followed up to the end of the AGB phase. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid mppnp code. The convective boundary mixing model leads to the formation of a 13C-pocket three times wider compared to the one obtained in the previous set of models, bringing the simulation results now in closer agreement with observations. We also discuss the potential impact of other processes inducing mixing, like rotation, adopting parametric models compatible with theory and observations. Complete yield data tables, derived data products and online analytic data access are provided.

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arXiv2019

Galactic Chemical Evolution of Radioactive Isotopes

The presence of short-lived ($\sim$\,Myr) radioactive isotopes in meteoritic inclusions at the time of their formation represents a unique opportunity to study the circumstances that led to the formation of the Solar System. To interpret these observations we need to calculate the evolution of radioactive-to-stable isotopic ratios in the Galaxy. We present an extension of the open-source galactic chemical evolution codes NuPyCEE and JINAPyCEE that enables to track the decay of radioactive isotopes in the interstellar medium. We show how the evolution of isotopic ratio depends on the star formation history and efficiency, star-to-gas mass ratio, and galactic outflows. Given the uncertainties in the observations used to calibrate our model, our predictions for isotopic ratios at the time of formation of the Sun are uncertain by a factor of 3.6. At that time, to recover the actual radioactive-to-stable isotopic ratios predicted by our model, one can multiply the steady-state solution (see Equation~1) by $2.3^{+3.4}_{-0.7}$. However, in the cases where the radioactive isotope has a half-life longer than $\sim$\,200\,Myr, or the target radioactive or stable isotopes have mass- and/or metallicity-depended production rates, or they originate from different sources with different delay-time distributions, or the reference isotope is radioactive, our codes should be used for more accurate solutions. Our preliminary calculations confirm the dichotomy between radioactive nuclei in the early Solar System with $r$- and $s$-process origin, and that $^{55}$Mn and $^{60}$Fe can be explained by galactic chemical evolution, while $^{26}$Al cannot.

astro-ph.GA↗

arXiv2019

Neutron Star Mergers Might not be the Only Source of r-Process Elements in the Milky Way

Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H]\,$>-1$ in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times ($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined with a strong burst of mergers before the onset of Type~Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type~Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this extra site could be responsible for roughly 50% of the Eu production in the early universe, before the onset of Type~Ia supernovae. Rare classes of supernovae could be this additional r-process source, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.

astro-ph.HE↗

arXiv2019

Nucleosynthetic Yields from Neutron Stars Accreting in Binary Common Envelopes

Massive-star binaries can undergo a phase where one of the two stars expands during its advanced evolutionary stage as a giant and envelops its companion, ejecting the hydrogen envelope and tightening its orbit. Such a common envelope phase is required to tighten the binary orbit in the formation of many of the observed X-ray binaries and merging compact binary systems. In the formation scenario for neutron star binaries, the system might pass through a phase where a neutron star spirals into the envelope of its giant star companion. These phases lead to mass accretion onto the neutron star. Accretion onto these common-envelope-phase neutron stars can eject matter that has undergone burning near to the neutron star surface. This paper presents nucleosynthetic yields of this ejected matter, using population synthesis models to study the importance of these nucleosynthetic yields in a galactic chemical evolution context. Depending on the extreme conditions in temperature and density found in the accreted material, both proton-rich and neutron-rich nucleosynthesis can be obtained, with efficient production of neutron rich isotopes of low Z material at the most extreme conditions, and proton rich isotopes, again at low Z, in lower density models. Final yields are found to be extremely sensitive to the physical modeling of the accretion phase. We show that neutron stars accreting in binary common envelopes might be a new relevant site for galactic chemical evolution, and therefore more comprehensive studies are needed to better constrain nucleosynthesis in these objects.

astro-ph.SR↗

arXiv2018

H-He Shell Interactions and Nucleosynthesis in Massive Population III Stars

We report on our ongoing investigation into the nucleosynthetic and hydrodynamic nature of mixing at the interface between the H- and He-convection zones in massive Pop III stars. Studying a grid of 26 1D stellar evolution simulations with different mixing assumptions, we find that H-He interactions occur in 23/26 cases. We demonstrate the nucleosynthesis expected in a H-He interaction in an 80M$_\odot$. Finally, we describe our progress in simulating a Pop III double convection zone in the PPMStar hydrodynamics code.

astro-ph.SR↗

arXiv2018

The impact of (n,$γ$) reaction rate uncertainties of unstable isotopes near $N=50$ on the i process nucleosynthesis in He-shell flash white dwarfs

The first peak s-process elements Rb, Sr, Y and Zr in the post-AGB star Sakurai's object (V4334 Sagittarii) have been proposed to be the result of i-process nucleosynthesis in a post-AGB very-late thermal pulse event. We estimate the nuclear physics uncertainties in the i-process model predictions to determine whether the remaining discrepancies with observations are significant and point to potential issues with the underlying astrophysical model. We find that the dominant source in the nuclear physics uncertainties are predictions of neutron capture rates on unstable neutron rich nuclei, which can have uncertainties of more than a factor 20 in the band of the i-process. We use a Monte Carlo variation of 52 neutron capture rates and a 1D multi-zone post-processing model for the i-process in Sakurai's object to determine the cumulative effect of these uncertainties on the final elemental abundance predictions. We find that the nuclear physics uncertainties are large and comparable to observational errors. Within these uncertainties the model predictions are consistent with observations. A correlation analysis of the results of our MC simulations reveals that the strongest impact on the predicted abundances of Rb, Sr, Y and Zr is made by the uncertainties in the (n,$γ$) reaction rates of $^{85}$Br, $^{86}$Br, $^{87}$Kr, $^{88}$Kr, $^{89}$Kr, $^{89}$Rb, $^{89}$Sr, and $^{92}$Sr. (abridged)

astro-ph.SR↗

arXiv2018

i-Process Contribution of Rapidly Accreting White Dwarfs to the Solar Composition of First-Peak Neutron-Capture Elements

Rapidly accreting white dwarfs (RAWDs) have been proposed as contributors to the chemical evolution of heavy elements in the Galaxy. Here, we test this scenario for the first time and determine the contribution of RAWDs to the solar composition of first-peak neutron-capture elements. We add the metallicity-dependent contribution of RAWDs to the one-zone galactic chemical evolution code OMEGA according to RAWD rates from binary stellar population models combined with metallicity-dependent $i$-process stellar yields calculated following the models of Denissenkov et al. (2017). With this approach we find that the contribution of RAWDs to the evolution of heavy elements in the Galaxy could be responsible for a significant fraction of the solar composition of Kr, Rb, Sr, Y, Zr, Nb, and Mo ranging from $2$ to $45\%$ depending on the element, the enrichment history of the Galactic gas, and the total mass ejected per RAWD. This contribution could explain the missing solar Lighter Element Primary Process for some elements (e.g., Sr, Y, and Zr). We do not overproduce any isotope relative to the solar composition, but $^{96}$Zr is produced in a similar amount. The $i$ process produces efficiently the Mo stable isotopes $^{95}$Mo and $^{97}$Mo. When nuclear reaction rate uncertainties are combined with our GCE uncertainties, the upper limits for the predicted RAWD contribution increase by a factor of $1.5-2$ for Rb, Sr, Y, and Zr, and by 3.8 and 2.4 for Nb and Mo, respectively. We discuss the implication of the RAWD stellar evolution properties on the single degenerate Type Ia supernova scenario.

astro-ph.SR↗

arXiv2017

Stellar origin of 15N-rich presolar SiC grains of type AB: supernovae with explosive hydrogen burning

We report C, N, and Si isotopic data for 59 highly 13C-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 15N-rich (14N/15N<=solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in 13C, 15N and 26Al with the PNGs showing more extreme enhancements. The 15N-rich AB grains show systematically higher 26Al and 30Si excesses than the 14N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (14N/15N =solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG, and found 32S and/or 50Ti enhancements. Interestingly, one AB1 grain had the largest 32S and 50Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the 32S excess and thus the initial presence of radiogenic 32Si (t1/2=153 yr). More importantly, we found that the 15N and 26Al excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from SNe that experienced H ingestion into the He/C zones of their progenitors.

astro-ph.SR↗

arXiv2017

Stellar yields and abundances: new directions from planetary nebulae

Planetary nebulae retain the signature of the nucleosynthesis and mixing events that occurred during the previous AGB phase. Observational signatures complement observations of AGB and post-AGB stars and their binary companions. The abundances of the elements heavier than iron such as Kr and Xe in planetary nebulae can be used to complement abundances of Sr/Y/Zr and Ba/La/Ce in AGB stars, respectively, to determine the operation of the slow neutron-capture process (the s process) in AGB stars. Additionally, observations of the Rb abundance in Type I planetary nebulae may allow us to infer the initial mass of the central star. Several noble gas components present in meteoritic stardust silicon carbide (SiC) grains are associated with implantation into the dust grains in the high-energy environment connected to the fast winds from the central stars during the planetary nebulae phase.

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